Head and tail in one line

Question

Is there a pythonic way to unpack a list in the first element and the  tail  in a single command   For example    gt  gt  head  tail     some magic applied to    1  1  2  3  5  8  13  21  34  55   gt  gt  head 1  gt  gt  gt  tail  1  2  3  5  8  13  21  34  55

User · Answer

Building on the Python 2 solution from  GarethLatty  the following is a way to get a single line equivalent without intermediate variables in Python 2     t iter  1  1  2  3  5  8  13  21  34  55   h t     h list t   for h in t  0    If you need it to be exception-proof  i e  supporting empty list   then add   t iter     h t      h list t   for h in t    None       0    If you want to do it without the semicolon  use   h t      h list t   for t in  iter  1 2 3 4    for h in t    None       0

User · Answer

Python 2  using lambda   gt  gt  gt  head  tail    lambda lst   lst 0   lst 1      1  1  2  3  5  8  13  21  34  55    gt  gt  gt  head 1  gt  gt  gt  tail  1  2  3  5  8  13  21  34  55

User · Answer

Under Python 3 x  you can do this nicely    gt  gt  gt  head   tail    1  1  2  3  5  8  13  21  34  55   gt  gt  gt  head 1  gt  gt  gt  tail  1  2  3  5  8  13  21  34  55    A new feature in 3 x is to use the   operator in unpacking  to mean any extra values  It is described in PEP 3132 - Extended Iterable Unpacking  This also has the advantage of working on any iterable  not just sequences   It s also really readable   As described in the PEP  if you want to do the equivalent under 2 x  without potentially making a temporary list   you have to do this   it   iter iterable  head  tail   next it   list it    As noted in the comments  this also provides an opportunity to get a default value for head rather than throwing an exception  If you want this behaviour  next   takes an optional second argument with a default value  so next it  None  would give you None if there was no head element   Naturally  if you are working on a list  the easiest way without the 3 x syntax is   head  tail   seq 0   seq 1

User · Answer

For O 1  complexity of head tail operation you should use deque however    Following way   from collections import deque l   deque  1 2 3 4 5 6 7 8 9   head  tail   l popleft    l   It s useful when you must iterate through all elements of the list  For example in naive merging 2 partitions in merge sort

User · Answer

gt  gt  gt  mylist    1  1  2  3  5  8  13  21  34  55   gt  gt  gt  head  tail   mylist 0   mylist 1    gt  gt  gt  head 1  gt  gt  gt  tail  1  2  3  5  8  13  21  34  55

[python] Head and tail in one line

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