How does python list.sort() function actually work?

Question

Look at the following code:

arr = [5, 4, 3, 2, 1]
arr1 = arr
arr1.sort()
print(arr, arr1)

The expected output is:

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

As, arr1 is getting sorted and not arr.

Although, the output is:

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

How come both lists are getting sorted?

arr1 = arr doesn't copy the list. It just makes a new reference to the same object in memory, so changes to one reflect on the other. Use arr1 = arr[:] to copy the list using slice notation. Then, when you call arr1.sort(), it will only mutate arr1 in-place, leaving arr unchanged. — ggorlen
– ggorlen, Commented Oct 22, 2019 at 16:59

Pratik · Accepted Answer · 2019-10-22 17:00:04Z

1

sort() is in-place function in python and lists are pass by reference. So, if one array is getting sorted in-place it will change another, too.

If you want to prevent that, you can use:

arr = arr1[:]

or

import copy
arr = copy.deepcopy(arr1)

answered Oct 22, 2019 at 17:00

Pratik

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... or arr = arr1.copy(), which is the One Obvious Way to do a list copy ...