why list reversing returns None?in python

Question

I use reverse, but it returns me None, can you help?

ls = [1,2,3,4,5,6,7,8,9]
print('List = ',ls)
print('Reversed List = ',ls.reverse())

this is result:

('List = ', [1, 2, 3, 4, 5, 6, 7, 8, 9])
('Reversed List = ', None)

Answer 1

Because reverse does not return a list, it mutates the list in place. A method with no return always returns None. Try:

ls = [1,2,3,4,5,6,7,8,9]
print('List = ',ls)
ls.reverse()
print('Reversed List = ',ls)

Alternatively you could use a step value with slice notation. This also has the effect of not mutating the input list:

ls = [1,2,3,4,5,6,7,8,9]
print('List = ',ls)
print('Reversed List = ',ls[::-1])

Answer 2

reverse method does the operations "in-place", so you need to do:

ls = [1,2,3,4,5,6,7,8,9]
print('List = ',ls)
ls.reverse()
print('Reversed List = ', ls)

Or use reversed that isn't "in-place":

ls = [1,2,3,4,5,6,7,8,9]
print('List = ',ls)
print('Reversed List = ', list(reversed(ls)))

Or a cool slicing method:

ls = [1,2,3,4,5,6,7,8,9]
print('List = ',ls)
print('Reversed List = ', ls[::-1])