What s the idiomatic syntax for prepending to a short python list

Question

list append   is the obvious choice for adding to the end of a list   Here s a reasonable explanation for the missing list prepend     Assuming my list is short and performance concerns are negligible  is   list insert 0  x    or   list 0 0     x    idiomatic

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Lets go over 4 methods  Using insert     gt  gt  gt    gt  gt  gt  l   list range 5    gt  gt  gt  l  0  1  2  3  4   gt  gt  gt  l insert 0  5   gt  gt  gt  l  5  0  1  2  3  4   gt  gt  gt     Using    and     gt  gt  gt    gt  gt  gt  l   list range 5    gt  gt  gt  l  0  1  2  3  4   gt  gt  gt  l    5    l  gt  gt  gt  l  5  0  1  2  3  4   gt  gt  gt     Using Slicing   gt  gt  gt    gt  gt  gt  l   list range 5    gt  gt  gt  l  0  1  2  3  4   gt  gt  gt  l  0     5   gt  gt  gt  l  5  0  1  2  3  4   gt  gt  gt     Using collections deque appendleft     gt  gt  gt    gt  gt  gt  from collections import deque  gt  gt  gt    gt  gt  gt  l   list range 5    gt  gt  gt  l  0  1  2  3  4   gt  gt  gt  l   deque l   gt  gt  gt  l appendleft 5   gt  gt  gt  l   list l   gt  gt  gt  l  5  0  1  2  3  4   gt  gt  gt

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In my opinion  the most elegant and idiomatic way of prepending an element or list to another list  in Python  is using the expansion operator    also called unpacking operator     Initial list l    4  5  6     Modification l    1  2  3   l   Where the resulting list after the modification is  1  2  3  4  5  6  I also like simply combining two lists with the operator    as shown    Prepends  1  2  3  to l l    1  2  3    l    Prepends element 42 to l l    42    l  I don t like the other common approach  l insert 0  value   as it requires a magic number  Moreover  insert   only allows prepending a single element  however the approach above has the same syntax for prepending a single element or multiple elements

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If you can go the functional way  the following is pretty clear  new list    x    your list   Of course you haven t inserted x into your list  rather you have created a new list with x preprended to it

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What s the idiomatic syntax for prepending to a short python list    You don t usually want to repetitively prepend to a list in Python   If it s short  and you re not doing it a lot    then ok   list insert  The list insert can be used this way   list insert 0  x    But this is inefficient  because in Python  a list is an array of pointers  and Python must now take every pointer in the list and move it down by one to insert the pointer to your object in the first slot  so this is really only efficient for rather short lists  as you ask   Here s a snippet from the CPython source where this is implemented - and as you can see  we start at the end of the array and move everything down by one for every insertion   for  i   n  --i  gt   where        items i 1    items i     If you want a container list that s efficient at prepending elements  you want a linked list  Python has a doubly linked list  which can insert at the beginning and end quickly - it s called a deque   deque appendleft  A collections deque has many of the methods of a list  list sort is an exception  making deque definitively not entirely Liskov substitutable for list     gt  gt  gt  set dir list   - set dir deque     sort     The deque also has an appendleft method  as well as popleft   The deque is a double-ended queue and a doubly-linked list - no matter the length  it always takes the same amount of time to preprend something  In big O notation  O 1  versus the O n  time for lists  Here s the usage    gt  gt  gt  import collections  gt  gt  gt  d   collections deque  1234    gt  gt  gt  d deque   1    2    3    4     gt  gt  gt  d appendleft  0    gt  gt  gt  d deque   0    1    2    3    4      deque extendleft  Also relevant is the deque s extendleft method  which iteratively prepends    gt  gt  gt  from collections import deque  gt  gt  gt  d2   deque  def    gt  gt  gt  d2 extendleft  cba    gt  gt  gt  d2 deque   a    b    c    d    e    f      Note that each element will be prepended one at a time  thus effectively reversing their order   Performance of list versus deque  First we setup with some iterative prepending   import timeit from collections import deque  def list insert 0        l          for i in range 20           l insert 0  i   def list slice insert        l          for i in range 20           l  0     i         semantically same as list insert 0  i   def list add        l          for i in range 20           l    i    l        caveat  new list each time  def deque appendleft        d   deque       for i in range 20           d appendleft i     semantically same as list insert 0  i   def deque extendleft        d   deque       d extendleft range 20     semantically same as deque appendleft above   and performance    gt  gt  gt  min timeit repeat list insert 0   2 8267281929729506  gt  gt  gt  min timeit repeat list slice insert   2 5210217320127413  gt  gt  gt  min timeit repeat list add   2 0641671380144544  gt  gt  gt  min timeit repeat deque appendleft   1 5863927800091915  gt  gt  gt  min timeit repeat deque extendleft   0 5352169770048931   The deque is much faster  As the lists get longer  I would expect a deque to perform even better  If you can use deque s extendleft you ll probably get the best performance that way

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The s insert 0  x  form is the most common   Whenever you see it though  it may be time to consider using a collections deque instead of a list

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If someone finds this question like me  here are my performance tests of proposed methods   Python 2 7 8  In  1    timeit   1  1000000  insert 0  0  100 loops  best of 3  4 62 ms per loop  In  2    timeit   1  1000000  0 0     0  100 loops  best of 3  4 55 ms per loop  In  3    timeit  0     1  1000000 100 loops  best of 3  8 04 ms per loop   As you can see  insert and slice assignment are as almost twice as fast than explicit adding and are very close in results  As Raymond Hettinger noted insert is more common option and I  personally prefer this way to prepend to list

[python] What's the idiomatic syntax for prepending to a short python list?

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