False negative assigned-from-none for value = dict.update({}, {})

[jamesbraza]

Current problem

This mistake recently bit a coworker of mine:

some_dict = {0: 1}.update({2: 3})
# This will fail, as `some_dict` is actually `None`
assert isinstance(some_dict, dict)

Obviously this type of error one should catch in unit testing, but we didn't have tests. mypy can catch this only if one adds type annotations.

I am wondering, can pylint catch this in the future, without type annotations?

Desired solution

pylint would:

1. Understand that some_dict is actually None
2. Know that its usage downstream as a dict will fail

In other words the request: pylint catching errors associated with None returns later being used as if not None.

Additional context

No response

[Pierre-Sassoulas]

Sounds great ! When switching between language you never know if a data structure is modified in place or if it's returned, so I think it's very useful to warn about it. Also I think it's a false negative of assignment-from-none.

[jamesbraza]

Oh wow, I forgot about assignment-from-none, so it sounds like this request already is handled by pylint 🥳.

Would you like me to rename this issue to just be about the false negative?