assign list.sort() instead of sorted(list)

[orSolocate]

Current problem

when you use the sort() method on a list object, it modifies the list and returns None.
A common mistake is to assign the return value of this assignment (i.e. None) to a variable thinking you assigned the sorted list instead.

lst=[3,2,1]
A=lst.sort()

Results with:
None in A and [1,2,3] in lst.

Desired solution

The checker should raise the WARNING message:
“A sort method of list object is assigned to a variable and equals None, consider using the sorted() function instead”.

Additional context

When i run pylint with all extensions on the example above I don't get any message.
But PyCharm's linter has a message for it so I am not sure if it was already implemented.

If not I could write it.

[DanielNoord]

I like the idea although I wonder how opinionated this might be. Personally I never use .sort() to return None, but others might actually rely on that behaviour.

@Pierre-Sassoulas Should this be a plugin or a standard check?

[Pierre-Sassoulas]

I also like this. Don't we have 'assignment-from-none' already? Is it a false negative?

[DanielNoord]

Yes, I think we can consider this a false-positive.

[orSolocate]

In the list.sort() method documentation they write:

Sort the list in ascending order and return None.

So I assume it always returns None.. therefore I don't see a case when someone relies on this value.
If they did, they should assign None to the variable/expression they want to evaluate..

@Pierre-Sassoulas I could try to expand the 'assignment-from-none' checker...