What is the difference between Python s list methods append and extend

Question

What s the difference between the list methods append   and extend

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I hope I can make a useful supplement to this question. If your list stores a specific type object, for example Info, here is a situation that extend method is not suitable: In a for loop and and generating an Info object every time and using extend to store it into your list, it will fail. The exception is like below:

TypeError: 'Info' object is not iterable

But if you use the append method, the result is OK. Because every time using the extend method, it will always treat it as a list or any other collection type, iterate it, and place it after the previous list. A specific object can not be iterated, obviously.

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append    It is basically used in Python to add one element      Example 1     gt  gt  a    1  2  3  4   gt  gt  a append 5   gt  gt  print a   gt  gt  a    1  2  3  4  5       Example 2     gt  gt  a    1  2  3  4   gt  gt  a append  5  6    gt  gt  print a   gt  gt  a    1  2  3  4   5  6     extend    Where extend    is used to merge two lists or insert multiple elements in one list      Example 1     gt  gt  a    1  2  3  4   gt  gt  b    5  6  7  8   gt  gt  a extend b   gt  gt  print a   gt  gt  a    1  2  3  4  5  6  7  8       Example 2     gt  gt  a    1  2  3  4   gt  gt  a extend  5  6    gt  gt  print a   gt  gt  a    1  2  3  4  5  6

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Append vs Extend    With append you can append a single element that will extend the list    gt  gt  gt  a    1 2   gt  gt  gt  a append 3   gt  gt  gt  a  1 2 3    If you want to extend more than one element you should use extend  because you can only append one elment or one list of element    gt  gt  gt  a append  4 5    gt  gt  gt  a  gt  gt  gt   1 2 3  4 5     So that you get a nested list  Instead with extend  you can extend a single element like this   gt  gt  gt  a    1 2   gt  gt  gt  a extend  3    gt  gt  gt  a  1 2 3    Or  differently  from append  extend more elements in one time without nesting the list into the original one  that s the reason of the name extend    gt  gt  gt  a extend  4 5 6    gt  gt  gt  a  1 2 3 4 5 6     Adding one element with both methods    Both append and extend can add one element to the end of the list  though append is simpler    append 1 element    gt  gt  gt  x    1 2   gt  gt  gt  x append 3   gt  gt  gt  x  1 2 3     extend one element    gt  gt  gt  x    1 2   gt  gt  gt  x extend  3    gt  gt  gt  x  1 2 3     Adding more elements    with different results   If you use append for more than one element  you have to pass a list of elements as arguments and you will obtain a NESTED list    gt  gt  gt  x    1 2   gt  gt  gt  x append  3 4    gt  gt  gt  x  1 2  3 4     With extend  instead  you pass a list as an argument  but you will obtain a list with the new element that is not nested in the old one    gt  gt  gt  z    1 2    gt  gt  gt  z extend  3 4    gt  gt  gt  z  1 2 3 4    So  with more elements  you will use extend to get a list with more items  However  appending a list will not add more elements to the list  but one element that is a nested list as you can clearly see in the output of the code

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An English dictionary defines the words append and extend as   append  add  something  to the end of a written document   extend  make larger  Enlarge or expand    With that knowledge  now let s understand  1  The difference between append and extend  append    Appends any Python object as-is to the end of the list  i e  as a the last element in the list   The resulting list may be nested and contain heterogeneous elements  i e  list  string  tuple  dictionary  set  etc     extend    Accepts any iterable as its argument and makes the list larger  The resulting list is always one-dimensional list  i e  no nesting  and it may contain heterogeneous elements in it  e g  characters  integers  float  as a result of applying list iterable     2  Similarity between append and extend   Both take exactly one argument  Both modify the list in-place  As a result  both returns None      Example  lis    1  2  3      extend  is equivalent to this lis   lis   list iterable      append  simply appends its argument as the last element to the list   as long as the argument is a valid Python object list append object

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Append adds the entire data at once  The whole data will be added to the newly created index  On the other hand  extend  as it name suggests  extends the current array    For example  list1    123  456  678  list2    111  222    With append we get   result    123  456  678   111  222     While on extend we get   result    123  456  678  111  222

User · Answer

I hope I can make a useful supplement to this question. If your list stores a specific type object, for example Info, here is a situation that extend method is not suitable: In a for loop and and generating an Info object every time and using extend to store it into your list, it will fail. The exception is like below:

TypeError: 'Info' object is not iterable

But if you use the append method, the result is OK. Because every time using the extend method, it will always treat it as a list or any other collection type, iterate it, and place it after the previous list. A specific object can not be iterated, obviously.

