Understanding slice notation

Question

I need a good explanation  references are a plus  on Python s slice notation    To me  this notation needs a bit of picking up    It looks extremely powerful  but I haven t quite got my head around it

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After using it a bit I realise that the simplest description is that it is exactly the same as the arguments in a for loop...

(from:to:step)

Any of them are optional:

(:to:step)
(from::step)
(from:to)

Then the negative indexing just needs you to add the length of the string to the negative indices to understand it.

This works for me anyway...

User · Answer

Python slicing notation   a start end step     For start and end  negative values are interpreted as being relative to the end of the sequence  Positive indices for end indicate the position after the last element to be included  Blank values are defaulted as follows    0 -0 1   Using a negative step reverses the interpretation of start and end   The notation extends to  numpy  matrices and multidimensional arrays   For example  to slice entire columns you can use   m    0 2      slice the first two columns   Slices hold references  not copies  of the array elements   If you want to make a separate copy an array  you can use deepcopy

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You can also use slice assignment to remove one or more elements from a list   r    1   blah   9  8  2  3  4   gt  gt  gt  r 1 4        gt  gt  gt  r  1  2  3  4

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The Python tutorial talks about it  scroll down a bit until you get to the part about slicing    The ASCII art diagram is helpful too for remembering how slices work     --- --- --- --- --- ---     P   y   t   h   o   n     --- --- --- --- --- ---   0   1   2   3   4   5   6 -6  -5  -4  -3  -2  -1      One way to remember how slices work is to think of the indices as pointing between characters  with the left edge of the first character numbered 0  Then the right edge of the last character of a string of n characters has index n

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The Python tutorial talks about it  scroll down a bit until you get to the part about slicing    The ASCII art diagram is helpful too for remembering how slices work     --- --- --- --- --- ---     P   y   t   h   o   n     --- --- --- --- --- ---   0   1   2   3   4   5   6 -6  -5  -4  -3  -2  -1      One way to remember how slices work is to think of the indices as pointing between characters  with the left edge of the first character numbered 0  Then the right edge of the last character of a string of n characters has index n

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In my opinion  you will understand and memorize better the Python string slicing notation if you look at it the following way  read on    Let s work with the following string      azString    abcdefghijklmnopqrstuvwxyz    For those who don t know  you can create any substring from azString using the notation azString x y   Coming from other programming languages  that s when the common sense gets compromised  What are x and y   I had to sit down and run several scenarios in my quest for a memorization technique that will help me remember what x and y are and help me slice strings properly at the first attempt   My conclusion is that x and y should be seen as the boundary indexes that are surrounding the strings that we want to extra  So we should see the expression as azString index1  index2  or even more clearer as azString index of first character  index after the last character    Here is an example visualization of that      Letters   a b c d e f g h i j                                                             Indexes  0 1 2 3 4 5 6 7 8 9                                cdefgh    index1       index2   So all you have to do is setting index1 and index2 to the values that will surround the desired substring  For instance  to get the substring  cdefgh   you can use azString 2 8   because the index on the left side of  c  is 2 and the one on the right size of  h  is 8   Remember that we are setting the boundaries  And those boundaries are the positions where you could place some brackets that will be wrapped around the substring like this      a b   c d e f g h   i j  That trick works all the time and is easy to memorize

User · Answer

After using it a bit I realise that the simplest description is that it is exactly the same as the arguments in a for loop...

(from:to:step)

Any of them are optional:

(:to:step)
(from::step)
(from:to)

Then the negative indexing just needs you to add the length of the string to the negative indices to understand it.

This works for me anyway...

User · Answer

It s pretty simple really   a start stop     items start through stop-1 a start          items start through the rest of the array a  stop          items from the beginning through stop-1 a                a copy of the whole array   There is also the step value  which can be used with any of the above   a start stop step    start through not past stop  by step   The key point to remember is that the  stop value represents the first value that is not in the selected slice  So  the difference between stop and start is the number of elements selected  if step is 1  the default    The other feature is that start or stop may be a negative number  which means it counts from the end of the array instead of the beginning  So   a -1       last item in the array a -2       last two items in the array a  -2      everything except the last two items   Similarly  step may be a negative number   a   -1       all items in the array  reversed a 1  -1      the first two items  reversed a  -3 -1     the last two items  reversed a -3  -1     everything except the last two items  reversed   Python is kind to the programmer if there are fewer items than you ask for  For example  if you ask for a  -2  and a only contains one element  you get an empty list instead of an error  Sometimes you would prefer the error  so you have to be aware that this may happen   Relation to slice   object  The slicing operator    is actually being used in the above code with a slice   object using the   notation  which is only valid within      i e    a start stop step    is equivalent to   a slice start  stop  step     Slice objects also behave slightly differently depending on the number of arguments  similarly to range    i e  both slice stop  and slice start  stop   step   are supported  To skip specifying a given argument  one might use None  so that e g  a start   is equivalent to a slice start  None   or a   -1  is equivalent to a slice None  None  -1     While the  -based notation is very helpful for simple slicing  the explicit use of slice   objects simplifies the programmatic generation of slicing

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Enumerating the possibilities allowed by the grammar    gt  gt  gt  seq                      seq 0     seq 1                 seq -1        gt  gt  gt  seq low                  seq low   seq low 1             seq -1        gt  gt  gt  seq  high                seq 0     seq 1                 seq high-1    gt  gt  gt  seq low high             seq low   seq low 1             seq high-1    gt  gt  gt  seq   stride             seq 0     seq stride            seq -1        gt  gt  gt  seq low  stride          seq low   seq low stride        seq -1        gt  gt  gt  seq  high stride         seq 0     seq stride            seq high-1    gt  gt  gt  seq low high stride      seq low   seq low stride        seq high-1     Of course  if  high-low  stride    0  then the end point will be a little lower than high-1   If stride is negative  the ordering is changed a bit since we re counting down    gt  gt  gt  seq   -stride            seq -1     seq -1-stride          seq 0        gt  gt  gt  seq high  -stride        seq high   seq high-stride        seq 0        gt  gt  gt  seq  low -stride         seq -1     seq -1-stride          seq low 1    gt  gt  gt  seq high low -stride     seq high   seq high-stride        seq low 1     Extended slicing  with commas and ellipses  are mostly used only by special data structures  like NumPy   the basic sequences don t support them    gt  gt  gt  class slicee          def   getitem   self  item               return repr item       gt  gt  gt  slicee   0  1 2    5         0  slice 1  2  None   slice None  None  5   Ellipsis

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In Python  the most basic form for slicing is the following   l start end    where l is some collection  start is an inclusive index  and end is an exclusive index   In  1   l   list range 10    In  2   l  5    First five elements Out 2    0  1  2  3  4   In  3   l -5     Last five elements Out 3    5  6  7  8  9    When slicing from the start  you can omit the zero index  and when slicing to the end  you can omit the final index since it is redundant  so do not be verbose   In  5   l  3     l 0 3  Out 5   True  In  6   l 7      l 7 len l   Out 6   True   Negative integers are useful when doing offsets relative to the end of a collection   In  7   l  -1    Include all elements but the last one Out 7    0  1  2  3  4  5  6  7  8   In  8   l -3     Take the last three elements Out 8    7  8  9    It is possible to provide indices that are out of bounds when slicing such as   In  9   l  20    20 is out of index bounds  and l 20  will raise an IndexError exception Out 9    0  1  2  3  4  5  6  7  8  9   In  11   l -20     -20 is out of index bounds  and l -20  will raise an IndexError exception Out 11    0  1  2  3  4  5  6  7  8  9    Keep in mind that the result of slicing a collection is a whole new collection  In addition  when using slice notation in assignments  the length of the slice assignments do not need to be the same  The values before and after the assigned slice will be kept  and the collection will shrink or grow to contain the new values   In  16   l 2 6    list  abc     Assigning fewer elements than the ones contained in the sliced collection l 2 6   In  17   l Out 17    0  1   a    b    c   6  7  8  9   In  18   l 2 5    list  hello     Assigning more elements than the ones contained in the sliced collection l  2 5   In  19   l Out 19    0  1   h    e    l    l    o   6  7  8  9    If you omit the start and end index  you will make a copy of the collection   In  14   l copy   l     In  15   l    l copy and l is not l copy Out 15   True   If the start and end indexes are omitted when performing an assignment operation  the entire content of the collection will be replaced with a copy of what is referenced   In  20   l      list  hello       In  21   l Out 21     h    e    l    l    o                    Besides basic slicing  it is also possible to apply the following notation   l start end step    where l is a collection  start is an inclusive index  end is an exclusive index  and step is a stride that can be used to take every nth item in l   In  22   l   list range 10    In  23   l   2    Take the elements which indexes are even Out 23    0  2  4  6  8   In  24   l 1  2    Take the elements which indexes are odd Out 24    1  3  5  7  9    Using step provides a useful trick to reverse a collection in Python   In  25   l   -1  Out 25    9  8  7  6  5  4  3  2  1  0    It is also possible to use negative integers for step as the following example   In 28    l   -2  Out 28    9  7  5  3  1    However  using a negative value for step could become very confusing  Moreover  in order to be Pythonic  you should avoid using start  end  and step in a single slice  In case this is required  consider doing this in two assignments  one to slice  and the other to stride    In  29   l   l   2    This step is for striding  In  30   l Out 30    0  2  4  6  8   In  31   l   l 1 -1    This step is for slicing  In  32   l Out 32    2  4  6

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The below is the example of an index of a string     --- --- --- --- ---     H   e   l   p   A     --- --- --- --- ---   0   1   2   3   4   5 -5  -4  -3  -2  -1  str  Name string    Slicing example   start end step   str start end    Items start through end-1 str start        Items start through the rest of the array str  end         Items from the beginning through end-1 str              A copy of the whole array   Below is the example usage   print str 0    N print str 0 2    Na print str 0 7    Name st print str 0 7 2    Nm t print str 0 -1 2    Nm ti

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It s pretty simple really   a start stop     items start through stop-1 a start          items start through the rest of the array a  stop          items from the beginning through stop-1 a                a copy of the whole array   There is also the step value  which can be used with any of the above   a start stop step    start through not past stop  by step   The key point to remember is that the  stop value represents the first value that is not in the selected slice  So  the difference between stop and start is the number of elements selected  if step is 1  the default    The other feature is that start or stop may be a negative number  which means it counts from the end of the array instead of the beginning  So   a -1       last item in the array a -2       last two items in the array a  -2      everything except the last two items   Similarly  step may be a negative number   a   -1       all items in the array  reversed a 1  -1      the first two items  reversed a  -3 -1     the last two items  reversed a -3  -1     everything except the last two items  reversed   Python is kind to the programmer if there are fewer items than you ask for  For example  if you ask for a  -2  and a only contains one element  you get an empty list instead of an error  Sometimes you would prefer the error  so you have to be aware that this may happen   Relation to slice   object  The slicing operator    is actually being used in the above code with a slice   object using the   notation  which is only valid within      i e    a start stop step    is equivalent to   a slice start  stop  step     Slice objects also behave slightly differently depending on the number of arguments  similarly to range    i e  both slice stop  and slice start  stop   step   are supported  To skip specifying a given argument  one might use None  so that e g  a start   is equivalent to a slice start  None   or a   -1  is equivalent to a slice None  None  -1     While the  -based notation is very helpful for simple slicing  the explicit use of slice   objects simplifies the programmatic generation of slicing

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In Python 2 7  Slicing in Python   a b c   len   length of string  tuple or list  c -- default is  1  The sign of c indicates forward or backward  absolute value of c indicates steps  Default is forward with step size 1  Positive means forward  negative means backward   a --  When c is positive or blank  default is 0  When c is negative  default is -1   b --  When c is positive or blank  default is len  When c is negative  default is - len 1     Understanding index assignment is very important   In forward direction  starts at 0 and ends at len-1  In backward direction  starts at -1 and ends at -len   When you say  a b c   you are saying depending on the sign of c  forward or backward   start at a and end at b  excluding element at bth index   Use the indexing rule above and remember you will only find elements in this range   -len  -len 1  -len 2       0  1  2 3 4   len -1   But this range continues in both directions infinitely       -len -2  -len-1 -len  -len 1  -len 2       0  1  2 3 4   len -1  len  len  1  len 2          For example                0    1    2   3    4   5   6   7   8   9   10   11              a    s    t   r    i   n   g     -9  -8  -7   -6   -5  -4   -3  -2  -1   If your choice of a  b  and c allows overlap with the range above as you traverse using rules for a b c above you will either get a list with elements  touched during traversal  or you will get an empty list   One last thing  if a and b are equal  then also you get an empty list    gt  gt  gt  l1  2  3  4    gt  gt  gt  l1     2  3  4    gt  gt  gt  l1   -1    a default is -1   b default is - len 1   4  3  2    gt  gt  gt  l1  -4 -1    a default is -1  4  3  2    gt  gt  gt  l1  -3 -1    a default is -1  4  3    gt  gt  gt  l1       c default is  1  so a default is 0  b default is len  2  3  4    gt  gt  gt  l1   -1    c is -1   so a default is -1 and b default is - len 1   4  3  2     gt  gt  gt  l1 -100 -200 -1    Interesting      gt  gt  gt  l1 -1 -200 -1    Interesting  4  3  2     gt  gt  gt  l1 -1 -1 1        gt  gt  gt  l1 -1 5 1    Interesting  4     gt  gt  gt  l1 1 -7 1       gt  gt  gt  l1 1 -7 -1    Interesting  3  2    gt  gt  gt  l1  -2 -2    a default is -1  stop b  at -2   step c  by 2 in reverse direction  4

