Abid Rizvi Abid Rizvi - 1 year ago 94
Python Question

Is removing an element from the front of a list cheap in Python?

I am writing a program, that does a lot of deletions at either the front or back of a list of data, never the middle.

I understand that deletion of the last element is cheap, but how about deletion of the first element? For example let's say list

A
's address is at
4000
, so element
0
is at
4000
and element
1
is at
4001
.

Would deleting element
0
then just make the compiler put list
A
's address at
4001
, or would it shift element
1
at
4001
to the location at
4000
, and shift all other elements down by
1
?

Answer Source

No, it isn't cheap. Removing an element from the front of the list (using list.pop(0) for example) is O(N) and should be avoided. Similarly, inserting elements at the beginning (using list.insert(0, <value>) is equally inefficient. Lists are built for fast random access and O(1) operations on their end.

Since you're doing this operation commonly, though, you should consider using a deque from collections as @ayhan suggested in a comment. The docs on deque also highlight how lists aren't suitable for these operations:

Though list objects support similar operations, they are optimized for fast fixed-length operations and incur O(n) memory movement costs for pop(0) and insert(0, v) operations which change both the size and position of the underlying data representation.

(Emphasis mine)

The deque data structure offers O(1) complexity for both sides (beginning and end) with appendleft/popleft and append/pop methods for the beginning and end respectively.

Of course, with small sizes this incurs some extra space requirements (due to the structure of the deque) which should generally be of no concern (and as @juanpa noted in a comment, doesn't always hold) as the sizes of the lists grow. Finally, as @ShadowRanger's insightful comment notes, with really small sequence sizes the problem of popping or inserting from the front is trivialized to the point that it becomes of really no concern.

So, in short, for lists with many items, use deque if you need fast appends/pops from both sides, else, if you're randomly accessing and appending to the end, use lists.

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