User · Answer

extend   can be used with an iterator argument  Here is an example  You wish to make a list out of a list of lists this way   From  list2d     1 2 3   4 5 6    7    8 9     you want   gt  gt  gt   1  2  3  4  5  6  7  8  9    You may use itertools chain from iterable   to do so  This method s output is an iterator  Its implementation is equivalent to  def from iterable iterables         chain from iterable   ABC    DEF    -- gt  A B C D E F     for it in iterables          for element in it              yield element   Back to our example  we can do  import itertools list2d     1 2 3   4 5 6    7    8 9   merged   list itertools chain from iterable list2d     and get the wanted list   Here is how equivalently extend   can be used with an iterator argument   merged      merged extend itertools chain from iterable list2d   print merged   gt  gt  gt   1  2  3  4  5  6  7  8  9

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append  extends  the list  in place  by only one item  the single object passed  as argument    extend  extends  the list  in place  by as many items as the object passed  as argument  contains   This may be slightly confusing for str objects    If you pass a string as argument  append will add a single string item at the end but extend will add as many  single   str  items as the length of that string  If you pass a list of strings as argument  append will still add a single  list  item at the end and extend will add as many  list  items as the length of the passed list     def append o a list  element       a list append element      print  append    end            for item in a list          print item  end            print    def extend o a list  element       a list extend element      print  extend    end            for item in a list          print item  end            print   append o   ab    cd    extend o   ab    cd   append o   ab     cd    ef    extend o   ab     cd    ef    append o   ab     cd    extend o   ab     cd       produces   append  ab cd  extend  ab c d  append  ab   cd    ef    extend  ab cd ef  append  ab   cd    extend  ab cd

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An interesting point that has been hinted  but not explained  is that extend is faster than append  For any loop that has append inside should be considered to be replaced by list extend processed elements    Bear in mind that apprending new elements might result in the realloaction of the whole list to a better location in memory  If this is done several times because we are appending 1 element at a time  overall performance suffers  In this sense  list extend is analogous to    join stringlist

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This is the equivalent of append and extend using the   operator    gt  gt  gt  x    1 2 3   gt  gt  gt  x  1  2  3   gt  gt  gt  x   x    4 5 6    Extend  gt  gt  gt  x  1  2  3  4  5  6   gt  gt  gt  x   x     7 8     Append  gt  gt  gt  x  1  2  3  4  5  6   7  8

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append appends a single element  extend appends a list of elements   Note that if you pass a list to append  it still adds one element    gt  gt  gt  a    1  2  3   gt  gt  gt  a append  4  5  6    gt  gt  gt  a  1  2  3   4  5  6

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append object  - Updates the list by adding an object to the list   x    20    List passed to the append object  method is treated as a single object  x append  21  22  23     Hence the resultant list length will be 2 print x  -- gt   20   21  22  23     extend list  - Essentially concatenates two lists   x    20    The parameter passed to extend list  method is treated as a list    Eventually it is two lists being concatenated  x extend  21  22  23     Here the resultant list s length is 4 print x   20  21  22  23

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append  Appends object at the end   x    1  2  3  x append  4  5   print  x    gives you   1  2  3   4  5      extend  Extends list by appending elements from the iterable   x    1  2  3  x extend  4  5   print  x    gives you   1  2  3  4  5