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In my opinion  you will understand and memorize better the Python string slicing notation if you look at it the following way  read on    Let s work with the following string      azString    abcdefghijklmnopqrstuvwxyz    For those who don t know  you can create any substring from azString using the notation azString x y   Coming from other programming languages  that s when the common sense gets compromised  What are x and y   I had to sit down and run several scenarios in my quest for a memorization technique that will help me remember what x and y are and help me slice strings properly at the first attempt   My conclusion is that x and y should be seen as the boundary indexes that are surrounding the strings that we want to extra  So we should see the expression as azString index1  index2  or even more clearer as azString index of first character  index after the last character    Here is an example visualization of that      Letters   a b c d e f g h i j                                                             Indexes  0 1 2 3 4 5 6 7 8 9                                cdefgh    index1       index2   So all you have to do is setting index1 and index2 to the values that will surround the desired substring  For instance  to get the substring  cdefgh   you can use azString 2 8   because the index on the left side of  c  is 2 and the one on the right size of  h  is 8   Remember that we are setting the boundaries  And those boundaries are the positions where you could place some brackets that will be wrapped around the substring like this      a b   c d e f g h   i j  That trick works all the time and is easy to memorize

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It s pretty simple really   a start stop     items start through stop-1 a start          items start through the rest of the array a  stop          items from the beginning through stop-1 a                a copy of the whole array   There is also the step value  which can be used with any of the above   a start stop step    start through not past stop  by step   The key point to remember is that the  stop value represents the first value that is not in the selected slice  So  the difference between stop and start is the number of elements selected  if step is 1  the default    The other feature is that start or stop may be a negative number  which means it counts from the end of the array instead of the beginning  So   a -1       last item in the array a -2       last two items in the array a  -2      everything except the last two items   Similarly  step may be a negative number   a   -1       all items in the array  reversed a 1  -1      the first two items  reversed a  -3 -1     the last two items  reversed a -3  -1     everything except the last two items  reversed   Python is kind to the programmer if there are fewer items than you ask for  For example  if you ask for a  -2  and a only contains one element  you get an empty list instead of an error  Sometimes you would prefer the error  so you have to be aware that this may happen   Relation to slice   object  The slicing operator    is actually being used in the above code with a slice   object using the   notation  which is only valid within      i e    a start stop step    is equivalent to   a slice start  stop  step     Slice objects also behave slightly differently depending on the number of arguments  similarly to range    i e  both slice stop  and slice start  stop   step   are supported  To skip specifying a given argument  one might use None  so that e g  a start   is equivalent to a slice start  None   or a   -1  is equivalent to a slice None  None  -1     While the  -based notation is very helpful for simple slicing  the explicit use of slice   objects simplifies the programmatic generation of slicing

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The basic slicing technique is to define the starting point  the stopping point  and the step size - also known as stride   First  we will create a list of values to use in our slicing   Create two lists to slice  The first is a numeric list from 1 to 9  List A   The second is also a numeric list  from 0 to 9  List B    A   list range 1  10  1     Start  stop  and step B   list range 9    print  This is List A    A  print  This is List B    B    Index the number 3 from A and the number 6 from B   print A 2   print B 6     Basic Slicing  Extended indexing syntax used for slicing is aList start stop step   The start argument and the step argument both default to none - the only required argument is stop  Did you notice this is similar to how range was used to define lists A and B  This is because the slice object represents the set of indices specified by range start  stop  step   Python 3 4 documentation   As you can see  defining only stop returns one element  Since the start defaults to none  this translates into retrieving only one element   It is important to note  the first element is index 0  not index 1  This is why we are using 2 lists for this exercise  List A s elements are numbered according to the ordinal position  the first element is 1  the second element is 2  etc   while List B s elements are the numbers that would be used to index them   0  for the first element 0  etc     With extended indexing syntax  we retrieve a range of values  For example  all values are retrieved with a colon   A      To retrieve a subset of elements  the start and stop positions need to be defined   Given the pattern aList start stop   retrieve the first two elements from List A

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Enumerating the possibilities allowed by the grammar    gt  gt  gt  seq                      seq 0     seq 1                 seq -1        gt  gt  gt  seq low                  seq low   seq low 1             seq -1        gt  gt  gt  seq  high                seq 0     seq 1                 seq high-1    gt  gt  gt  seq low high             seq low   seq low 1             seq high-1    gt  gt  gt  seq   stride             seq 0     seq stride            seq -1        gt  gt  gt  seq low  stride          seq low   seq low stride        seq -1        gt  gt  gt  seq  high stride         seq 0     seq stride            seq high-1    gt  gt  gt  seq low high stride      seq low   seq low stride        seq high-1     Of course  if  high-low  stride    0  then the end point will be a little lower than high-1   If stride is negative  the ordering is changed a bit since we re counting down    gt  gt  gt  seq   -stride            seq -1     seq -1-stride          seq 0        gt  gt  gt  seq high  -stride        seq high   seq high-stride        seq 0        gt  gt  gt  seq  low -stride         seq -1     seq -1-stride          seq low 1    gt  gt  gt  seq high low -stride     seq high   seq high-stride        seq low 1     Extended slicing  with commas and ellipses  are mostly used only by special data structures  like NumPy   the basic sequences don t support them    gt  gt  gt  class slicee          def   getitem   self  item               return repr item       gt  gt  gt  slicee   0  1 2    5         0  slice 1  2  None   slice None  None  5   Ellipsis

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The basic slicing technique is to define the starting point  the stopping point  and the step size - also known as stride   First  we will create a list of values to use in our slicing   Create two lists to slice  The first is a numeric list from 1 to 9  List A   The second is also a numeric list  from 0 to 9  List B    A   list range 1  10  1     Start  stop  and step B   list range 9    print  This is List A    A  print  This is List B    B    Index the number 3 from A and the number 6 from B   print A 2   print B 6     Basic Slicing  Extended indexing syntax used for slicing is aList start stop step   The start argument and the step argument both default to none - the only required argument is stop  Did you notice this is similar to how range was used to define lists A and B  This is because the slice object represents the set of indices specified by range start  stop  step   Python 3 4 documentation   As you can see  defining only stop returns one element  Since the start defaults to none  this translates into retrieving only one element   It is important to note  the first element is index 0  not index 1  This is why we are using 2 lists for this exercise  List A s elements are numbered according to the ordinal position  the first element is 1  the second element is 2  etc   while List B s elements are the numbers that would be used to index them   0  for the first element 0  etc     With extended indexing syntax  we retrieve a range of values  For example  all values are retrieved with a colon   A      To retrieve a subset of elements  the start and stop positions need to be defined   Given the pattern aList start stop   retrieve the first two elements from List A

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I find it easier to remember how it works  and then I can figure out any specific start stop step combination   It s instructive to understand range   first   def range start 0  stop  step 1      Illegal syntax  but that s the effect     i   start     while  i  lt  stop if step  gt  0 else i  gt  stop           yield i         i    step   Begin from start  increment by step  do not reach stop   Very simple   The thing to remember about negative step is that stop is always the excluded end  whether it s higher or lower  If you want same slice in opposite order  it s much cleaner to do the reversal separately  e g   abcde  1 -2    -1  slices off one char from left  two from right  then reverses   See also reversed      Sequence slicing is same  except it first normalizes negative indexes  and it can never go outside the sequence   TODO  The code below had a bug with  never go outside the sequence  when abs step  1  I think I patched it to be correct  but it s hard to understand   def this is how slicing works seq  start None  stop None  step 1       if start is None          start    0 if step  gt  0 else len seq -1      elif start  lt  0          start    len seq      if not 0  lt   start  lt  len seq      clip if still outside bounds         start    0 if step  gt  0 else len seq -1      if stop is None          stop    len seq  if step  gt  0 else -1     really -1  not last element     elif stop  lt  0          stop    len seq      for i in range start  stop  step           if 0  lt   i  lt  len seq               yield seq i    Don t worry about the is None details - just remember that omitting start and or stop always does the right thing to give you the whole sequence   Normalizing negative indexes first allows start and or stop to be counted from the end independently   abcde  1 -2      abcde  1 3      bc  despite range 1 -2         The normalization is sometimes thought of as  modulo the length   but note it adds the length just once  e g   abcde  -53 42  is just the whole string

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This is just for some extra info    Consider the list below    gt  gt  gt  l  12 23 345 456 67 7 945 467    Few other tricks for reversing the list    gt  gt  gt  l len l  -len l -1 -1   467  945  7  67  456  345  23  12    gt  gt  gt  l  -len l -1 -1   467  945  7  67  456  345  23  12    gt  gt  gt  l len l   -1   467  945  7  67  456  345  23  12    gt  gt  gt  l   -1   467  945  7  67  456  345  23  12    gt  gt  gt  l -1 -len l -1 -1   467  945  7  67  456  345  23  12

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This is how I teach slices to newbies   Understanding the difference between indexing and slicing   Wiki Python has this amazing picture which clearly distinguishes indexing and slicing     It is a list with six elements in it  To understand slicing better  consider that list as a set of six boxes placed together  Each box has an alphabet in it   Indexing is like dealing with the contents of box  You can check contents of any box  But you can t check the contents of multiple boxes at once  You can even replace the contents of the box  But you can t place two balls in one box or replace two balls at a time   In  122   alpha     a    b    c    d    e    f    In  123   alpha Out 123     a    b    c    d    e    f    In  124   alpha 0  Out 124    a   In  127   alpha 0     A   In  128   alpha Out 128     A    b    c    d    e    f    In  129   alpha 0 1  --------------------------------------------------------------------------- TypeError                                 Traceback  most recent call last   lt ipython-input-129-c7eb16585371 gt  in  lt module gt    ---- gt  1 alpha 0 1   TypeError  list indices must be integers  not tuple   Slicing is like dealing with boxes themselves  You can pick up the first box and place it on another table  To pick up the box  all you need to know is the position of beginning and ending of the box   You can even pick up the first three boxes or the last two boxes or all boxes between 1 and 4  So  you can pick any set of boxes if you know the beginning and ending  These positions are called start and stop positions   The interesting thing is that you can replace multiple boxes at once  Also you can place multiple boxes wherever you like   In  130   alpha 0 1  Out 130     A    In  131   alpha 0 1     a   In  132   alpha Out 132     a    b    c    d    e    f    In  133   alpha 0 2      A    B    In  134   alpha Out 134     A    B    c    d    e    f    In  135   alpha 2 2      x    xx    In  136   alpha Out 136     A    B    x    xx    c    d    e    f     Slicing With Step   Till now you have picked boxes continuously  But sometimes you need to pick up discretely  For example  you can pick up every second box  You can even pick up every third box from the end  This value is called step size  This represents the gap between your successive pickups  The step size should be positive if You are picking boxes from the beginning to end and vice versa   In  137   alpha     a    b    c    d    e    f    In  142   alpha 1 5 2  Out 142     b    d    In  143   alpha -1 -5 -2  Out 143     f    d    In  144   alpha 1 5 -2  Out 144       In  145   alpha -1 -5 2  Out 145        How Python Figures Out Missing Parameters   When slicing  if you leave out any parameter  Python tries to figure it out automatically   If you check the source code of CPython  you will find a function called PySlice GetIndicesEx   which figures out indices to a slice for any given parameters  Here is the logical equivalent code in Python   This function takes a Python object and optional parameters for slicing and returns the start  stop  step  and slice length for the requested slice   def py slice get indices ex obj  start None  stop None  step None        length   len obj       if step is None          step   1     if step    0          raise Exception  Step cannot be zero         if start is None          start   0 if step  gt  0 else length - 1     else          if start  lt  0              start    length         if start  lt  0              start   0 if step  gt  0 else -1         if start  gt   length              start   length if step  gt  0 else length - 1      if stop is None          stop   length if step  gt  0 else -1     else          if stop  lt  0              stop    length         if stop  lt  0              stop   0 if step  gt  0 else -1         if stop  gt   length              stop   length if step  gt  0 else length - 1      if  step  lt  0 and stop  gt   start  or  step  gt  0 and start  gt   stop           slice length   0     elif step  lt  0          slice length    stop - start   1   step    1     else          slice length    stop - start - 1   step    1      return  start  stop  step  slice length    This is the intelligence that is present behind slices  Since Python has an built-in function called slice  you can pass some parameters and check how smartly it calculates missing parameters   In  21   alpha     a    b    c    d    e    f    In  22   s   slice None  None  None   In  23   s Out 23   slice None  None  None   In  24   s indices len alpha   Out 24    0  6  1   In  25   range  s indices len alpha    Out 25    0  1  2  3  4  5   In  26   s   slice None  None  -1   In  27   range  s indices len alpha    Out 27    5  4  3  2  1  0   In  28   s   slice None  3  -1   In  29   range  s indices len alpha    Out 29    5  4    Note  This post was originally written in my blog  The Intelligence Behind Python Slices

