MyPy not acknowledging __all__

[TimSimpsonR]

When MyPy encounters star (*) imports it considers every symbol defined in the given module as being imported even if an __all__ attribute is present on the module.

For example, say you have mypkg with two files, main.py:

from datetime import datetime
from mypkg.widgets import *

and widget.py:

from datetime import datetime

__all__ = [
    "Gadget"
]

class Gadget(object):
    pass

Running "mypy mypkg/app.py" issues the following error: "mypkg/app.py:2: error: Name 'datetime' already defined", though datetime should not be re-imported to the app module.

[gvanrossum]

Thanks for the report! For whoever wants to tackle this, apparently mypy ignore the value of __all__ when computing the effect of from ... import *. That feels like it shouldn't be too hard to fix.

[refi64]

Was trying to tackle this. Given a SymbolTableNode, is it possible to get the variable's initializer somehow? Can't seem to figure out anything...

[JukkaL]

The initializer is not accessible via SymbolTableNode. One way would be to hook into the semantic analysis of assignment statements (special case __all__ at module top level).

[gvanrossum]

You probably have to special-case assignment to `__all__`. ISTR that's how type aliases are done.

[refi64]

How should messes like this be handled:

from typing import List, Union

__all__ = None # type: Union[int, str, bytes, List[int], List[str]]

if True:
    __all__ = 1
elif True:
    __all__ = 'a'
elif True:
    __all__ = [1]
elif True:
    __all__ = ['a']

My guess is to make it an error if __all__ has any type other than Sequence[str], and, if __all__ is assigned to multiple times, then just try to merge them all or something. So this would be an error:

__all__ = 1 # ERROR: type of __all__ must be Sequence[str]

and this would attempt to make __all__ ['x', 'y']:

if True:
    __all__ = ['x']
else:
    __all__ = ['y']

Any attempts to mutate it via append and such would be ignored (at least for now). How does all that sound?

[gvanrossum]

Check the stdlib for patterns. IIRC asyncio has dynamic all.

But yes, it should be Sequence of stress (or Any).

--Guido (mobile)