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You can use     for returning extend  instead of extending in place   l1 range 10   l1  11    0  1  2  3  4  5  6  7  8  9  11   l2 range 10 1 -1   l1 l2   0  1  2  3  4  5  6  7  8  9  10  9  8  7  6  5  4  3  2    Similarly    for in place behavior  but with slight differences from append  amp  extend  One of the biggest differences of    from append and extend is when it is used in function scopes  see this blog post

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Append and extend are one of the extensibility mechanisms in python    Append  Adds an element to the end of the list    my list    1 2 3 4    To add a new element to the list  we can use append method in the following way   my list append 5    The default location that the new element will be added is always in the  length 1  position    Insert  The insert method was used to overcome the limitations of append  With insert  we can explicitly define the exact position we want our new element to be inserted at    Method descriptor of insert index  object   It takes two arguments  first being the index we want to insert our element and second the element itself    Example  my list    1 2 3 4  my list 4   a   my list  1 2 3 4  a     Extend  This is very useful when we want to join two or more lists into a single list  Without extend  if we want to join two lists  the resulting object will contain a list of lists    a    1 2  b    3  a append b  print  a   1 2  3     If we try to access the element at pos 2  we get a list   3    instead of the element  To join two lists  we ll have to use append    a    1 2  b    3  a extend b  print  a   1 2 3    To join multiple lists  a    1  b    2  c    3  a extend b c  print  a   1 2 3

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append  Appends object at the end   x    1  2  3  x append  4  5   print  x    gives you   1  2  3   4  5      extend  Extends list by appending elements from the iterable   x    1  2  3  x extend  4  5   print  x    gives you   1  2  3  4  5

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The append   method adds a single item to the end of the list   x    1  2  3  x append  4  5   x append  abc   print x    gives you  1  2  3   4  5    abc     The extend   method takes one argument  a list  and appends each of the items of the argument to the original list   Lists are implemented as classes     Creating    a list is really instantiating a class  As such  a list has methods that operate on it    x    1  2  3  x extend  4  5   x extend  abc   print x    gives you  1  2  3  4  5   a    b    c     From Dive Into Python

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This is the equivalent of append and extend using the   operator    gt  gt  gt  x    1 2 3   gt  gt  gt  x  1  2  3   gt  gt  gt  x   x    4 5 6    Extend  gt  gt  gt  x  1  2  3  4  5  6   gt  gt  gt  x   x     7 8     Append  gt  gt  gt  x  1  2  3  4  5  6   7  8

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The following two snippets are semantically equivalent   for item in iterator      a list append item    and  a list extend iterator    The latter may be faster as the loop is implemented in C