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Python slicing notation   a start end step     For start and end  negative values are interpreted as being relative to the end of the sequence  Positive indices for end indicate the position after the last element to be included  Blank values are defaulted as follows    0 -0 1   Using a negative step reverses the interpretation of start and end   The notation extends to  numpy  matrices and multidimensional arrays   For example  to slice entire columns you can use   m    0 2      slice the first two columns   Slices hold references  not copies  of the array elements   If you want to make a separate copy an array  you can use deepcopy

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Found this great table at http   wiki python org moin MovingToPythonFromOtherLanguages  Python indexes and slices for a six-element list  Indexes enumerate the elements  slices enumerate the spaces between the elements   Index from rear     -6  -5  -4  -3  -2  -1      a  0 1 2 3 4 5     a 1     1 2 3 4 5  Index from front     0   1   2   3   4   5      len a   6          a  5    0 1 2 3 4                      --- --- --- --- --- ---     a 0   0            a  -2    0 1 2 3                       a   b   c   d   e   f      a 5   5            a 1 2    1                      --- --- --- --- --- ---     a -1   5           a 1 -1    1 2 3 4  Slice from front       1   2   3   4   5        a -2   4 Slice from rear       -5  -4  -3  -2  -1                                                     b a                                                    b   0 1 2 3 4 5   shallow copy of a

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And a couple of things that weren t immediately obvious to me when I first saw the slicing syntax    gt  gt  gt  x    1 2 3 4 5 6   gt  gt  gt  x   -1   6 5 4 3 2 1    Easy way to reverse sequences   And if you wanted  for some reason  every second item in the reversed sequence    gt  gt  gt  x    1 2 3 4 5 6   gt  gt  gt  x   -2   6 4 2

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Index        ------------ gt    0   1   2   3   4  --- --- --- --- ---    a   b   c   d   e    --- --- --- --- ---    0  -4  -3  -2  -1        lt ------------  Slice       lt ---------------   --------------- gt      1   2   3   4      --- --- --- --- ---    a   b   c   d   e    --- --- --- --- ---     -4  -3  -2  -1      --------------- gt       lt ---------------    I hope this will help you to model the list in Python   Reference  http   wiki python org moin MovingToPythonFromOtherLanguages

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Enumerating the possibilities allowed by the grammar    gt  gt  gt  seq                      seq 0     seq 1                 seq -1        gt  gt  gt  seq low                  seq low   seq low 1             seq -1        gt  gt  gt  seq  high                seq 0     seq 1                 seq high-1    gt  gt  gt  seq low high             seq low   seq low 1             seq high-1    gt  gt  gt  seq   stride             seq 0     seq stride            seq -1        gt  gt  gt  seq low  stride          seq low   seq low stride        seq -1        gt  gt  gt  seq  high stride         seq 0     seq stride            seq high-1    gt  gt  gt  seq low high stride      seq low   seq low stride        seq high-1     Of course  if  high-low  stride    0  then the end point will be a little lower than high-1   If stride is negative  the ordering is changed a bit since we re counting down    gt  gt  gt  seq   -stride            seq -1     seq -1-stride          seq 0        gt  gt  gt  seq high  -stride        seq high   seq high-stride        seq 0        gt  gt  gt  seq  low -stride         seq -1     seq -1-stride          seq low 1    gt  gt  gt  seq high low -stride     seq high   seq high-stride        seq low 1     Extended slicing  with commas and ellipses  are mostly used only by special data structures  like NumPy   the basic sequences don t support them    gt  gt  gt  class slicee          def   getitem   self  item               return repr item       gt  gt  gt  slicee   0  1 2    5         0  slice 1  2  None   slice None  None  5   Ellipsis

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As a general rule  writing code with a lot of hardcoded index values leads to a readability and maintenance mess  For example  if you come back to the code a year later  you   ll look at it and wonder what you were thinking when you wrote it  The solution shown is simply a way of more clearly stating what your code is actually doing  In general  the built-in slice   creates a slice object that can be used anywhere a slice is allowed  For example    gt  gt  gt  items    0  1  2  3  4  5  6   gt  gt  gt  a   slice 2  4   gt  gt  gt  items 2 4   2  3   gt  gt  gt  items a   2  3   gt  gt  gt  items a     10 11   gt  gt  gt  items  0  1  10  11  4  5  6   gt  gt  gt  del items a   gt  gt  gt  items  0  1  4  5  6    If you have a slice instance s  you can get more information about it by looking at its s start  s stop  and s step attributes  respectively  For example     gt  gt  gt  a   slice 10  50  2   gt  gt  gt  a start 10  gt  gt  gt  a stop 50  gt  gt  gt  a step 2  gt  gt  gt

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In Python  the most basic form for slicing is the following   l start end    where l is some collection  start is an inclusive index  and end is an exclusive index   In  1   l   list range 10    In  2   l  5    First five elements Out 2    0  1  2  3  4   In  3   l -5     Last five elements Out 3    5  6  7  8  9    When slicing from the start  you can omit the zero index  and when slicing to the end  you can omit the final index since it is redundant  so do not be verbose   In  5   l  3     l 0 3  Out 5   True  In  6   l 7      l 7 len l   Out 6   True   Negative integers are useful when doing offsets relative to the end of a collection   In  7   l  -1    Include all elements but the last one Out 7    0  1  2  3  4  5  6  7  8   In  8   l -3     Take the last three elements Out 8    7  8  9    It is possible to provide indices that are out of bounds when slicing such as   In  9   l  20    20 is out of index bounds  and l 20  will raise an IndexError exception Out 9    0  1  2  3  4  5  6  7  8  9   In  11   l -20     -20 is out of index bounds  and l -20  will raise an IndexError exception Out 11    0  1  2  3  4  5  6  7  8  9    Keep in mind that the result of slicing a collection is a whole new collection  In addition  when using slice notation in assignments  the length of the slice assignments do not need to be the same  The values before and after the assigned slice will be kept  and the collection will shrink or grow to contain the new values   In  16   l 2 6    list  abc     Assigning fewer elements than the ones contained in the sliced collection l 2 6   In  17   l Out 17    0  1   a    b    c   6  7  8  9   In  18   l 2 5    list  hello     Assigning more elements than the ones contained in the sliced collection l  2 5   In  19   l Out 19    0  1   h    e    l    l    o   6  7  8  9    If you omit the start and end index  you will make a copy of the collection   In  14   l copy   l     In  15   l    l copy and l is not l copy Out 15   True   If the start and end indexes are omitted when performing an assignment operation  the entire content of the collection will be replaced with a copy of what is referenced   In  20   l      list  hello       In  21   l Out 21     h    e    l    l    o                    Besides basic slicing  it is also possible to apply the following notation   l start end step    where l is a collection  start is an inclusive index  end is an exclusive index  and step is a stride that can be used to take every nth item in l   In  22   l   list range 10    In  23   l   2    Take the elements which indexes are even Out 23    0  2  4  6  8   In  24   l 1  2    Take the elements which indexes are odd Out 24    1  3  5  7  9    Using step provides a useful trick to reverse a collection in Python   In  25   l   -1  Out 25    9  8  7  6  5  4  3  2  1  0    It is also possible to use negative integers for step as the following example   In 28    l   -2  Out 28    9  7  5  3  1    However  using a negative value for step could become very confusing  Moreover  in order to be Pythonic  you should avoid using start  end  and step in a single slice  In case this is required  consider doing this in two assignments  one to slice  and the other to stride    In  29   l   l   2    This step is for striding  In  30   l Out 30    0  2  4  6  8   In  31   l   l 1 -1    This step is for slicing  In  32   l Out 32    2  4  6

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I want to add one Hello  World  example that explains the basics of slices for the very beginners  It helped me a lot   Let s have a list with six values   P    Y    T    H    O    N      --- --- --- --- --- ---    P   Y   T   H   O   N    --- --- --- --- --- ---    0   1   2   3   4   5   Now the simplest slices of that list are its sublists  The notation is   lt index gt   lt index gt   and the key is to read it like this     start cutting before this index   end cutting before this index     Now if you make a slice  2 5  of the list above  this will happen                          --- --- --- --- --- ---    P   Y   T   H   O   N    --- --- --- --- --- ---    0   1   2   3   4   5   You made a cut before the element with index 2 and another cut before the element with index 5  So the result will be a slice between those two cuts  a list   T    H    O

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It is easy to understand if we could relate slicing to range  which gives the indexes  We can categorize slicing into the following two categories     1  No step or step   0  For example   i j  or  i j k   k 0   Suppose the sequence is s  1 2 3 4 5     if 0 lt i lt len s  and 0 lt j lt len s   then  i j k  - gt  range i j k    For example   0 3 2  - gt  range 0 3 2  - gt  0  2   if i gt len s  or j gt len s   then i len s  or j len s    For example   0 100 2  - gt  range 0 len s  2  - gt  range 0 5 2  - gt  0  2  4   if i lt 0 or j lt 0  then i max 0 len s  i  or j max 0 len s  j    For example   0 -3 2  - gt  range 0 len s -3 2  - gt  range 0 2 2  - gt  0  For another example   0 -1 2  - gt  range 0 len s -1 2  - gt  range 0 4 2  - gt  0  2   if i is not specified  then i 0   For example    4 2  - gt  range 0 4 2  - gt  range 0 4 2  - gt  0  2   if j is not specified  then j len s    For example   0  2  - gt  range 0 len s  2  - gt  range 0 5 2  - gt  0  2  4    2  Step  lt  0  For example   i j k   k lt 0   Suppose the sequence is s  1 2 3 4 5     if 0 lt i lt len s  and 0 lt j lt len s   then  i j k  - gt  range i j k    For example   5 0 -2  - gt  range 5 0 -2  - gt  5  3  1   if i gt len s  or j gt len s   then i len s -1 or j len s -1   For example   100 0 -2  - gt  range len s -1 0 -2  - gt  range 4 0 -2  - gt  4  2   if i lt 0 or j lt 0  then i max -1 len s  i  or j max -1 len s  j    For example   -2 -10 -2  - gt  range len s -2 -1 -2  - gt  range 3 -1 -2  - gt  3  1   if i is not specified  then i len s -1   For example    0 -2  - gt  range len s -1 0 -2  - gt  range 4 0 -2  - gt  4  2   if j is not specified  then j -1   For example   2  -2  - gt  range 2 -1 -2  - gt  2  0  For another example     -1  - gt  range len s -1 -1 -1  - gt  range 4 -1 -1  - gt  4  3  2  1  0    In summary

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The rules of slicing are as follows   lower bound   upper bound   step size   I- Convert upper bound  and lower bound into common signs  II- Then check if the step size is a positive or a negative value   i  If the step size is a positive value  upper bound should be greater than lower bound  otherwise empty string is printed  For example  s  quot Welcome quot  s1 s 0 3 1  print s1   The output  Wel  However if we run the following code  s  quot Welcome quot  s1 s 3 0 1  print s1   It will return an empty string   ii  If the step size if a negative value  upper bound should be lesser than lower bound  otherwise empty string will be printed  For example  s  quot Welcome quot  s1 s 3 0 -1  print s1   The output  cle  But if we run the following code  s  quot Welcome quot  s1 s 0 5 -1  print s1   The output will be an empty string  Thus in the code  str    abcd  l   len str  str2   str l-1 0 -1      str 3 0 -1   print str2  str2   str l-1 -1 -1      str 3 -1 -1  print str2   In the first str2 str l-1 0 -1   the upper bound is lesser than the lower bound  thus dcb is printed  However in str2 str l-1 -1 -1   the upper bound is not less than the lower bound  upon converting lower bound into negative value which is -1  since index of last element is -1 as well as 3

