No overload variant of "get" … for os.environ with unicode_literals

[posita]

This works:

# test.py
import os
print('PWD: {}'.format(os.environ.get('PWD', '<not set>')))

$ python --version
Python 2.7.13
$ python test.py
PWD: …
$ python3 --version
Python 3.6.0
$ python3 test.py
PWD: …
$ mypy test.py
$ mypy --py2 test.py

This fails validation with --py2:

# test_unicode_literals.py
from __future__ import unicode_literals  # this is new
import os
print('PWD: {}'.format(os.environ.get('PWD', '<not set>')))

$ python test_unicode_literals.py
PWD: …
$ python3 test_unicode_literals.py
PWD: …
$ mypy test_unicode_literals.py
$ mypy --py2 test_unicode_literals.py
test_unicode_literals.py:3: error: No overload variant of "get" of "Mapping" matches argument types [builtins.unicode, builtins.unicode]

See stdlib/2/os/__init__.pyi#L67.

[posita]

This seems to address the issue for Python 2:

# test_unicode_literals_workaound.py
from __future__ import unicode_literals
import os
import typing
_EnvironWorkAround = typing.MutableMapping[typing.AnyStr, typing.AnyStr]
environ = typing.cast(_EnvironWorkAround, os.environ)
print('PWD: {}'.format(environ.get('PWD', '<not set>')))

$ mypy --py2 test_unicode_literals_workaound.py

[JelleZijlstra]

I'm not sure this is fixable in typeshed. python/mypy#3619 might provide a suitable workaround.

[JelleZijlstra]

Duplicate of #872.