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What is the difference between the list methods append and extend     append adds its argument as a single element to the end of a list  The length of the list itself will increase by one  extend iterates over its argument adding each element to the list  extending the list  The length of the list will increase by however many elements were in the iterable argument    append  The list append method appends an object to the end of the list   my list append object     Whatever the object is  whether a number  a string  another list  or something else  it gets added onto the end of my list as a single entry on the list     gt  gt  gt  my list   foo    bar    gt  gt  gt  my list append  baz    gt  gt  gt  my list   foo    bar    baz     So keep in mind that a list is an object  If you append another list onto a list  the first list will be a single object at the end of the list  which may not be what you want     gt  gt  gt  another list    1  2  3   gt  gt  gt  my list append another list   gt  gt  gt  my list   foo    bar    baz    1  2  3                                  --- single item at the end of the list    extend  The list extend method extends a list by appending elements from an iterable   my list extend iterable    So with extend  each element of the iterable gets appended onto the list  For example    gt  gt  gt  my list   foo    bar    gt  gt  gt  another list    1  2  3   gt  gt  gt  my list extend another list   gt  gt  gt  my list   foo    bar   1  2  3    Keep in mind that a string is an iterable  so if you extend a list with a string  you ll append each character as you iterate over the string  which may not be what you want     gt  gt  gt  my list extend  baz    gt  gt  gt  my list   foo    bar   1  2  3   b    a    z     Operator Overload    add       and   iadd         Both   and    operators are defined for list  They are semantically similar to extend   my list   another list creates a third list in memory  so you can return the result of it  but it requires that the second iterable be a list    my list    another list modifies the list in-place  it is the in-place operator  and lists are mutable objects  as we ve seen  so it does not create a new list  It also works like extend  in that the second iterable can be any kind of iterable   Don t get confused - my list   my list   another list is not equivalent to    - it gives you a brand new list assigned to my list   Time Complexity  Append has constant time complexity  O 1     Extend has time complexity  O k     Iterating through the multiple calls to append adds to the complexity  making it equivalent to that of extend  and since extend s iteration is implemented in C  it will always be faster if you intend to append successive items from an iterable onto a list   Performance  You may wonder what is more performant  since append can be used to achieve the same outcome as extend  The following functions do the same thing   def append alist  iterable       for item in iterable          alist append item   def extend alist  iterable       alist extend iterable    So let s time them   import timeit   gt  gt  gt  min timeit repeat lambda  append      abcdefghijklmnopqrstuvwxyz     2 867846965789795  gt  gt  gt  min timeit repeat lambda  extend      abcdefghijklmnopqrstuvwxyz     0 8060121536254883   Addressing a comment on timings  A commenter said      Perfect answer  I just miss the timing of comparing adding only one element   Do the semantically correct thing  If you want to append all elements in an iterable  use extend  If you re just adding one element  use append   Ok  so let s create an experiment to see how this works out in time   def append one a list  element       a list append element   def extend one a list  element          creating a new list is semantically the most direct     way to create an iterable to give to extend        a list extend  element    import timeit   And we see that going out of our way to create an iterable just to use extend is a  minor  waste of time    gt  gt  gt  min timeit repeat lambda  append one     0    0 2082819009956438  gt  gt  gt  min timeit repeat lambda  extend one     0    0 2397019260097295   We learn from this that there s nothing gained from using extend when we have only one element to append   Also  these timings are not that important  I am just showing them to make the point that  in Python  doing the semantically correct thing is doing things the Right Way      It s conceivable that you might test timings on two comparable operations and get an ambiguous or inverse result  Just focus on doing the semantically correct thing   Conclusion  We see that extend is semantically clearer  and that it can run much faster than append  when you intend to append each element in an iterable to a list    If you only have a single element  not in an iterable  to add to the list  use append

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append  Appends object at the end   x    1  2  3  x append  4  5   print  x    gives you   1  2  3   4  5      extend  Extends list by appending elements from the iterable   x    1  2  3  x extend  4  5   print  x    gives you   1  2  3  4  5

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Append vs Extend    With append you can append a single element that will extend the list    gt  gt  gt  a    1 2   gt  gt  gt  a append 3   gt  gt  gt  a  1 2 3    If you want to extend more than one element you should use extend  because you can only append one elment or one list of element    gt  gt  gt  a append  4 5    gt  gt  gt  a  gt  gt  gt   1 2 3  4 5     So that you get a nested list  Instead with extend  you can extend a single element like this   gt  gt  gt  a    1 2   gt  gt  gt  a extend  3    gt  gt  gt  a  1 2 3    Or  differently  from append  extend more elements in one time without nesting the list into the original one  that s the reason of the name extend    gt  gt  gt  a extend  4 5 6    gt  gt  gt  a  1 2 3 4 5 6     Adding one element with both methods    Both append and extend can add one element to the end of the list  though append is simpler    append 1 element    gt  gt  gt  x    1 2   gt  gt  gt  x append 3   gt  gt  gt  x  1 2 3     extend one element    gt  gt  gt  x    1 2   gt  gt  gt  x extend  3    gt  gt  gt  x  1 2 3     Adding more elements    with different results   If you use append for more than one element  you have to pass a list of elements as arguments and you will obtain a NESTED list    gt  gt  gt  x    1 2   gt  gt  gt  x append  3 4    gt  gt  gt  x  1 2  3 4     With extend  instead  you pass a list as an argument  but you will obtain a list with the new element that is not nested in the old one    gt  gt  gt  z    1 2    gt  gt  gt  z extend  3 4    gt  gt  gt  z  1 2 3 4    So  with more elements  you will use extend to get a list with more items  However  appending a list will not add more elements to the list  but one element that is a nested list as you can clearly see in the output of the code