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The answers above don t discuss slice assignment  To understand slice assignment  it s helpful to add another concept to the ASCII art                    --- --- --- --- --- ---                    P   y   t   h   o   n                    --- --- --- --- --- ---  Slice position  0   1   2   3   4   5   6 Index position    0   1   2   3   4   5   gt  gt  gt  p     P   y   t   h   o   n     Why the two sets of numbers    indexing gives items  not lists  gt  gt  gt  p 0    P   gt  gt  gt  p 5    n     Slicing gives lists  gt  gt  gt  p 0 1     P    gt  gt  gt  p 0 2     P   y     One heuristic is  for a slice from zero to n  think   zero is the beginning  start at the beginning and take n items in a list     gt  gt  gt  p 5    the last of six items  indexed from zero   n   gt  gt  gt  p 0 5    does NOT include the last item     P   y   t   h   o    gt  gt  gt  p 0 6    not p 0 5        P   y   t   h   o   n     Another heuristic is   for any slice  replace the start by zero  apply the previous heuristic to get the end of the list  then count the first number back up to chop items off the beginning    gt  gt  gt  p 0 4    Start at the beginning and count out 4 items    P   y   t   h    gt  gt  gt  p 1 4    Take one item off the front    y   t   h    gt  gt  gt  p 2 4    Take two items off the front    t   h     etc    The first rule of slice assignment is that since slicing returns a list  slice assignment requires a list  or other iterable     gt  gt  gt  p 2 3     t    gt  gt  gt  p 2 3      T    gt  gt  gt  p    P   y   T   h   o   n    gt  gt  gt  p 2 3     t  Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  TypeError  can only assign an iterable   The second rule of slice assignment  which you can also see above  is that whatever portion of the list is returned by slice indexing  that s the same portion that is changed by slice assignment    gt  gt  gt  p 2 4     T   h    gt  gt  gt  p 2 4      t   r    gt  gt  gt  p    P   y   t   r   o   n     The third rule of slice assignment is  the assigned list  iterable  doesn t have to have the same length  the indexed slice is simply sliced out and replaced en masse by whatever is being assigned    gt  gt  gt  p     P   y   t   h   o   n     Start over  gt  gt  gt  p 2 4      s   p   a   m    gt  gt  gt  p    P   y   s   p   a   m   o   n     The trickiest part to get used to is assignment to empty slices  Using heuristic 1 and 2 it s easy to get your head around indexing an empty slice    gt  gt  gt  p     P   y   t   h   o   n    gt  gt  gt  p 0 4     P   y   t   h    gt  gt  gt  p 1 4     y   t   h    gt  gt  gt  p 2 4     t   h    gt  gt  gt  p 3 4     h    gt  gt  gt  p 4 4        And then once you ve seen that  slice assignment to the empty slice makes sense too    gt  gt  gt  p     P   y   t   h   o   n    gt  gt  gt  p 2 4      x   y     Assigned list is same length as slice  gt  gt  gt  p    P   y   x   y   o   n     Result is same length  gt  gt  gt  p     P   y   t   h   o   n    gt  gt  gt  p 3 4      x   y     Assigned list is longer than slice  gt  gt  gt  p    P   y   t   x   y   o   n     The result is longer  gt  gt  gt  p     P   y   t   h   o   n    gt  gt  gt  p 4 4      x   y    gt  gt  gt  p    P   y   t   h   x   y   o   n     The result is longer still   Note that  since we are not changing the second number of the slice  4   the inserted items always stack right up against the  o   even when we re assigning to the empty slice  So the position for the empty slice assignment is the logical extension of the positions for the non-empty slice assignments   Backing up a little bit  what happens when you keep going with our procession of counting up the slice beginning    gt  gt  gt  p     P   y   t   h   o   n    gt  gt  gt  p 0 4     P   y   t   h    gt  gt  gt  p 1 4     y   t   h    gt  gt  gt  p 2 4     t   h    gt  gt  gt  p 3 4     h    gt  gt  gt  p 4 4       gt  gt  gt  p 5 4       gt  gt  gt  p 6 4        With slicing  once you re done  you re done  it doesn t start slicing backwards  In Python you don t get negative strides unless you explicitly ask for them by using a negative number    gt  gt  gt  p 5 3 -1     n   o     There are some weird consequences to the  once you re done  you re done  rule    gt  gt  gt  p 4 4       gt  gt  gt  p 5 4       gt  gt  gt  p 6 4       gt  gt  gt  p 6  Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  IndexError  list index out of range   In fact  compared to indexing  Python slicing is bizarrely error-proof    gt  gt  gt  p 100 200       gt  gt  gt  p int 2e99  int 1e99         This can come in handy sometimes  but it can also lead to somewhat strange behavior    gt  gt  gt  p    P    y    t    h    o    n    gt  gt  gt  p int 2e99  int 1e99       p   o   w   e   r    gt  gt  gt  p    P    y    t    h    o    n    p    o    w    e    r     Depending on your application  that might    or might not    be what you were hoping for there     Below is the text of my original answer  It has been useful to many people  so I didn t want to delete it    gt  gt  gt  r  1 2 3 4   gt  gt  gt  r 1 1      gt  gt  gt  r 1 1   9 8   gt  gt  gt  r  1  9  8  2  3  4   gt  gt  gt  r 1 1    blah    gt  gt  gt  r  1   blah   9  8  2  3  4    This may also clarify the difference between slicing and indexing

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This is just for some extra info    Consider the list below    gt  gt  gt  l  12 23 345 456 67 7 945 467    Few other tricks for reversing the list    gt  gt  gt  l len l  -len l -1 -1   467  945  7  67  456  345  23  12    gt  gt  gt  l  -len l -1 -1   467  945  7  67  456  345  23  12    gt  gt  gt  l len l   -1   467  945  7  67  456  345  23  12    gt  gt  gt  l   -1   467  945  7  67  456  345  23  12    gt  gt  gt  l -1 -len l -1 -1   467  945  7  67  456  345  23  12

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The rules of slicing are as follows   lower bound   upper bound   step size   I- Convert upper bound  and lower bound into common signs  II- Then check if the step size is a positive or a negative value   i  If the step size is a positive value  upper bound should be greater than lower bound  otherwise empty string is printed  For example  s  quot Welcome quot  s1 s 0 3 1  print s1   The output  Wel  However if we run the following code  s  quot Welcome quot  s1 s 3 0 1  print s1   It will return an empty string   ii  If the step size if a negative value  upper bound should be lesser than lower bound  otherwise empty string will be printed  For example  s  quot Welcome quot  s1 s 3 0 -1  print s1   The output  cle  But if we run the following code  s  quot Welcome quot  s1 s 0 5 -1  print s1   The output will be an empty string  Thus in the code  str    abcd  l   len str  str2   str l-1 0 -1      str 3 0 -1   print str2  str2   str l-1 -1 -1      str 3 -1 -1  print str2   In the first str2 str l-1 0 -1   the upper bound is lesser than the lower bound  thus dcb is printed  However in str2 str l-1 -1 -1   the upper bound is not less than the lower bound  upon converting lower bound into negative value which is -1  since index of last element is -1 as well as 3

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You can also use slice assignment to remove one or more elements from a list   r    1   blah   9  8  2  3  4   gt  gt  gt  r 1 4        gt  gt  gt  r  1  2  3  4

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Enumerating the possibilities allowed by the grammar    gt  gt  gt  seq                      seq 0     seq 1                 seq -1        gt  gt  gt  seq low                  seq low   seq low 1             seq -1        gt  gt  gt  seq  high                seq 0     seq 1                 seq high-1    gt  gt  gt  seq low high             seq low   seq low 1             seq high-1    gt  gt  gt  seq   stride             seq 0     seq stride            seq -1        gt  gt  gt  seq low  stride          seq low   seq low stride        seq -1        gt  gt  gt  seq  high stride         seq 0     seq stride            seq high-1    gt  gt  gt  seq low high stride      seq low   seq low stride        seq high-1     Of course  if  high-low  stride    0  then the end point will be a little lower than high-1   If stride is negative  the ordering is changed a bit since we re counting down    gt  gt  gt  seq   -stride            seq -1     seq -1-stride          seq 0        gt  gt  gt  seq high  -stride        seq high   seq high-stride        seq 0        gt  gt  gt  seq  low -stride         seq -1     seq -1-stride          seq low 1    gt  gt  gt  seq high low -stride     seq high   seq high-stride        seq low 1     Extended slicing  with commas and ellipses  are mostly used only by special data structures  like NumPy   the basic sequences don t support them    gt  gt  gt  class slicee          def   getitem   self  item               return repr item       gt  gt  gt  slicee   0  1 2    5         0  slice 1  2  None   slice None  None  5   Ellipsis

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1  Slice Notation  To make it simple  remember slice has only one form   s start end step    and here is how it works    s  an object that can be sliced start  first index to start iteration end  last index  NOTE that end index will not be included in the resulted slice step  pick element every step index   Another import thing  all start end  step can be omitted  And if they are omitted  their default value will be used  0 len s  1 accordingly   So possible variations are     Mostly used variations s start end  s start   s  end     Step-related variations s  end step  s start  step  s   step     Make a copy s      NOTE  If start  gt   end  considering only when step gt 0   Python will return a empty slice      2  Pitfalls  The above part explains the core features on how slice works  and it will work on most occasions  However  there can be pitfalls you should watch out  and this part explains them   Negative indexes  The very first thing that confuses Python learners is that an index can be negative  Don t panic  a negative index means count backwards   For example   s -5        Start at the 5th index from the end of array              thus returning the last 5 elements  s  -5       Start at index 0  and end until the 5th index from end of array              thus returning s 0 len s -5     Negative step  Making things more confusing is that step can be negative too   A negative step means iterate the array backwards  from the end to start  with the end index included  and the start index excluded from the result   NOTE  when step is negative  the default value for start is len s   while end does not equal to 0  because s   -1  contains s 0    For example   s   -1               Reversed slice s len s   -1         The same as above  reversed slice s 0 len s  -1        Empty list   Out of range error   Be surprised  slice does not raise an IndexError when the index is out of range   If the index is out of range  Python will try its best to set the index to 0 or len s  according to the situation  For example   s  len s  5         The same as s  len s   s -len s -5         The same as s 0   s len s  5  -1      The same as s len s   -1   and the same as s   -1    3  Examples  Let s finish this answer with examples  explaining everything we have discussed     Create our array for demonstration In  1   s    i for i in range 10    In  2   s Out 2    0  1  2  3  4  5  6  7  8  9   In  3   s 2       From index 2 to last index Out 3    2  3  4  5  6  7  8  9   In  4   s  8      From index 0 up to index 8 Out 4    0  1  2  3  4  5  6  7   In  5   s 4 7     From index 4  included  up to index 7 excluded  Out 5    4  5  6   In  6   s  -2     Up to second last index  negative index  Out 6    0  1  2  3  4  5  6  7   In  7   s -2      From second last index  negative index  Out 7    8  9   In  8   s   -1    From last to first in reverse order  negative step  Out 8    9  8  7  6  5  4  3  2  1  0   In  9   s   -2    All odd numbers in reversed order Out 9    9  7  5  3  1   In  11   s -2  -2    All even numbers in reversed order Out 11    8  6  4  2  0   In  12   s 3 15      End is out of range  and Python will set it to len s   Out 12    3  4  5  6  7  8  9   In  14   s 5 1       Start  gt  end  return empty list Out 14       In  15   s 11        Access index 11  greater than len s   will raise an IndexError --------------------------------------------------------------------------- IndexError                                Traceback  most recent call last   lt ipython-input-15-79ffc22473a3 gt  in  lt module gt    ---- gt  1 s 11   IndexError  list index out of range

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My brain seems happy to accept that lst start end  contains the start-th item  I might even say that it is a  natural assumption    But occasionally a doubt creeps in and my brain asks for reassurance that it does not contain the end-th element   In these moments I rely on this simple theorem   for any n     lst   lst  n    lst n     This pretty property tells me that lst start end  does not contain the end-th item because it is in lst end     Note that this theorem is true for any n at all  For example  you can check that  lst   range 10  lst  -42    lst -42      lst   returns True

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The previous answers don t discuss multi-dimensional array slicing which is possible using the famous NumPy package   Slicing can also be applied to multi-dimensional arrays     Here  a is a NumPy array   gt  gt  gt  a array    1   2   3   4            5   6   7   8            9  10  11  12     gt  gt  gt  a  2  0 3 2  array   1  3           5  7      The   2  before the comma operates on the first dimension and the  0 3 2  after the comma operates on the second dimension

