How are these return statements different

Question

Take these two sample code statments:

result = []
return result.append(feed.entries[0])

&

result = []
result.append(feed.entries[0])
return result

The first gives me an error because the method that result is passed to complains of a NonType not being iterable. Why is this? To me both statements are equivalent

score 2 · Accepted Answer · edited Jun 18 '14 at 14:43

2

The append method of a list does not return anything

>>> a = []
>>> type(a.append(12))
<type 'NoneType'>

So when you're doing:

return result.append(feed.entries[0])

You're actually returning None in all cases, whereas when you do:

result.append(....)
return result

you're returning the list after it has been mutated (modified) hence giving a the expected result.

edited Jun 18 '14 at 14:43

Tim Pietzcker

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answered Jun 18 '14 at 14:40

Ketouem

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How does the second return None? – Padraic Cunningham Jun 18 '14 at 14:42
The second statement given by OP does not return None but the list itself after it has been mutated. – Ketouem Jun 18 '14 at 14:43
How does `you're actually returning None in all cases` make sense then? – Padraic Cunningham Jun 18 '14 at 14:44
What I've wanted to say (even if I badly did it) is that if a function uses `return result.append(feed.entries[0])` it will always return the `None` pointer. – Ketouem Jun 18 '14 at 14:46

How are these return statements different

1 Answers1