Pylint: false positive: unsupported-membership-test

[larsch]

Steps to reproduce

# pylint: disable=missing-docstring,too-few-public-methods

class MyClass:
    _all = None

    @classmethod
    def all(cls):
        if not cls._all:
            cls._all = find_all()
        return cls._all

    @classmethod
    def exist(cls, number):
        return number in cls.all()

def find_all():
    return [1, 2, 3]


if __name__ == '__main__':
    assert MyClass.exist(2)

Current behavior

************* Module clsmemb
clsmemb.py:14:25: E1135: Value 'cls.all()' doesn't support membership test (unsupported-membership-test)

Expected behavior

--------------------------------------------------------------------
Your code has been rated at 10.00/10 (previous run: 10.00/10, +0.00)

pylint --version output

pylint 2.3.1
astroid 2.2.5
Python 3.7.3 (default, Apr 24 2019, 15:29:51) [MSC v.1915 64 bit (AMD64)]

[PCManticore]

This looks like an issue that could be solved with better control flow inference, so that pylint could know that all could be either None or some other iterable value.

[ncoghlan]

I'm not sure if it's the same issue or a separate one, but I'm also getting this false positive with subprocess.check_output on pylint 2.3.1 and astroid 2.2.5 (old Python version, though - 3.5.3 on Debian 9):

```$ cat check_pylint_e1135.py
import subprocess

bytes_data = subprocess.check_output(['echo', 'hello']);
text_data = subprocess.check_output(['echo', 'hello'], universal_newlines=True);

print(b'bytes' in bytes_data) # Prints False
print('text' in text_data) # Prints False
'text' in bytes_data # Runtime TypeError
b'bytes' in text_data # Runtime TypeError

$ pylint -E check_pylint_e1135.py
*** Module check_pylint_e1135
check_pylint_e1135.py:6:18: E1135: Value 'bytes_data' doesn't support membership test (unsupported-membership-test)
check_pylint_e1135.py:7:16: E1135: Value 'text_data' doesn't support membership test (unsupported-membership-test)
check_pylint_e1135.py:8:10: E1135: Value 'bytes_data' doesn't support membership test (unsupported-membership-test)
check_pylint_e1135.py:9:12: E1135: Value 'text_data' doesn't support membership test (unsupported-membership-test)
```

[PCManticore]

Thanks for that report @ncoghlan It's probably a separate issue which we can resolve easier with a brain transform https://github.com/PyCQA/astroid/tree/master/astroid/brain so it's probably better to move the report to a separate issue.

[tuukkamustonen]

Also bumped into this. Even simpler example to reproduce:

from typing import Optional, Tuple

prop = None  # type: Optional[Tuple]

exists = prop and 'a' in prop

Out:

violation.py:5:27: E1135: Value 'prop' doesn't support membership test (unsupported-membership-test)

[Jamie-]

Equally adapting the code from the example above also generates a false positive for unsubscriptable-object in both Python 2, and Python 3 on latest version.

Python 2 & Pylint 1.6.5:

from typing import Optional

prop = None  # type: Optional[dict]

if prop and 'a' in prop:
    thing = prop['a']

Python 3 & Pylint 2.5.3:

from typing import Optional

prop: Optional[dict] = None

if prop and 'a' in prop:
    thing = prop['a']