User · Answer

Append and extend are one of the extensibility mechanisms in python    Append  Adds an element to the end of the list    my list    1 2 3 4    To add a new element to the list  we can use append method in the following way   my list append 5    The default location that the new element will be added is always in the  length 1  position    Insert  The insert method was used to overcome the limitations of append  With insert  we can explicitly define the exact position we want our new element to be inserted at    Method descriptor of insert index  object   It takes two arguments  first being the index we want to insert our element and second the element itself    Example  my list    1 2 3 4  my list 4   a   my list  1 2 3 4  a     Extend  This is very useful when we want to join two or more lists into a single list  Without extend  if we want to join two lists  the resulting object will contain a list of lists    a    1 2  b    3  a append b  print  a   1 2  3     If we try to access the element at pos 2  we get a list   3    instead of the element  To join two lists  we ll have to use append    a    1 2  b    3  a extend b  print  a   1 2 3    To join multiple lists  a    1  b    2  c    3  a extend b c  print  a   1 2 3

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append appends a single element  extend appends a list of elements   Note that if you pass a list to append  it still adds one element    gt  gt  gt  a    1  2  3   gt  gt  gt  a append  4  5  6    gt  gt  gt  a  1  2  3   4  5  6

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What is the difference between the list methods append and extend     append adds its argument as a single element to the end of a list  The length of the list itself will increase by one  extend iterates over its argument adding each element to the list  extending the list  The length of the list will increase by however many elements were in the iterable argument    append  The list append method appends an object to the end of the list   my list append object     Whatever the object is  whether a number  a string  another list  or something else  it gets added onto the end of my list as a single entry on the list     gt  gt  gt  my list   foo    bar    gt  gt  gt  my list append  baz    gt  gt  gt  my list   foo    bar    baz     So keep in mind that a list is an object  If you append another list onto a list  the first list will be a single object at the end of the list  which may not be what you want     gt  gt  gt  another list    1  2  3   gt  gt  gt  my list append another list   gt  gt  gt  my list   foo    bar    baz    1  2  3                                  --- single item at the end of the list    extend  The list extend method extends a list by appending elements from an iterable   my list extend iterable    So with extend  each element of the iterable gets appended onto the list  For example    gt  gt  gt  my list   foo    bar    gt  gt  gt  another list    1  2  3   gt  gt  gt  my list extend another list   gt  gt  gt  my list   foo    bar   1  2  3    Keep in mind that a string is an iterable  so if you extend a list with a string  you ll append each character as you iterate over the string  which may not be what you want     gt  gt  gt  my list extend  baz    gt  gt  gt  my list   foo    bar   1  2  3   b    a    z     Operator Overload    add       and   iadd         Both   and    operators are defined for list  They are semantically similar to extend   my list   another list creates a third list in memory  so you can return the result of it  but it requires that the second iterable be a list    my list    another list modifies the list in-place  it is the in-place operator  and lists are mutable objects  as we ve seen  so it does not create a new list  It also works like extend  in that the second iterable can be any kind of iterable   Don t get confused - my list   my list   another list is not equivalent to    - it gives you a brand new list assigned to my list   Time Complexity  Append has constant time complexity  O 1     Extend has time complexity  O k     Iterating through the multiple calls to append adds to the complexity  making it equivalent to that of extend  and since extend s iteration is implemented in C  it will always be faster if you intend to append successive items from an iterable onto a list   Performance  You may wonder what is more performant  since append can be used to achieve the same outcome as extend  The following functions do the same thing   def append alist  iterable       for item in iterable          alist append item   def extend alist  iterable       alist extend iterable    So let s time them   import timeit   gt  gt  gt  min timeit repeat lambda  append      abcdefghijklmnopqrstuvwxyz     2 867846965789795  gt  gt  gt  min timeit repeat lambda  extend      abcdefghijklmnopqrstuvwxyz     0 8060121536254883   Addressing a comment on timings  A commenter said      Perfect answer  I just miss the timing of comparing adding only one element   Do the semantically correct thing  If you want to append all elements in an iterable  use extend  If you re just adding one element  use append   Ok  so let s create an experiment to see how this works out in time   def append one a list  element       a list append element   def extend one a list  element          creating a new list is semantically the most direct     way to create an iterable to give to extend        a list extend  element    import timeit   And we see that going out of our way to create an iterable just to use extend is a  minor  waste of time    gt  gt  gt  min timeit repeat lambda  append one     0    0 2082819009956438  gt  gt  gt  min timeit repeat lambda  extend one     0    0 2397019260097295   We learn from this that there s nothing gained from using extend when we have only one element to append   Also  these timings are not that important  I am just showing them to make the point that  in Python  doing the semantically correct thing is doing things the Right Way      It s conceivable that you might test timings on two comparable operations and get an ambiguous or inverse result  Just focus on doing the semantically correct thing   Conclusion  We see that extend is semantically clearer  and that it can run much faster than append  when you intend to append each element in an iterable to a list    If you only have a single element  not in an iterable  to add to the list  use append

