False positive for unsubscriptable-object

[JohnVillalovos]

Bug description

With this file:

#!/usr/bin/python3 -ttu

from typing import Optional

class Reproducer:
    def __init__(self, name):
        self.http_username: Optional[str] = None
        self.set_values(name)

    def _get_values_from_helper(self) -> None:
        value = getattr(self, "http_username")
        if not isinstance(value, str):
            return

        helper = value[2:].strip()
        print(helper)


    def set_values(self, value: str) -> None:
        self.http_username = value

Configuration

No response

Command used

pylint reproducer.py

Pylint output

************* Module reproducer
reproducer.py:15:17: E1136: Value 'value' is unsubscriptable (unsubscriptable-object)

------------------------------------------------------------------
Your code has been rated at 6.15/10 (previous run: 3.08/10, +3.08)

Expected behavior

Would expect it to realize value is a string.

Pylint version

pylint 2.14.0
astroid 2.11.5
Python 3.10.4 (main, Mar 25 2022, 00:00:00) [GCC 11.2.1 20220127 (Red Hat 11.2.1-9)]

OS / Environment

No response

Additional dependencies

No response

[jacobtylerwalls]

Thanks for the report. In one sense this is a duplicate of #1162, so I'll be closing it, but in another sense this is outside the scope of pylint:

pylint uses astroid as an inference engine to statically analyze your code to find what values variables actually take, not what they promise to take from type annotations. In this sample, only None is assigned to http_username. Of course, you have a setter, but despite the type annotation, it could take anything at runtime. If you want type-checking that performs sophisticated type-narrowing and incorporates your type annotations, mypy is your tool.

That does mean, unfortunately, you will run into issues where you have to disable no-member, unsubscriptable-object in pylint from time to time.

We are open to improvements where we quiet pylint warnings when we detect ambiguity, and there are some interesting proposals in astroid, see pylint-dev/astroid#1189, but I don't see astroid implementing full-fledged type-narrowing and type annotation analysis like mypy--which is what would be needed to find "ambiguity" here in code that only ever sets http_username to None. I hope that makes sense.

[jacobtylerwalls]

Ah ha! Wait a darn minute! I feel like this is something that could be discussed in pylint-dev/astroid#1189, which introduces Constraint. If something previously inferable becomes Uninferable only in virtue of a Constraint, we should be able to pass back another sentinel like UninferableOnlyBecauseOfConstraintSoShutUpAboutNoMember.

[jacobtylerwalls]

I'll go echo that comment on #1162 and pylint-dev/astroid#1189.

Despite my excitement I guess this is still a duplicate of either #1162 or #1498 or #4920.