Unexpected Behavior of Extend with a list in Python [duplicate]

Question

I am trying to understand how extend works in Python and it is not quite doing what I would expect. For instance:

>>> a = [1, 2, 3]
>>> b = [4, 5, 6].extend(a)
>>> b
>>> 

But I would have expected:

[4, 5, 6, 1, 2, 3]

Why is that returning a None instead of extending the list?

Answer 1

The extend() method appends to the existing array and returns None. In your case, you are creating an array — [4, 5, 6] — on the fly, extending it and then discarding it. The variable b ends up with the return value of None.

Answer 2

list methods operate in-place for the most part, and return None.

>>> a = [1, 2, 3]
>>> b = [4, 5, 6]
>>> b.extend(a)
>>> b
[4, 5, 6, 1, 2, 3]

Answer 3

Others have pointed out many list methods, particularly those that mutate the list, return None rather than a reference to the list. The reason they do this is so that you don't get confused about whether a copy of the list is made. If you could write a = b.extend([4, 5, 6]) then is a a reference to the same list as b? Was b modified by the statement? By returning None instead of the mutated list, such a statement is made useless, you figure out quickly that a doesn't have in it what you thought it did, and you learn to just write b.extend(...) instead. Thus the lack of clarity is removed.