Python extend with an empty list bug?

python list extend
Doug T. picture Doug T. · Feb 14, 2009 · Viewed 23.9k times · Source

Why does python 2.5.2 have the following behavior

>>>[2].extend([]) == [2]
False

>>> [2].extend([]) == None
True

$ python --version
Python 2.5.2

I assume I'm not understanding something here, but intuitively I'd think that [2].extend([]) should yield [2]

Answer

Rafał Dowgird picture Rafał Dowgird · Feb 14, 2009

Extend is a method of list, which modifies it but doesn't return self (returning None instead). If you need the modified value as the expression value, use +, as in [2]+[].

Related questions

What is the difference between Python's list methods append and extend?

What's the difference between the list methods append() and extend()?

Read More
What is the syntax to insert one list into another list in python?

Given two lists: x = [1,2,3] y = [4,5,6] What is the syntax to: Insert x into y such that y now looks like [1, 2, 3, [4, 5, 6]]? Insert all the items of x into y such that y now looks like [1, 2, 3, 4, 5, 6]?

Read More
Python: understanding difference between append and extend

The code below will not run in its current state. However, if I change sum_vec.extend( vec1[i] + vec2[i] ) to sum_vec.append( vec1[i] + vec2[i] ) it works just fine. I understand the basic different between append …

Read More