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Found this great table at http   wiki python org moin MovingToPythonFromOtherLanguages  Python indexes and slices for a six-element list  Indexes enumerate the elements  slices enumerate the spaces between the elements   Index from rear     -6  -5  -4  -3  -2  -1      a  0 1 2 3 4 5     a 1     1 2 3 4 5  Index from front     0   1   2   3   4   5      len a   6          a  5    0 1 2 3 4                      --- --- --- --- --- ---     a 0   0            a  -2    0 1 2 3                       a   b   c   d   e   f      a 5   5            a 1 2    1                      --- --- --- --- --- ---     a -1   5           a 1 -1    1 2 3 4  Slice from front       1   2   3   4   5        a -2   4 Slice from rear       -5  -4  -3  -2  -1                                                     b a                                                    b   0 1 2 3 4 5   shallow copy of a

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I use the  an index points between elements  method of thinking about it myself  but one way of describing it which sometimes helps others get it is this   mylist X Y    X is the index of the first element you want  Y is the index of the first element you don t want

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I use the  an index points between elements  method of thinking about it myself  but one way of describing it which sometimes helps others get it is this   mylist X Y    X is the index of the first element you want  Y is the index of the first element you don t want

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And a couple of things that weren t immediately obvious to me when I first saw the slicing syntax    gt  gt  gt  x    1 2 3 4 5 6   gt  gt  gt  x   -1   6 5 4 3 2 1    Easy way to reverse sequences   And if you wanted  for some reason  every second item in the reversed sequence    gt  gt  gt  x    1 2 3 4 5 6   gt  gt  gt  x   -2   6 4 2

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Most of the previous answers clears up questions about slice notation   The extended indexing syntax used for slicing is aList start stop step   and basic examples are      More slicing examples  15 Extended Slices

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It s pretty simple really   a start stop     items start through stop-1 a start          items start through the rest of the array a  stop          items from the beginning through stop-1 a                a copy of the whole array   There is also the step value  which can be used with any of the above   a start stop step    start through not past stop  by step   The key point to remember is that the  stop value represents the first value that is not in the selected slice  So  the difference between stop and start is the number of elements selected  if step is 1  the default    The other feature is that start or stop may be a negative number  which means it counts from the end of the array instead of the beginning  So   a -1       last item in the array a -2       last two items in the array a  -2      everything except the last two items   Similarly  step may be a negative number   a   -1       all items in the array  reversed a 1  -1      the first two items  reversed a  -3 -1     the last two items  reversed a -3  -1     everything except the last two items  reversed   Python is kind to the programmer if there are fewer items than you ask for  For example  if you ask for a  -2  and a only contains one element  you get an empty list instead of an error  Sometimes you would prefer the error  so you have to be aware that this may happen   Relation to slice   object  The slicing operator    is actually being used in the above code with a slice   object using the   notation  which is only valid within      i e    a start stop step    is equivalent to   a slice start  stop  step     Slice objects also behave slightly differently depending on the number of arguments  similarly to range    i e  both slice stop  and slice start  stop   step   are supported  To skip specifying a given argument  one might use None  so that e g  a start   is equivalent to a slice start  None   or a   -1  is equivalent to a slice None  None  -1     While the  -based notation is very helpful for simple slicing  the explicit use of slice   objects simplifies the programmatic generation of slicing

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And a couple of things that weren t immediately obvious to me when I first saw the slicing syntax    gt  gt  gt  x    1 2 3 4 5 6   gt  gt  gt  x   -1   6 5 4 3 2 1    Easy way to reverse sequences   And if you wanted  for some reason  every second item in the reversed sequence    gt  gt  gt  x    1 2 3 4 5 6   gt  gt  gt  x   -2   6 4 2

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I got a little frustrated in not finding an online source  or Python documentation that describes precisely what slicing does   I took Aaron Hall s suggestion  read the relevant parts of the CPython source code  and wrote some Python code that performs slicing similarly to how it s done in CPython  I ve tested my code in Python 3 on millions of random tests on integer lists   You may find the references in my code to the relevant functions in CPython helpful     Return the result of slicing list x   See the part of list subscript   in listobject c that pertains   to when the indexing item is a PySliceObject def slicer x  start None  stop None  step None         Handle slicing index values of None  and a step value of 0        See PySlice Unpack   in sliceobject c  which       extracts start  stop  step from a PySliceObject      maxint   10000000         A hack to simulate PY SSIZE T MAX     if step    None          step   1     elif step    0          raise ValueError  slice step cannot be zero        if start    None          start   maxint if step  lt  0 else 0     if stop    None          stop   -maxint if step  lt  0 else maxint        Handle negative slice indexes and bad slice indexes        Compute number of elements in the slice as slice length        See PySlice AdjustIndices   in sliceobject c     length   len x      slice length   0      if start  lt  0          start    length         if start  lt  0              start   -1 if step  lt  0 else 0     elif start  gt   length          start   length - 1 if step  lt  0 else length      if stop  lt  0          stop    length         if stop  lt  0              stop   -1 if step  lt  0 else 0     elif stop  gt  length          stop   length - 1 if step  lt  0 else length      if step  lt  0          if stop  lt  start              slice length    start - stop - 1      -step    1     else          if start  lt  stop              slice length    stop - start - 1     step   1        Cases of step   1 and step    1 are treated separately     if slice length  lt   0          return        elif step    1            See list slice   in listobject c         result              for i in range stop - start               result append x i start           return result     else          result              cur   start         for i in range slice length               result append x cur               cur    step         return result

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I want to add one Hello  World  example that explains the basics of slices for the very beginners  It helped me a lot   Let s have a list with six values   P    Y    T    H    O    N      --- --- --- --- --- ---    P   Y   T   H   O   N    --- --- --- --- --- ---    0   1   2   3   4   5   Now the simplest slices of that list are its sublists  The notation is   lt index gt   lt index gt   and the key is to read it like this     start cutting before this index   end cutting before this index     Now if you make a slice  2 5  of the list above  this will happen                          --- --- --- --- --- ---    P   Y   T   H   O   N    --- --- --- --- --- ---    0   1   2   3   4   5   You made a cut before the element with index 2 and another cut before the element with index 5  So the result will be a slice between those two cuts  a list   T    H    O

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Index        ------------ gt    0   1   2   3   4  --- --- --- --- ---    a   b   c   d   e    --- --- --- --- ---    0  -4  -3  -2  -1        lt ------------  Slice       lt ---------------   --------------- gt      1   2   3   4      --- --- --- --- ---    a   b   c   d   e    --- --- --- --- ---     -4  -3  -2  -1      --------------- gt       lt ---------------    I hope this will help you to model the list in Python   Reference  http   wiki python org moin MovingToPythonFromOtherLanguages

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Explain Python s slice notation  In short  the colons     in subscript notation  subscriptable subscriptarg   make slice notation - which has the optional arguments  start  stop  step  sliceable start stop step   Python slicing is a computationally fast way to methodically access parts of your data  In my opinion  to be even an intermediate Python programmer  it s one aspect of the language that it is necessary to be familiar with  Important Definitions To begin with  let s define a few terms   start  the beginning index of the slice  it will include the element at this index unless it is the same as stop  defaults to 0  i e  the first index  If it s negative  it means to start n items from the end  stop  the ending index of the slice  it does not include the element at this index  defaults to length of the sequence being sliced  that is  up to and including the end  step  the amount by which the index increases  defaults to 1  If it s negative  you re slicing over the iterable in reverse   How Indexing Works You can make any of these positive or negative numbers  The meaning of the positive numbers is straightforward  but for negative numbers  just like indexes in Python  you count backwards from the end for the start and stop  and for the step  you simply decrement your index  This example is from the documentation s tutorial  but I ve modified it slightly to indicate which item in a sequence each index references    --- --- --- --- --- ---     P   y   t   h   o   n     --- --- --- --- --- ---     0   1   2   3   4   5    -6  -5  -4  -3  -2  -1  How Slicing Works To use slice notation with a sequence that supports it  you must include at least one colon in the square brackets that follow the sequence  which actually implement the   getitem   method of the sequence  according to the Python data model   Slice notation works like this  sequence start stop step   And recall that there are defaults for start  stop  and step  so to access the defaults  simply leave out the argument  Slice notation to get the last nine elements from a list  or any other sequence that supports it  like a string  would look like this  my list -9    When I see this  I read the part in the brackets as  quot 9th from the end  to the end  quot   Actually  I abbreviate it mentally as  quot -9  on quot   Explanation  The full notation is my list -9 None None   and to substitute the defaults  actually when step is negative  stop s default is -len my list  - 1  so None for stop really just means it goes to whichever end step takes it to   my list -9 len my list  1   The colon      is what tells Python you re giving it a slice and not a regular index  That s why the idiomatic way of making a shallow copy of lists in Python 2 is list copy   sequence     And clearing them is with  del my list      Python 3 gets a list copy and list clear method   When step is negative  the defaults for start and stop change By default  when the step argument is empty  or None   it is assigned to  1  But you can pass in a negative integer  and the list  or most other standard slicables  will be sliced from the end to the beginning  Thus a negative slice will change the defaults for start and stop  Confirming this in the source I like to encourage users to read the source as well as the documentation  The source code for slice objects and this logic is found here  First we determine if step is negative    step is negative   step sign  lt  0    If so  the lower bound is -1  meaning we slice all the way up to and including the beginning  and the upper bound is the length minus 1  meaning we start at the end   Note that the semantics of this -1 is different from a -1 that users may pass indexes in Python indicating the last item    if  step is negative        lower   PyLong FromLong -1L       if  lower    NULL          goto error       upper   PyNumber Add length  lower       if  upper    NULL          goto error      Otherwise step is positive  and the lower bound will be zero and the upper bound  which we go up to but not including  the length of the sliced list   else       lower    PyLong Zero      Py INCREF lower       upper   length      Py INCREF upper       Then  we may need to apply the defaults for start and stop - the default then for start is calculated as the upper bound when step is negative   if  self- gt start    Py None        start   step is negative   upper   lower      Py INCREF start       and stop  the lower bound   if  self- gt stop    Py None        stop   step is negative   lower   upper      Py INCREF stop       Give your slices a descriptive name  You may find it useful to separate forming the slice from passing it to the list   getitem   method  that s what the square brackets do   Even if you re not new to it  it keeps your code more readable so that others that may have to read your code can more readily understand what you re doing  However  you can t just assign some integers separated by colons to a variable  You need to use the slice object  last nine slice   slice -9  None   The second argument  None  is required  so that the first argument is interpreted as the start argument otherwise it would be the stop argument  You can then pass the slice object to your sequence   gt  gt  gt  list range 100   last nine slice   91  92  93  94  95  96  97  98  99   It s interesting that ranges also take slices   gt  gt  gt  range 100  last nine slice  range 91  100   Memory Considerations  Since slices of Python lists create new objects in memory  another important function to be aware of is itertools islice  Typically you ll want to iterate over a slice  not just have it created statically in memory  islice is perfect for this  A caveat  it doesn t support negative arguments to start  stop  or step  so if that s an issue you may need to calculate indices or reverse the iterable in advance  length   100 last nine iter   itertools islice list range length    length-9  None  1  list last nine   list last nine iter   and now   gt  gt  gt  list last nine  91  92  93  94  95  96  97  98  99   The fact that list slices make a copy is a feature of lists themselves  If you re slicing advanced objects like a Pandas DataFrame  it may return a view on the original  and not a copy

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My brain seems happy to accept that lst start end  contains the start-th item  I might even say that it is a  natural assumption    But occasionally a doubt creeps in and my brain asks for reassurance that it does not contain the end-th element   In these moments I rely on this simple theorem   for any n     lst   lst  n    lst n     This pretty property tells me that lst start end  does not contain the end-th item because it is in lst end     Note that this theorem is true for any n at all  For example  you can check that  lst   range 10  lst  -42    lst -42      lst   returns True

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And a couple of things that weren t immediately obvious to me when I first saw the slicing syntax    gt  gt  gt  x    1 2 3 4 5 6   gt  gt  gt  x   -1   6 5 4 3 2 1    Easy way to reverse sequences   And if you wanted  for some reason  every second item in the reversed sequence    gt  gt  gt  x    1 2 3 4 5 6   gt  gt  gt  x   -2   6 4 2