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append  Appends object at the end   x    1  2  3  x append  4  5   print  x    gives you   1  2  3   4  5      extend  Extends list by appending elements from the iterable   x    1  2  3  x extend  4  5   print  x    gives you   1  2  3  4  5

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An interesting point that has been hinted  but not explained  is that extend is faster than append  For any loop that has append inside should be considered to be replaced by list extend processed elements    Bear in mind that apprending new elements might result in the realloaction of the whole list to a better location in memory  If this is done several times because we are appending 1 element at a time  overall performance suffers  In this sense  list extend is analogous to    join stringlist

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To distinguish them intuitively  l1     a    b    c   l2     d    e    f   l1 append l2  l1   a    b    c     d    e    f      It s like l1 reproduce a body inside her body nested      Reset l1     a    b    c   l1 extend l2  l1   a    b    c    d    e    f     It s like that two separated individuals get married and construct an united family   Besides I make an exhaustive cheatsheet of all list s methods for your reference   list methods     Add     extend    append    insert                     Remove     pop    remove    clear                    Sort     reverse    sort                     Search     count    index                     Copy     copy

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You can use     for returning extend  instead of extending in place   l1 range 10   l1  11    0  1  2  3  4  5  6  7  8  9  11   l2 range 10 1 -1   l1 l2   0  1  2  3  4  5  6  7  8  9  10  9  8  7  6  5  4  3  2    Similarly    for in place behavior  but with slight differences from append  amp  extend  One of the biggest differences of    from append and extend is when it is used in function scopes  see this blog post

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append object  - Updates the list by adding an object to the list   x    20    List passed to the append object  method is treated as a single object  x append  21  22  23     Hence the resultant list length will be 2 print x  -- gt   20   21  22  23     extend list  - Essentially concatenates two lists   x    20    The parameter passed to extend list  method is treated as a list    Eventually it is two lists being concatenated  x extend  21  22  23     Here the resultant list s length is 4 print x   20  21  22  23

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append appends a single element  extend appends a list of elements   Note that if you pass a list to append  it still adds one element    gt  gt  gt  a    1  2  3   gt  gt  gt  a append  4  5  6    gt  gt  gt  a  1  2  3   4  5  6

User · Answer

The following two snippets are semantically equivalent   for item in iterator      a list append item    and  a list extend iterator    The latter may be faster as the loop is implemented in C

User · Answer

Append adds the entire data at once  The whole data will be added to the newly created index  On the other hand  extend  as it name suggests  extends the current array    For example  list1    123  456  678  list2    111  222    With append we get   result    123  456  678   111  222     While on extend we get   result    123  456  678  111  222

