list.reverse() is not working

Question

I honestly just don't understand why this is returning None rather than a reversed list:

>>> l = range(10)
>>> print l
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> print l.reverse()
None

Why is this happening? According to the docs, I am doing nothing wrong.

Answer 1

reverse modifies the list in place and returns None. If you do

l.reverse()
print l

you will see your list has been modified.

Answer 2

L.reverse() modifies L in place. As a general rule, Python builtin methods will either mutate or return something but not both

The usual way to reverse a list is to use

print L[::-1]

reversed(L) returns a listreverseiterator object which is fine for iterating over, but not so good if you really want a list

[::-1] is just a normal slice - the step is -1 so you get a copy starting from the end and ending with the start

Answer 3

list.reverse() reverses the list in place. It doesn't return the reversed list. For that, use reversed() function:

print reversed(l)

Or just use the extended slice notation:

print l[::-1]

Answer 4

The docs say

list.reverse() : Reverse the elements of the list, in place.

in place means the original list gets changed, rather than returning a new list, so the return you ask it to print is None.