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I got a little frustrated in not finding an online source  or Python documentation that describes precisely what slicing does   I took Aaron Hall s suggestion  read the relevant parts of the CPython source code  and wrote some Python code that performs slicing similarly to how it s done in CPython  I ve tested my code in Python 3 on millions of random tests on integer lists   You may find the references in my code to the relevant functions in CPython helpful     Return the result of slicing list x   See the part of list subscript   in listobject c that pertains   to when the indexing item is a PySliceObject def slicer x  start None  stop None  step None         Handle slicing index values of None  and a step value of 0        See PySlice Unpack   in sliceobject c  which       extracts start  stop  step from a PySliceObject      maxint   10000000         A hack to simulate PY SSIZE T MAX     if step    None          step   1     elif step    0          raise ValueError  slice step cannot be zero        if start    None          start   maxint if step  lt  0 else 0     if stop    None          stop   -maxint if step  lt  0 else maxint        Handle negative slice indexes and bad slice indexes        Compute number of elements in the slice as slice length        See PySlice AdjustIndices   in sliceobject c     length   len x      slice length   0      if start  lt  0          start    length         if start  lt  0              start   -1 if step  lt  0 else 0     elif start  gt   length          start   length - 1 if step  lt  0 else length      if stop  lt  0          stop    length         if stop  lt  0              stop   -1 if step  lt  0 else 0     elif stop  gt  length          stop   length - 1 if step  lt  0 else length      if step  lt  0          if stop  lt  start              slice length    start - stop - 1      -step    1     else          if start  lt  stop              slice length    stop - start - 1     step   1        Cases of step   1 and step    1 are treated separately     if slice length  lt   0          return        elif step    1            See list slice   in listobject c         result              for i in range stop - start               result append x i start           return result     else          result              cur   start         for i in range slice length               result append x cur               cur    step         return result

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1  Slice Notation  To make it simple  remember slice has only one form   s start end step    and here is how it works    s  an object that can be sliced start  first index to start iteration end  last index  NOTE that end index will not be included in the resulted slice step  pick element every step index   Another import thing  all start end  step can be omitted  And if they are omitted  their default value will be used  0 len s  1 accordingly   So possible variations are     Mostly used variations s start end  s start   s  end     Step-related variations s  end step  s start  step  s   step     Make a copy s      NOTE  If start  gt   end  considering only when step gt 0   Python will return a empty slice      2  Pitfalls  The above part explains the core features on how slice works  and it will work on most occasions  However  there can be pitfalls you should watch out  and this part explains them   Negative indexes  The very first thing that confuses Python learners is that an index can be negative  Don t panic  a negative index means count backwards   For example   s -5        Start at the 5th index from the end of array              thus returning the last 5 elements  s  -5       Start at index 0  and end until the 5th index from end of array              thus returning s 0 len s -5     Negative step  Making things more confusing is that step can be negative too   A negative step means iterate the array backwards  from the end to start  with the end index included  and the start index excluded from the result   NOTE  when step is negative  the default value for start is len s   while end does not equal to 0  because s   -1  contains s 0    For example   s   -1               Reversed slice s len s   -1         The same as above  reversed slice s 0 len s  -1        Empty list   Out of range error   Be surprised  slice does not raise an IndexError when the index is out of range   If the index is out of range  Python will try its best to set the index to 0 or len s  according to the situation  For example   s  len s  5         The same as s  len s   s -len s -5         The same as s 0   s len s  5  -1      The same as s len s   -1   and the same as s   -1    3  Examples  Let s finish this answer with examples  explaining everything we have discussed     Create our array for demonstration In  1   s    i for i in range 10    In  2   s Out 2    0  1  2  3  4  5  6  7  8  9   In  3   s 2       From index 2 to last index Out 3    2  3  4  5  6  7  8  9   In  4   s  8      From index 0 up to index 8 Out 4    0  1  2  3  4  5  6  7   In  5   s 4 7     From index 4  included  up to index 7 excluded  Out 5    4  5  6   In  6   s  -2     Up to second last index  negative index  Out 6    0  1  2  3  4  5  6  7   In  7   s -2      From second last index  negative index  Out 7    8  9   In  8   s   -1    From last to first in reverse order  negative step  Out 8    9  8  7  6  5  4  3  2  1  0   In  9   s   -2    All odd numbers in reversed order Out 9    9  7  5  3  1   In  11   s -2  -2    All even numbers in reversed order Out 11    8  6  4  2  0   In  12   s 3 15      End is out of range  and Python will set it to len s   Out 12    3  4  5  6  7  8  9   In  14   s 5 1       Start  gt  end  return empty list Out 14       In  15   s 11        Access index 11  greater than len s   will raise an IndexError --------------------------------------------------------------------------- IndexError                                Traceback  most recent call last   lt ipython-input-15-79ffc22473a3 gt  in  lt module gt    ---- gt  1 s 11   IndexError  list index out of range

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This is how I teach slices to newbies   Understanding the difference between indexing and slicing   Wiki Python has this amazing picture which clearly distinguishes indexing and slicing     It is a list with six elements in it  To understand slicing better  consider that list as a set of six boxes placed together  Each box has an alphabet in it   Indexing is like dealing with the contents of box  You can check contents of any box  But you can t check the contents of multiple boxes at once  You can even replace the contents of the box  But you can t place two balls in one box or replace two balls at a time   In  122   alpha     a    b    c    d    e    f    In  123   alpha Out 123     a    b    c    d    e    f    In  124   alpha 0  Out 124    a   In  127   alpha 0     A   In  128   alpha Out 128     A    b    c    d    e    f    In  129   alpha 0 1  --------------------------------------------------------------------------- TypeError                                 Traceback  most recent call last   lt ipython-input-129-c7eb16585371 gt  in  lt module gt    ---- gt  1 alpha 0 1   TypeError  list indices must be integers  not tuple   Slicing is like dealing with boxes themselves  You can pick up the first box and place it on another table  To pick up the box  all you need to know is the position of beginning and ending of the box   You can even pick up the first three boxes or the last two boxes or all boxes between 1 and 4  So  you can pick any set of boxes if you know the beginning and ending  These positions are called start and stop positions   The interesting thing is that you can replace multiple boxes at once  Also you can place multiple boxes wherever you like   In  130   alpha 0 1  Out 130     A    In  131   alpha 0 1     a   In  132   alpha Out 132     a    b    c    d    e    f    In  133   alpha 0 2      A    B    In  134   alpha Out 134     A    B    c    d    e    f    In  135   alpha 2 2      x    xx    In  136   alpha Out 136     A    B    x    xx    c    d    e    f     Slicing With Step   Till now you have picked boxes continuously  But sometimes you need to pick up discretely  For example  you can pick up every second box  You can even pick up every third box from the end  This value is called step size  This represents the gap between your successive pickups  The step size should be positive if You are picking boxes from the beginning to end and vice versa   In  137   alpha     a    b    c    d    e    f    In  142   alpha 1 5 2  Out 142     b    d    In  143   alpha -1 -5 -2  Out 143     f    d    In  144   alpha 1 5 -2  Out 144       In  145   alpha -1 -5 2  Out 145        How Python Figures Out Missing Parameters   When slicing  if you leave out any parameter  Python tries to figure it out automatically   If you check the source code of CPython  you will find a function called PySlice GetIndicesEx   which figures out indices to a slice for any given parameters  Here is the logical equivalent code in Python   This function takes a Python object and optional parameters for slicing and returns the start  stop  step  and slice length for the requested slice   def py slice get indices ex obj  start None  stop None  step None        length   len obj       if step is None          step   1     if step    0          raise Exception  Step cannot be zero         if start is None          start   0 if step  gt  0 else length - 1     else          if start  lt  0              start    length         if start  lt  0              start   0 if step  gt  0 else -1         if start  gt   length              start   length if step  gt  0 else length - 1      if stop is None          stop   length if step  gt  0 else -1     else          if stop  lt  0              stop    length         if stop  lt  0              stop   0 if step  gt  0 else -1         if stop  gt   length              stop   length if step  gt  0 else length - 1      if  step  lt  0 and stop  gt   start  or  step  gt  0 and start  gt   stop           slice length   0     elif step  lt  0          slice length    stop - start   1   step    1     else          slice length    stop - start - 1   step    1      return  start  stop  step  slice length    This is the intelligence that is present behind slices  Since Python has an built-in function called slice  you can pass some parameters and check how smartly it calculates missing parameters   In  21   alpha     a    b    c    d    e    f    In  22   s   slice None  None  None   In  23   s Out 23   slice None  None  None   In  24   s indices len alpha   Out 24    0  6  1   In  25   range  s indices len alpha    Out 25    0  1  2  3  4  5   In  26   s   slice None  None  -1   In  27   range  s indices len alpha    Out 27    5  4  3  2  1  0   In  28   s   slice None  3  -1   In  29   range  s indices len alpha    Out 29    5  4    Note  This post was originally written in my blog  The Intelligence Behind Python Slices

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It is easy to understand if we could relate slicing to range  which gives the indexes  We can categorize slicing into the following two categories     1  No step or step   0  For example   i j  or  i j k   k 0   Suppose the sequence is s  1 2 3 4 5     if 0 lt i lt len s  and 0 lt j lt len s   then  i j k  - gt  range i j k    For example   0 3 2  - gt  range 0 3 2  - gt  0  2   if i gt len s  or j gt len s   then i len s  or j len s    For example   0 100 2  - gt  range 0 len s  2  - gt  range 0 5 2  - gt  0  2  4   if i lt 0 or j lt 0  then i max 0 len s  i  or j max 0 len s  j    For example   0 -3 2  - gt  range 0 len s -3 2  - gt  range 0 2 2  - gt  0  For another example   0 -1 2  - gt  range 0 len s -1 2  - gt  range 0 4 2  - gt  0  2   if i is not specified  then i 0   For example    4 2  - gt  range 0 4 2  - gt  range 0 4 2  - gt  0  2   if j is not specified  then j len s    For example   0  2  - gt  range 0 len s  2  - gt  range 0 5 2  - gt  0  2  4    2  Step  lt  0  For example   i j k   k lt 0   Suppose the sequence is s  1 2 3 4 5     if 0 lt i lt len s  and 0 lt j lt len s   then  i j k  - gt  range i j k    For example   5 0 -2  - gt  range 5 0 -2  - gt  5  3  1   if i gt len s  or j gt len s   then i len s -1 or j len s -1   For example   100 0 -2  - gt  range len s -1 0 -2  - gt  range 4 0 -2  - gt  4  2   if i lt 0 or j lt 0  then i max -1 len s  i  or j max -1 len s  j    For example   -2 -10 -2  - gt  range len s -2 -1 -2  - gt  range 3 -1 -2  - gt  3  1   if i is not specified  then i len s -1   For example    0 -2  - gt  range len s -1 0 -2  - gt  range 4 0 -2  - gt  4  2   if j is not specified  then j -1   For example   2  -2  - gt  range 2 -1 -2  - gt  2  0  For another example     -1  - gt  range len s -1 -1 -1  - gt  range 4 -1 -1  - gt  4  3  2  1  0    In summary