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extend L  extends the list by appending all the items in the given list L    gt  gt  gt  a  1  2  3  a extend  4     is eqivalent of a len a       4   gt  gt  gt  a  1  2  3  4  a    1  2  3   gt  gt  gt  a  1  2  3   gt  gt  gt  a len a       4   gt  gt  gt  a  1  2  3  4

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extend L  extends the list by appending all the items in the given list L    gt  gt  gt  a  1  2  3  a extend  4     is eqivalent of a len a       4   gt  gt  gt  a  1  2  3  4  a    1  2  3   gt  gt  gt  a  1  2  3   gt  gt  gt  a len a       4   gt  gt  gt  a  1  2  3  4

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append adds an element to a list  and extend concatenates the first list with another list  or another iterable  not necessarily a list     gt  gt  gt  li     a    b    mpilgrim    z    example    gt  gt  gt  li   a    b    mpilgrim    z    example     gt  gt  gt  li append  new    gt  gt  gt  li   a    b    mpilgrim    z    example    new     gt  gt  gt  li append   new   2    gt  gt  gt  li   a    b    mpilgrim    z    example    new     new   2     gt  gt  gt  li insert 2   new    gt  gt  gt  li   a    b    new    mpilgrim    z    example    new     new   2     gt  gt  gt  li extend   two    elements     gt  gt  gt  li   a    b    new    mpilgrim    z    example    new     new   2    two    elements

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append    It is basically used in Python to add one element      Example 1     gt  gt  a    1  2  3  4   gt  gt  a append 5   gt  gt  print a   gt  gt  a    1  2  3  4  5       Example 2     gt  gt  a    1  2  3  4   gt  gt  a append  5  6    gt  gt  print a   gt  gt  a    1  2  3  4   5  6     extend    Where extend    is used to merge two lists or insert multiple elements in one list      Example 1     gt  gt  a    1  2  3  4   gt  gt  b    5  6  7  8   gt  gt  a extend b   gt  gt  print a   gt  gt  a    1  2  3  4  5  6  7  8       Example 2     gt  gt  a    1  2  3  4   gt  gt  a extend  5  6    gt  gt  print a   gt  gt  a    1  2  3  4  5  6

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append adds an element to a list  and extend concatenates the first list with another list  or another iterable  not necessarily a list     gt  gt  gt  li     a    b    mpilgrim    z    example    gt  gt  gt  li   a    b    mpilgrim    z    example     gt  gt  gt  li append  new    gt  gt  gt  li   a    b    mpilgrim    z    example    new     gt  gt  gt  li append   new   2    gt  gt  gt  li   a    b    mpilgrim    z    example    new     new   2     gt  gt  gt  li insert 2   new    gt  gt  gt  li   a    b    new    mpilgrim    z    example    new     new   2     gt  gt  gt  li extend   two    elements     gt  gt  gt  li   a    b    new    mpilgrim    z    example    new     new   2    two    elements

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The append   method adds a single item to the end of the list   x    1  2  3  x append  4  5   x append  abc   print x    gives you  1  2  3   4  5    abc     The extend   method takes one argument  a list  and appends each of the items of the argument to the original list   Lists are implemented as classes     Creating    a list is really instantiating a class  As such  a list has methods that operate on it    x    1  2  3  x extend  4  5   x extend  abc   print x    gives you  1  2  3  4  5   a    b    c     From Dive Into Python

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extend   can be used with an iterator argument  Here is an example  You wish to make a list out of a list of lists this way   From  list2d     1 2 3   4 5 6    7    8 9     you want   gt  gt  gt   1  2  3  4  5  6  7  8  9    You may use itertools chain from iterable   to do so  This method s output is an iterator  Its implementation is equivalent to  def from iterable iterables         chain from iterable   ABC    DEF    -- gt  A B C D E F     for it in iterables          for element in it              yield element   Back to our example  we can do  import itertools list2d     1 2 3   4 5 6    7    8 9   merged   list itertools chain from iterable list2d     and get the wanted list   Here is how equivalently extend   can be used with an iterator argument   merged      merged extend itertools chain from iterable list2d   print merged   gt  gt  gt   1  2  3  4  5  6  7  8  9

