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Python Python Sequences Sequence Operations Sequence Operations Cheat Sheet

Posted by Shady Habashy
Shady Habashy
2,139 Points

Hi.. I tried the .reverse() method but it always returns ==> None why is this happening ?

Hi.. I tried the .reverse() method to reverse my list but it always returns ==> None why is this happening ?

Josh Keenan
Josh Keenan
20,315 Points

Can you post some code please so I can try and understand what's going on.

Shady Habashy
Shady Habashy
2,139 Points

my_list = [1, 2, 3, 4, 5]

rev_list = my_list.reverse()

print(rev_list)

2 Answers

Josh Keenan
Josh Keenan
20,315 Points

So list.reverse() uses in place modification, what this means is that the method does not return a new object and instead modifies the already existing one.

So if you want to create a new list object that is the reversed version of your original list, use reversed() and pass in your list, this will create a new object.

my_list = [1, 2, 3, 4, 5]
my_list.reverse()
print("list now: " + str(my_list))

This is what you must to if you want to use list.reverse()

Shady Habashy
Shady Habashy
2,139 Points

ok got it . Thank you :)

jessicamurr
jessicamurr
3,313 Points

Hi there! Thanks for answering the same question I had. However, I'm a bit confused. Why do we have to use str() for my_list. I know that not using it makes the return None or have an error pop up, but I'm confused as to why we have to make a list a string. Thanks ahead of time!

Jay zhang
Jay zhang
4,109 Points

hello, just to answer the jessicamurr's question, if you test the code you can quickly understand that :

You can only concatenate same value to each other, in this case since "list now: " is a string, you can't concatenate a list to a string

Muhammad khan
.a{fill-rule:evenodd;}techdegree
Muhammad khan
Python Development Techdegree Student 1,836 Points

can someone please explain why am I not getting a reversed list printed? getting some strange out: <list_reverseiterator object at 0x7feb3cf019b0> 

my_list = [1, 2, 3, 4, 5]
reversed_list = reversed(my_list)
print(reversed_list)
Alfonso Quijano
Alfonso Quijano
15,137 Points

try "reversed ()" like this: "obj = [6, 7, 8, 9, 10] inverted = reversed (obj) print (inverted) " and I got this result that I understand was stored in that place but it doesn't show me the inverted list. "<list_reverseiterator object at 0x00000268A9B44E20>"

Josh Keenan
Josh Keenan
20,315 Points
Josh Keenan
Josh Keenan
20,315 Points

Muhammad khan you need to do 

my_list.reverse()
Leonard Peris
Leonard Peris
18,204 Points
Leonard Peris
Leonard Peris
18,204 Points

Muhammad khan if you want to obtain a list that is the reverse of your list, without modifying your existing list, see the following code example.

my_list = [1, 2, 3, 4, 5]
reverse_list = list(reversed(my_list))
print(reverse_list)

reversed() returns a <list_reverseiterator object>. In order to obtain a list, we have to pass the return value to list().

Alternatively, you can use slices to obtain the reverse of a list without modifying the original.

my_list = [1, 2, 3, 4, 5]
reverse_list = my_list[::-1]
print(reverse_list)