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The answers above don t discuss slice assignment  To understand slice assignment  it s helpful to add another concept to the ASCII art                    --- --- --- --- --- ---                    P   y   t   h   o   n                    --- --- --- --- --- ---  Slice position  0   1   2   3   4   5   6 Index position    0   1   2   3   4   5   gt  gt  gt  p     P   y   t   h   o   n     Why the two sets of numbers    indexing gives items  not lists  gt  gt  gt  p 0    P   gt  gt  gt  p 5    n     Slicing gives lists  gt  gt  gt  p 0 1     P    gt  gt  gt  p 0 2     P   y     One heuristic is  for a slice from zero to n  think   zero is the beginning  start at the beginning and take n items in a list     gt  gt  gt  p 5    the last of six items  indexed from zero   n   gt  gt  gt  p 0 5    does NOT include the last item     P   y   t   h   o    gt  gt  gt  p 0 6    not p 0 5        P   y   t   h   o   n     Another heuristic is   for any slice  replace the start by zero  apply the previous heuristic to get the end of the list  then count the first number back up to chop items off the beginning    gt  gt  gt  p 0 4    Start at the beginning and count out 4 items    P   y   t   h    gt  gt  gt  p 1 4    Take one item off the front    y   t   h    gt  gt  gt  p 2 4    Take two items off the front    t   h     etc    The first rule of slice assignment is that since slicing returns a list  slice assignment requires a list  or other iterable     gt  gt  gt  p 2 3     t    gt  gt  gt  p 2 3      T    gt  gt  gt  p    P   y   T   h   o   n    gt  gt  gt  p 2 3     t  Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  TypeError  can only assign an iterable   The second rule of slice assignment  which you can also see above  is that whatever portion of the list is returned by slice indexing  that s the same portion that is changed by slice assignment    gt  gt  gt  p 2 4     T   h    gt  gt  gt  p 2 4      t   r    gt  gt  gt  p    P   y   t   r   o   n     The third rule of slice assignment is  the assigned list  iterable  doesn t have to have the same length  the indexed slice is simply sliced out and replaced en masse by whatever is being assigned    gt  gt  gt  p     P   y   t   h   o   n     Start over  gt  gt  gt  p 2 4      s   p   a   m    gt  gt  gt  p    P   y   s   p   a   m   o   n     The trickiest part to get used to is assignment to empty slices  Using heuristic 1 and 2 it s easy to get your head around indexing an empty slice    gt  gt  gt  p     P   y   t   h   o   n    gt  gt  gt  p 0 4     P   y   t   h    gt  gt  gt  p 1 4     y   t   h    gt  gt  gt  p 2 4     t   h    gt  gt  gt  p 3 4     h    gt  gt  gt  p 4 4        And then once you ve seen that  slice assignment to the empty slice makes sense too    gt  gt  gt  p     P   y   t   h   o   n    gt  gt  gt  p 2 4      x   y     Assigned list is same length as slice  gt  gt  gt  p    P   y   x   y   o   n     Result is same length  gt  gt  gt  p     P   y   t   h   o   n    gt  gt  gt  p 3 4      x   y     Assigned list is longer than slice  gt  gt  gt  p    P   y   t   x   y   o   n     The result is longer  gt  gt  gt  p     P   y   t   h   o   n    gt  gt  gt  p 4 4      x   y    gt  gt  gt  p    P   y   t   h   x   y   o   n     The result is longer still   Note that  since we are not changing the second number of the slice  4   the inserted items always stack right up against the  o   even when we re assigning to the empty slice  So the position for the empty slice assignment is the logical extension of the positions for the non-empty slice assignments   Backing up a little bit  what happens when you keep going with our procession of counting up the slice beginning    gt  gt  gt  p     P   y   t   h   o   n    gt  gt  gt  p 0 4     P   y   t   h    gt  gt  gt  p 1 4     y   t   h    gt  gt  gt  p 2 4     t   h    gt  gt  gt  p 3 4     h    gt  gt  gt  p 4 4       gt  gt  gt  p 5 4       gt  gt  gt  p 6 4        With slicing  once you re done  you re done  it doesn t start slicing backwards  In Python you don t get negative strides unless you explicitly ask for them by using a negative number    gt  gt  gt  p 5 3 -1     n   o     There are some weird consequences to the  once you re done  you re done  rule    gt  gt  gt  p 4 4       gt  gt  gt  p 5 4       gt  gt  gt  p 6 4       gt  gt  gt  p 6  Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  IndexError  list index out of range   In fact  compared to indexing  Python slicing is bizarrely error-proof    gt  gt  gt  p 100 200       gt  gt  gt  p int 2e99  int 1e99         This can come in handy sometimes  but it can also lead to somewhat strange behavior    gt  gt  gt  p    P    y    t    h    o    n    gt  gt  gt  p int 2e99  int 1e99       p   o   w   e   r    gt  gt  gt  p    P    y    t    h    o    n    p    o    w    e    r     Depending on your application  that might    or might not    be what you were hoping for there     Below is the text of my original answer  It has been useful to many people  so I didn t want to delete it    gt  gt  gt  r  1 2 3 4   gt  gt  gt  r 1 1      gt  gt  gt  r 1 1   9 8   gt  gt  gt  r  1  9  8  2  3  4   gt  gt  gt  r 1 1    blah    gt  gt  gt  r  1   blah   9  8  2  3  4    This may also clarify the difference between slicing and indexing

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usr bin env python  def slicegraphical s  lista        if len s   gt  9          print    Enter a string of maximum 9 characters      so the printig would looki nice            return 0        print          print        ---    len s           print          for letter in s          print        format letter       print         print       print   ---    len s            print          for letter in range len s   1           print        format letter       print        for letter in range -1  len s    0           print       format letter       print        print          for triada in lista          if len triada     3              if triada 0   None and triada 1     None and triada 2     None                    000                 print s                         s triada 0  triada 1  triada 2               elif triada 0     None and triada 1     None and triada 2     None                    001                 print s            0 2d     format triada 2                  s triada 0  triada 1  triada 2               elif triada 0     None and triada 1     None and triada 2     None                    010                 print s        0 2d         format triada 1           s triada 0  triada 1  triada 2               elif triada 0     None and triada 1     None and triada 2     None                    011                 print s        0 2d    1 2d     format triada 1   triada 2           s triada 0  triada 1  triada 2               elif triada 0     None and triada 1     None and triada 2     None                    100                 print s    0 2d             format triada 0           s triada 0  triada 1  triada 2               elif triada 0     None and triada 1     None and triada 2     None                    101                 print s    0 2d        1 2d     format triada 0   triada 2           s triada 0  triada 1  triada 2               elif triada 0     None and triada 1     None and triada 2     None                    110                 print s    0 2d    1 2d         format triada 0   triada 1           s triada 0  triada 1  triada 2               elif triada 0     None and triada 1     None and triada 2     None                    111                 print s    0 2d    1 2d    2 2d     format triada 0   triada 1   triada 2           s triada 0  triada 1  triada 2            elif len triada     2              if triada 0     None and triada 1     None                    00                 print s                          s triada 0  triada 1               elif triada 0     None and triada 1     None                    01                 print s        0 2d         format triada 1            s triada 0  triada 1               elif triada 0     None and triada 1     None                    10                 print s    0 2d             format triada 0            s triada 0  triada 1               elif triada 0     None and triada 1     None                    11                 print s    0 2d    1 2d         format triada 0  triada 1            s triada 0  triada 1            elif len triada     1              print s    0 2d             format triada 0            s triada 0     if   name         main           Change  s  to what ever string you like  make it 9 characters for       better representation      s    COMPUTERS         add to this list different lists to experement with indexes       to represent ex  s      use s None  None None   otherwise you get an error       for s 2   use s 2 None       lista     4 7   2 5 2   -5 1 -1   4   -4 -6 -1    2 -3 1   2 -3 -1    None None -1   -5 None   -5 0 -1   -5 None -1   -1 1 -2        slicegraphical s  lista    You can run this script and experiment with it  below is some samples that I got from the script      --- --- --- --- --- --- --- --- ---      C   O   M   P   U   T   E   R   S      --- --- --- --- --- --- --- --- ---    0   1   2   3   4   5   6   7   8   9     -9  -8  -7  -6  -5  -4  -3  -2  -1   COMPUTERS  4   7          UTE COMPUTERS  2   5   2      MU COMPUTERS -5   1  -1      UPM COMPUTERS  4              U COMPUTERS -4  -6  -1      TU COMPUTERS  2  -3   1      MPUT COMPUTERS  2  -3  -1       COMPUTERS         -1      SRETUPMOC COMPUTERS -5              UTERS COMPUTERS -5   0  -1      UPMO COMPUTERS -5      -1      UPMOC COMPUTERS -1   1  -2      SEUM  Finished in 0 9s    When using a negative step  notice that the answer is shifted to the right by 1

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If you feel negative indices in slicing is confusing  here s a very easy way to think about it  just replace the negative index with len - index  So for example  replace -3 with len list  - 3   The best way to illustrate what slicing does internally is just show it in code that implements this operation   def slice list  start   None  end   None  step   1       Take care of missing start end parameters   start   0 if start is None else start   end   len list  if end is None else end      Take care of negative start end parameters   start   len list    start if start  lt  0 else start   end   len list    end if end  lt  0 else end      Now just execute a for-loop with start  end and step   return  list i  for i in range start  end  step

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I don t think that the Python tutorial diagram  cited in various other answers  is good as this suggestion works for positive stride  but does not for a negative stride   This is the diagram     --- --- --- --- --- ---     P   y   t   h   o   n     --- --- --- --- --- ---   0   1   2   3   4   5   6 -6  -5  -4  -3  -2  -1    From the diagram  I expect a -4 -6 -1  to be yP but it is ty    gt  gt  gt  a    Python   gt  gt  gt  a 2 4 1    as expected  th   gt  gt  gt  a -4 -6 -1    off by 1  ty    What always work is to think in characters or slots and use indexing as a half-open interval -- right-open if positive stride  left-open if negative stride   This way  I can think of a -4 -6 -1  as a -6 -4  in interval terminology     --- --- --- --- --- ---     P   y   t   h   o   n     --- --- --- --- --- ---     0   1   2   3   4   5     -6  -5  -4  -3  -2  -1    --- --- --- --- --- --- --- --- --- --- --- ---     P   y   t   h   o   n   P   y   t   h   o   n     --- --- --- --- --- --- --- --- --- --- --- ---    -6  -5  -4  -3  -2  -1   0   1   2   3   4   5

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In Python 2 7  Slicing in Python   a b c   len   length of string  tuple or list  c -- default is  1  The sign of c indicates forward or backward  absolute value of c indicates steps  Default is forward with step size 1  Positive means forward  negative means backward   a --  When c is positive or blank  default is 0  When c is negative  default is -1   b --  When c is positive or blank  default is len  When c is negative  default is - len 1     Understanding index assignment is very important   In forward direction  starts at 0 and ends at len-1  In backward direction  starts at -1 and ends at -len   When you say  a b c   you are saying depending on the sign of c  forward or backward   start at a and end at b  excluding element at bth index   Use the indexing rule above and remember you will only find elements in this range   -len  -len 1  -len 2       0  1  2 3 4   len -1   But this range continues in both directions infinitely       -len -2  -len-1 -len  -len 1  -len 2       0  1  2 3 4   len -1  len  len  1  len 2          For example                0    1    2   3    4   5   6   7   8   9   10   11              a    s    t   r    i   n   g     -9  -8  -7   -6   -5  -4   -3  -2  -1   If your choice of a  b  and c allows overlap with the range above as you traverse using rules for a b c above you will either get a list with elements  touched during traversal  or you will get an empty list   One last thing  if a and b are equal  then also you get an empty list    gt  gt  gt  l1  2  3  4    gt  gt  gt  l1     2  3  4    gt  gt  gt  l1   -1    a default is -1   b default is - len 1   4  3  2    gt  gt  gt  l1  -4 -1    a default is -1  4  3  2    gt  gt  gt  l1  -3 -1    a default is -1  4  3    gt  gt  gt  l1       c default is  1  so a default is 0  b default is len  2  3  4    gt  gt  gt  l1   -1    c is -1   so a default is -1 and b default is - len 1   4  3  2     gt  gt  gt  l1 -100 -200 -1    Interesting      gt  gt  gt  l1 -1 -200 -1    Interesting  4  3  2     gt  gt  gt  l1 -1 -1 1        gt  gt  gt  l1 -1 5 1    Interesting  4     gt  gt  gt  l1 1 -7 1       gt  gt  gt  l1 1 -7 -1    Interesting  3  2    gt  gt  gt  l1  -2 -2    a default is -1  stop b  at -2   step c  by 2 in reverse direction  4

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The Python tutorial talks about it  scroll down a bit until you get to the part about slicing    The ASCII art diagram is helpful too for remembering how slices work     --- --- --- --- --- ---     P   y   t   h   o   n     --- --- --- --- --- ---   0   1   2   3   4   5   6 -6  -5  -4  -3  -2  -1      One way to remember how slices work is to think of the indices as pointing between characters  with the left edge of the first character numbered 0  Then the right edge of the last character of a string of n characters has index n

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The Python tutorial talks about it  scroll down a bit until you get to the part about slicing    The ASCII art diagram is helpful too for remembering how slices work     --- --- --- --- --- ---     P   y   t   h   o   n     --- --- --- --- --- ---   0   1   2   3   4   5   6 -6  -5  -4  -3  -2  -1      One way to remember how slices work is to think of the indices as pointing between characters  with the left edge of the first character numbered 0  Then the right edge of the last character of a string of n characters has index n