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append appends a single element  extend appends a list of elements   Note that if you pass a list to append  it still adds one element    gt  gt  gt  a    1  2  3   gt  gt  gt  a append  4  5  6    gt  gt  gt  a  1  2  3   4  5  6

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append adds an element to a list  and extend concatenates the first list with another list  or another iterable  not necessarily a list     gt  gt  gt  li     a    b    mpilgrim    z    example    gt  gt  gt  li   a    b    mpilgrim    z    example     gt  gt  gt  li append  new    gt  gt  gt  li   a    b    mpilgrim    z    example    new     gt  gt  gt  li append   new   2    gt  gt  gt  li   a    b    mpilgrim    z    example    new     new   2     gt  gt  gt  li insert 2   new    gt  gt  gt  li   a    b    new    mpilgrim    z    example    new     new   2     gt  gt  gt  li extend   two    elements     gt  gt  gt  li   a    b    new    mpilgrim    z    example    new     new   2    two    elements

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To distinguish them intuitively  l1     a    b    c   l2     d    e    f   l1 append l2  l1   a    b    c     d    e    f      It s like l1 reproduce a body inside her body nested      Reset l1     a    b    c   l1 extend l2  l1   a    b    c    d    e    f     It s like that two separated individuals get married and construct an united family   Besides I make an exhaustive cheatsheet of all list s methods for your reference   list methods     Add     extend    append    insert                     Remove     pop    remove    clear                    Sort     reverse    sort                     Search     count    index                     Copy     copy

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An English dictionary defines the words append and extend as   append  add  something  to the end of a written document   extend  make larger  Enlarge or expand    With that knowledge  now let s understand  1  The difference between append and extend  append    Appends any Python object as-is to the end of the list  i e  as a the last element in the list   The resulting list may be nested and contain heterogeneous elements  i e  list  string  tuple  dictionary  set  etc     extend    Accepts any iterable as its argument and makes the list larger  The resulting list is always one-dimensional list  i e  no nesting  and it may contain heterogeneous elements in it  e g  characters  integers  float  as a result of applying list iterable     2  Similarity between append and extend   Both take exactly one argument  Both modify the list in-place  As a result  both returns None      Example  lis    1  2  3      extend  is equivalent to this lis   lis   list iterable      append  simply appends its argument as the last element to the list   as long as the argument is a valid Python object list append object

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append adds an element to a list  and extend concatenates the first list with another list  or another iterable  not necessarily a list     gt  gt  gt  li     a    b    mpilgrim    z    example    gt  gt  gt  li   a    b    mpilgrim    z    example     gt  gt  gt  li append  new    gt  gt  gt  li   a    b    mpilgrim    z    example    new     gt  gt  gt  li append   new   2    gt  gt  gt  li   a    b    mpilgrim    z    example    new     new   2     gt  gt  gt  li insert 2   new    gt  gt  gt  li   a    b    new    mpilgrim    z    example    new     new   2     gt  gt  gt  li extend   two    elements     gt  gt  gt  li   a    b    new    mpilgrim    z    example    new     new   2    two    elements

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append  extends  the list  in place  by only one item  the single object passed  as argument    extend  extends  the list  in place  by as many items as the object passed  as argument  contains   This may be slightly confusing for str objects    If you pass a string as argument  append will add a single string item at the end but extend will add as many  single   str  items as the length of that string  If you pass a list of strings as argument  append will still add a single  list  item at the end and extend will add as many  list  items as the length of the passed list     def append o a list  element       a list append element      print  append    end            for item in a list          print item  end            print    def extend o a list  element       a list extend element      print  extend    end            for item in a list          print item  end            print   append o   ab    cd    extend o   ab    cd   append o   ab     cd    ef    extend o   ab     cd    ef    append o   ab     cd    extend o   ab     cd       produces   append  ab cd  extend  ab c d  append  ab   cd    ef    extend  ab cd ef  append  ab   cd    extend  ab cd

[python] What is the difference between Python's list methods append and extend?

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