The usage of reverse() in Python

Question

I used two different ways to create a reverse list.
The first way:

>>> a=list(range(3))  
>>> a.reverse()  
>>> print(a) 
[2,1,0]   

The second way:

>>> a=list(range(3)).reverse()
>>> print(a)
None   

Why does the second way does not work?Thanks for any help.

Answer 1

It fails because reverse changes the list in place (i.e. it does not create a new list) and like most functions that operate in place it returns None.

In your first example

a=list(range(3))  
a.reverse()
print a 

It works fine as you capture a as the return value from list(range(3)), which returns a list. You then reverse that list in place and print it. All fine!

In your second example

a=list(range(3)).reverse()
print(a)

a is equal to the return value of reverse() (not list(range(3))). reverse is called on a temporary list and returns None, which is the value assigned to a.

Answer 2

Why does the second way does not work?

.reverse() doesn't return a list (it returns None). It modifies the argument you pass into it.

You need to do it the same as the first method:

a=list(range(3))
a.reverse()
print(a)

Answer 3

Because .reverse() mutates the list, but does not return the list.

Why they chose to implement it this way, I don't know. Perhaps to save memory.