How do I reverse a part (slice) of a list in Python?

Question

Why doesn\'t this work?

# to reverse a part of the string in place 
a = [1,2,3,4,5]
a[2:4] = reversed(a[2:4])  # This works!
a[2:4] = [0,0]             # Thi         


        

Answer 1

I believe it's worth further elaborating the reasons why

a[2:4].reverse()

does not seem to work.

In addition to the fact that a[2:4] creates a copy in the memory, it should be noted that

list.reverse()

reverses the list in place and its return type is None. If you do

print(a[2:4].reverse())

None is printed. Obviously, if you forcefully do,

a[2:4] = a[2:4].reverse()

Python would raise a TypeError. However, there does exist a copy of a[2:4] in the memory which has been reversed. It's just you have no reference to access it.

On the other hand, a[2:4] = reversed(a[2:4]) works because reversed() returns a reverse iterator.

See comments of the accepted answer for how a[2:4] being different when appearing on the left of assignment.