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

Question

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• Why does list.append evaluate to false in a boolean context? [duplicate] 7 answers

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

extend extends its operand, but doesn't return a value. If you had done:

b = [4, 5, 6]
b.extend(a)

Then you would get the expected result.

Answer 4

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.

Answer 5

I had this problem and while the other answers provide correct explanations, the solution/workaround I liked isn't here. Using the addition operator will concatenate lists together and return the result. In my case I was bookkeeping color as a 3-digit list and opacity as a float, but the library needed color as a 4 digit list with opacity as the 4th digit. I didn't want to name a throwaway variable, so this syntax suited my needs:

color = [1, 1, 0]
opacity = 0.75
plot.setColor(color + [opacity])

This creates a new list for opacity on the fly and a new list after the concatenation, but that's fine for my purposes. I just wanted compact syntax for extending a list with a float and returning the resulting list without affecting the original list or float.