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.