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Explain Python s slice notation  In short  the colons     in subscript notation  subscriptable subscriptarg   make slice notation - which has the optional arguments  start  stop  step  sliceable start stop step   Python slicing is a computationally fast way to methodically access parts of your data  In my opinion  to be even an intermediate Python programmer  it s one aspect of the language that it is necessary to be familiar with  Important Definitions To begin with  let s define a few terms   start  the beginning index of the slice  it will include the element at this index unless it is the same as stop  defaults to 0  i e  the first index  If it s negative  it means to start n items from the end  stop  the ending index of the slice  it does not include the element at this index  defaults to length of the sequence being sliced  that is  up to and including the end  step  the amount by which the index increases  defaults to 1  If it s negative  you re slicing over the iterable in reverse   How Indexing Works You can make any of these positive or negative numbers  The meaning of the positive numbers is straightforward  but for negative numbers  just like indexes in Python  you count backwards from the end for the start and stop  and for the step  you simply decrement your index  This example is from the documentation s tutorial  but I ve modified it slightly to indicate which item in a sequence each index references    --- --- --- --- --- ---     P   y   t   h   o   n     --- --- --- --- --- ---     0   1   2   3   4   5    -6  -5  -4  -3  -2  -1  How Slicing Works To use slice notation with a sequence that supports it  you must include at least one colon in the square brackets that follow the sequence  which actually implement the   getitem   method of the sequence  according to the Python data model   Slice notation works like this  sequence start stop step   And recall that there are defaults for start  stop  and step  so to access the defaults  simply leave out the argument  Slice notation to get the last nine elements from a list  or any other sequence that supports it  like a string  would look like this  my list -9    When I see this  I read the part in the brackets as  quot 9th from the end  to the end  quot   Actually  I abbreviate it mentally as  quot -9  on quot   Explanation  The full notation is my list -9 None None   and to substitute the defaults  actually when step is negative  stop s default is -len my list  - 1  so None for stop really just means it goes to whichever end step takes it to   my list -9 len my list  1   The colon      is what tells Python you re giving it a slice and not a regular index  That s why the idiomatic way of making a shallow copy of lists in Python 2 is list copy   sequence     And clearing them is with  del my list      Python 3 gets a list copy and list clear method   When step is negative  the defaults for start and stop change By default  when the step argument is empty  or None   it is assigned to  1  But you can pass in a negative integer  and the list  or most other standard slicables  will be sliced from the end to the beginning  Thus a negative slice will change the defaults for start and stop  Confirming this in the source I like to encourage users to read the source as well as the documentation  The source code for slice objects and this logic is found here  First we determine if step is negative    step is negative   step sign  lt  0    If so  the lower bound is -1  meaning we slice all the way up to and including the beginning  and the upper bound is the length minus 1  meaning we start at the end   Note that the semantics of this -1 is different from a -1 that users may pass indexes in Python indicating the last item    if  step is negative        lower   PyLong FromLong -1L       if  lower    NULL          goto error       upper   PyNumber Add length  lower       if  upper    NULL          goto error      Otherwise step is positive  and the lower bound will be zero and the upper bound  which we go up to but not including  the length of the sliced list   else       lower    PyLong Zero      Py INCREF lower       upper   length      Py INCREF upper       Then  we may need to apply the defaults for start and stop - the default then for start is calculated as the upper bound when step is negative   if  self- gt start    Py None        start   step is negative   upper   lower      Py INCREF start       and stop  the lower bound   if  self- gt stop    Py None        stop   step is negative   lower   upper      Py INCREF stop       Give your slices a descriptive name  You may find it useful to separate forming the slice from passing it to the list   getitem   method  that s what the square brackets do   Even if you re not new to it  it keeps your code more readable so that others that may have to read your code can more readily understand what you re doing  However  you can t just assign some integers separated by colons to a variable  You need to use the slice object  last nine slice   slice -9  None   The second argument  None  is required  so that the first argument is interpreted as the start argument otherwise it would be the stop argument  You can then pass the slice object to your sequence   gt  gt  gt  list range 100   last nine slice   91  92  93  94  95  96  97  98  99   It s interesting that ranges also take slices   gt  gt  gt  range 100  last nine slice  range 91  100   Memory Considerations  Since slices of Python lists create new objects in memory  another important function to be aware of is itertools islice  Typically you ll want to iterate over a slice  not just have it created statically in memory  islice is perfect for this  A caveat  it doesn t support negative arguments to start  stop  or step  so if that s an issue you may need to calculate indices or reverse the iterable in advance  length   100 last nine iter   itertools islice list range length    length-9  None  1  list last nine   list last nine iter   and now   gt  gt  gt  list last nine  91  92  93  94  95  96  97  98  99   The fact that list slices make a copy is a feature of lists themselves  If you re slicing advanced objects like a Pandas DataFrame  it may return a view on the original  and not a copy

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The previous answers don t discuss multi-dimensional array slicing which is possible using the famous NumPy package   Slicing can also be applied to multi-dimensional arrays     Here  a is a NumPy array   gt  gt  gt  a array    1   2   3   4            5   6   7   8            9  10  11  12     gt  gt  gt  a  2  0 3 2  array   1  3           5  7      The   2  before the comma operates on the first dimension and the  0 3 2  after the comma operates on the second dimension

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The below is the example of an index of a string     --- --- --- --- ---     H   e   l   p   A     --- --- --- --- ---   0   1   2   3   4   5 -5  -4  -3  -2  -1  str  Name string    Slicing example   start end step   str start end    Items start through end-1 str start        Items start through the rest of the array str  end         Items from the beginning through end-1 str              A copy of the whole array   Below is the example usage   print str 0    N print str 0 2    Na print str 0 7    Name st print str 0 7 2    Nm t print str 0 -1 2    Nm ti

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If you feel negative indices in slicing is confusing  here s a very easy way to think about it  just replace the negative index with len - index  So for example  replace -3 with len list  - 3   The best way to illustrate what slicing does internally is just show it in code that implements this operation   def slice list  start   None  end   None  step   1       Take care of missing start end parameters   start   0 if start is None else start   end   len list  if end is None else end      Take care of negative start end parameters   start   len list    start if start  lt  0 else start   end   len list    end if end  lt  0 else end      Now just execute a for-loop with start  end and step   return  list i  for i in range start  end  step

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usr bin env python  def slicegraphical s  lista        if len s   gt  9          print    Enter a string of maximum 9 characters      so the printig would looki nice            return 0        print          print        ---    len s           print          for letter in s          print        format letter       print         print       print   ---    len s            print          for letter in range len s   1           print        format letter       print        for letter in range -1  len s    0           print       format letter       print        print          for triada in lista          if len triada     3              if triada 0   None and triada 1     None and triada 2     None                    000                 print s                         s triada 0  triada 1  triada 2               elif triada 0     None and triada 1     None and triada 2     None                    001                 print s            0 2d     format triada 2                  s triada 0  triada 1  triada 2               elif triada 0     None and triada 1     None and triada 2     None                    010                 print s        0 2d         format triada 1           s triada 0  triada 1  triada 2               elif triada 0     None and triada 1     None and triada 2     None                    011                 print s        0 2d    1 2d     format triada 1   triada 2           s triada 0  triada 1  triada 2               elif triada 0     None and triada 1     None and triada 2     None                    100                 print s    0 2d             format triada 0           s triada 0  triada 1  triada 2               elif triada 0     None and triada 1     None and triada 2     None                    101                 print s    0 2d        1 2d     format triada 0   triada 2           s triada 0  triada 1  triada 2               elif triada 0     None and triada 1     None and triada 2     None                    110                 print s    0 2d    1 2d         format triada 0   triada 1           s triada 0  triada 1  triada 2               elif triada 0     None and triada 1     None and triada 2     None                    111                 print s    0 2d    1 2d    2 2d     format triada 0   triada 1   triada 2           s triada 0  triada 1  triada 2            elif len triada     2              if triada 0     None and triada 1     None                    00                 print s                          s triada 0  triada 1               elif triada 0     None and triada 1     None                    01                 print s        0 2d         format triada 1            s triada 0  triada 1               elif triada 0     None and triada 1     None                    10                 print s    0 2d             format triada 0            s triada 0  triada 1               elif triada 0     None and triada 1     None                    11                 print s    0 2d    1 2d         format triada 0  triada 1            s triada 0  triada 1            elif len triada     1              print s    0 2d             format triada 0            s triada 0     if   name         main           Change  s  to what ever string you like  make it 9 characters for       better representation      s    COMPUTERS         add to this list different lists to experement with indexes       to represent ex  s      use s None  None None   otherwise you get an error       for s 2   use s 2 None       lista     4 7   2 5 2   -5 1 -1   4   -4 -6 -1    2 -3 1   2 -3 -1    None None -1   -5 None   -5 0 -1   -5 None -1   -1 1 -2        slicegraphical s  lista    You can run this script and experiment with it  below is some samples that I got from the script      --- --- --- --- --- --- --- --- ---      C   O   M   P   U   T   E   R   S      --- --- --- --- --- --- --- --- ---    0   1   2   3   4   5   6   7   8   9     -9  -8  -7  -6  -5  -4  -3  -2  -1   COMPUTERS  4   7          UTE COMPUTERS  2   5   2      MU COMPUTERS -5   1  -1      UPM COMPUTERS  4              U COMPUTERS -4  -6  -1      TU COMPUTERS  2  -3   1      MPUT COMPUTERS  2  -3  -1       COMPUTERS         -1      SRETUPMOC COMPUTERS -5              UTERS COMPUTERS -5   0  -1      UPMO COMPUTERS -5      -1      UPMOC COMPUTERS -1   1  -2      SEUM  Finished in 0 9s    When using a negative step  notice that the answer is shifted to the right by 1

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I don t think that the Python tutorial diagram  cited in various other answers  is good as this suggestion works for positive stride  but does not for a negative stride   This is the diagram     --- --- --- --- --- ---     P   y   t   h   o   n     --- --- --- --- --- ---   0   1   2   3   4   5   6 -6  -5  -4  -3  -2  -1    From the diagram  I expect a -4 -6 -1  to be yP but it is ty    gt  gt  gt  a    Python   gt  gt  gt  a 2 4 1    as expected  th   gt  gt  gt  a -4 -6 -1    off by 1  ty    What always work is to think in characters or slots and use indexing as a half-open interval -- right-open if positive stride  left-open if negative stride   This way  I can think of a -4 -6 -1  as a -6 -4  in interval terminology     --- --- --- --- --- ---     P   y   t   h   o   n     --- --- --- --- --- ---     0   1   2   3   4   5     -6  -5  -4  -3  -2  -1    --- --- --- --- --- --- --- --- --- --- --- ---     P   y   t   h   o   n   P   y   t   h   o   n     --- --- --- --- --- --- --- --- --- --- --- ---    -6  -5  -4  -3  -2  -1   0   1   2   3   4   5

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I find it easier to remember how it works  and then I can figure out any specific start stop step combination   It s instructive to understand range   first   def range start 0  stop  step 1      Illegal syntax  but that s the effect     i   start     while  i  lt  stop if step  gt  0 else i  gt  stop           yield i         i    step   Begin from start  increment by step  do not reach stop   Very simple   The thing to remember about negative step is that stop is always the excluded end  whether it s higher or lower  If you want same slice in opposite order  it s much cleaner to do the reversal separately  e g   abcde  1 -2    -1  slices off one char from left  two from right  then reverses   See also reversed      Sequence slicing is same  except it first normalizes negative indexes  and it can never go outside the sequence   TODO  The code below had a bug with  never go outside the sequence  when abs step  1  I think I patched it to be correct  but it s hard to understand   def this is how slicing works seq  start None  stop None  step 1       if start is None          start    0 if step  gt  0 else len seq -1      elif start  lt  0          start    len seq      if not 0  lt   start  lt  len seq      clip if still outside bounds         start    0 if step  gt  0 else len seq -1      if stop is None          stop    len seq  if step  gt  0 else -1     really -1  not last element     elif stop  lt  0          stop    len seq      for i in range start  stop  step           if 0  lt   i  lt  len seq               yield seq i    Don t worry about the is None details - just remember that omitting start and or stop always does the right thing to give you the whole sequence   Normalizing negative indexes first allows start and or stop to be counted from the end independently   abcde  1 -2      abcde  1 3      bc  despite range 1 -2         The normalization is sometimes thought of as  modulo the length   but note it adds the length just once  e g   abcde  -53 42  is just the whole string

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I personally think about it like a for loop   a start end step    for i   start  i  lt  end  i    step    Also  note that negative values for start and end are relative to the end of the list and computed in the example above by given index   a shape 0

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I personally think about it like a for loop   a start end step    for i   start  i  lt  end  i    step    Also  note that negative values for start and end are relative to the end of the list and computed in the example above by given index   a shape 0

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Most of the previous answers clears up questions about slice notation   The extended indexing syntax used for slicing is aList start stop step   and basic examples are      More slicing examples  15 Extended Slices

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As a general rule  writing code with a lot of hardcoded index values leads to a readability and maintenance mess  For example  if you come back to the code a year later  you   ll look at it and wonder what you were thinking when you wrote it  The solution shown is simply a way of more clearly stating what your code is actually doing  In general  the built-in slice   creates a slice object that can be used anywhere a slice is allowed  For example    gt  gt  gt  items    0  1  2  3  4  5  6   gt  gt  gt  a   slice 2  4   gt  gt  gt  items 2 4   2  3   gt  gt  gt  items a   2  3   gt  gt  gt  items a     10 11   gt  gt  gt  items  0  1  10  11  4  5  6   gt  gt  gt  del items a   gt  gt  gt  items  0  1  4  5  6    If you have a slice instance s  you can get more information about it by looking at its s start  s stop  and s step attributes  respectively  For example     gt  gt  gt  a   slice 10  50  2   gt  gt  gt  a start 10  gt  gt  gt  a stop 50  gt  gt  gt  a step 2  gt  gt  gt

[python] Understanding slice notation

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