Python slice first and last element in list

Question

Is there a way to slice only the first and last item in a list   For example  If this is my list    gt  gt  gt  some list   1    B    3    D    5    F     I want to do this  obviously  0 -1  is not valid syntax     gt  gt  gt  first item  last item   some list 0 -1   gt  gt  gt  print first item  1   gt  gt  gt  print last item  F    Some things I have tried   In  3   some list   -1  Out 3     F    5    D    3    B    1    In  4   some list -1 1 -1  Out 4     F    5    D    3    In  5   some list 0 -1 -1  Out 5

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Python 3 only answer  that doesn t use slicing or throw away the rest of the list  but might be good enough anyway  is use unpacking generalizations to get first and last separate from the middle   first      last   some list   The choice of   as the catchall for the  rest  of the arguments is arbitrary  they ll be stored in the name   which is often used as a stand-in for  stuff I don t care about    Unlike many other solutions  this one will ensure there are at least two elements in the sequence  if there is only one  so first and last would be identical   it will raise an exception  ValueError

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One way   some list   len some list -1    A better way  Doesn t use slicing  but is easier to read     some list 0   some list -1

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These are all interesting but what if you have a version number and you don t know the size of any one segment in string from and you want to drop the last segment   Something like 20 0 1 300 and I want to end up with 20 0 1 without the 300 on the end   I have this so far   str  20 0 1 300  split       3     which returns in list form as     20    0    1     How do I get it back to into a single string separated by periods  20 0 1

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This isn t a  slice   but it is a general solution that doesn t use explicit indexing  and works for the scenario where the sequence in question is anonymous  so you can create and  slice  on the same line  without creating twice and indexing twice   operator itemgetter  import operator    Done once and reused first and last   operator itemgetter 0  -1        first  last   first and last some list    You could just inline it as  after from operator import itemgetter for brevity at time of use    first  last   itemgetter 0  -1  some list    but if you ll be reusing the getter a lot  you can save the work of recreating it  and give it a useful  self-documenting name  by creating it once ahead of time   Thus  for your specific use case  you can replace   x  y   a split  -   0   a split  -   -1    with   x  y   itemgetter 0  -1  a split  -      and split only once without storing the complete list in a persistent name for len checking or double-indexing or the like   Note that itemgetter for multiple items returns a tuple  not a list  so if you re not just unpacking it to specific names  and need a true list  you d have to wrap the call in the list constructor

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def recall x        num1   x -4       num2   x   -1      num3   num2 -4       num4    num3  num1      return num4   Now just make an variable outside the function and recall the function   like this   avg   recall  idreesjaneqand    print avg

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first  last   some list 0   some list -1

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I found this might do this   list  0 -1

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You can use something like   y   max 1  len y -1     if you really want to use slicing  The advantage of this is that it cannot give index errors and works with length 1 or 0 lists as well

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Fun new approach to  one-lining  the case of an anonymously split thing such that you don t split it twice  but do all the work in one line is using the walrus operator      to perform assignment as an expression  allowing both   first  last    split str    a split  -    0   split str -1    and   first  last    split str    a split  -      len split str -1    Mind you  in both cases it s essentially exactly equivalent to doing on one line   split str   a split  -     then following up with one of   first  last   split str 0   split str -1  first  last   split str   len split str -1    including the fact that split str persists beyond the line it was used and accessed on  It s just technically meeting the requirements of one-lining  while being fairly ugly  I d never recommend it over unpacking or itemgetter solutions  even if one-lining was mandatory  ruling out the non-walrus versions that explicitly index or slice a named variable and must refer to said named variable twice

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You can do it like this   some list 0  len some list -1

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More General Case  Return N points from each end of list  The answers work for the specific first and last  but some  like myself  may be looking for a solution that can be applied to a more general case in which you can return the top N points from either side of the list  say you have a sorted list and only want the 5 highest or lowest   i came up with the following solution   In  1  def GetWings inlist winglen       if len inlist  lt  winglen 2          outlist inlist     else          outlist list inlist  winglen           outlist extend list inlist -winglen         return outlist   and an example to return bottom and top 3 numbers from list 1-10   In  2  GetWings  1 2 3 4 5 6 7 8 9 10  3    Out 2    1  2  3  8  9  10

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Utilize the packing unpacking operator to pack the middle of the list into a single variable    gt  gt  gt  l     1    B    3    D    5    F    gt  gt  gt  first   middle  last   l  gt  gt  gt  first  1   gt  gt  gt  middle   B    3    D    5    gt  gt  gt  last  F   gt  gt  gt     Or  if you want to discard the middle    gt  gt  gt  l     1    B    3    D    5    F    gt  gt  gt  first      last   l  gt  gt  gt  first  1   gt  gt  gt  last  F   gt  gt  gt

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Some people are answering the wrong question  it seems   You said you want to do    gt  gt  gt  first item  last item   some list 0 -1   gt  gt  gt  print first item  1   gt  gt  gt  print last item  F    Ie   you want to extract the first and last elements each into separate variables   In this case  the answers by Matthew Adams  pemistahl  and katrielalex are valid   This is just a compound assignment   first item  last item   some list 0   some list -1    But later you state a complication   I am splitting it in the same line  and that would have to spend time splitting it twice    x  y   a split  -   0   a split  -   -1    So in order to avoid two split   calls  you must only operate on the list which results from splitting once   In this case  attempting to do too much in one line is a detriment to clarity and simplicity   Use a variable to hold the split result   lst   a split  -   first item  last item   lst 0   lst -1    Other responses answered the question of  how to get a new list  consisting of the first and last elements of a list    They were probably inspired by your title  which mentions slicing  which you actually don t want  according to a careful reading of your question   AFAIK are 3 ways to get a new list with the 0th and last elements of a list    gt  gt  gt  s    Python ver  3 4   gt  gt  gt  a   s split    gt  gt  gt  a   Python    ver     3 4     gt  gt  gt    a 0   a -1             mentioned above   Python    3 4     gt  gt  gt  a   len a -1             also mentioned above   Python    3 4     gt  gt  gt    a e  for e in  0 -1      list comprehension  nobody mentioned    Python    3 4      Or  if you insist on doing it in one line   gt  gt  gt    s split   e  for e in  0 -1      Python    3 4     The advantage of the list comprehension approach  is that the set of indices in the tuple can be arbitrary and programmatically generated

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Just thought I d show how to do this with numpy s fancy indexing    gt  gt  gt  import numpy  gt  gt  gt  some list     1    B    3    D    5    F    gt  gt  gt  numpy array some list   0 -1   array   1    F           dtype   S1     Note that it also supports arbitrary index locations  which the    len some list -1  method would not work for    gt  gt  gt  numpy array some list   0 2 -1   array   1    3    F           dtype   S1     As DSM points out  you can do something similar with itemgetter    gt  gt  gt  import operator  gt  gt  gt  operator itemgetter 0  2  -1  some list    1    3    F

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Another python3 solution uses tuple unpacking with the     character   first      last   range 1  10

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Actually  I just figured it out   In  20   some list   len some list  - 1  Out 20     1    F

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What about this    gt  gt  gt  first element  last element   some list 0   some list -1

[python] Python slice first and last element in list

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