Unexpected List Behavior In Python

I wanted to reverse a list, and I managed to do so, but in the middle of the work I noticed something strange. The following program works as expected but uncommeting line list_rev

Solution 1:

The following:

list_reversed = list 

makes the two variables refer to the same list. When you change one, they both change.

To make a copy, use

list_reversed = list[:]

Better still, use the builtin function instead of writing your own:

list_reversed = reversed(list)

P.S. I'd recommend against using list as a variable name, since it shadows the builtin.

Solution 2:

When you do:

list_reversed = list

You don't create a copy of list, instead, you create a new name (variable) which references the same list you had before. You can see this by adding:

print(id(list_reversed), id(list))  # Notice, the same value!!

Solution 3:

list_reversed = list does not make a copy of list. It just makes list_reversed a new name pointing to the same list. You can see any number of other questions about this on this site, some list in the related questions to the right.

Solution 4:

list and reversed_list are the same list. Therefore, changing one also changes the other.

What you should do is this:

reversed_list = list[::-1]

This reverses and copies the list in one fell swoop.

Solution 5:

This is about general behaviour of lists in python. Doing:

list_reversed = list

doesn't copy the list, but a reference to it. You can run:

print(id(list_reversed))
print(id(list))

Both would output the same, meaning they are the same object. You can copy lists by:

a = [1,2]
b = a.copy()

or

a = [1,2]
b = a[:]