Tuple[AnyStr] always get converted into Tuple[unicode] in Py2

[gsnedders]

• Are you reporting a bug, or opening a feature request?
  A bug.

• Please insert below the code you are checking with mypy,
  or a mock-up repro if the source is private. We would appreciate
  if you try to simplify your case to a minimal repro.

from typing import AnyStr, Union, Text, Tuple


def anystr_func(s):
    # type: (AnyStr) -> AnyStr
    return s

def anystr_tuple_func(s):
    # type: (AnyStr) -> Tuple[AnyStr]
    return (s,)

class Foo(object):
    def __init__(self, s):
        # type: (AnyStr) -> None
        reveal_type(s)
        self.s = s  # type: Union[bytes, Text]
        reveal_type(self.s)
        self.t = anystr_func(s)  # type: Union[bytes, Text]
        reveal_type(self.t)
        self.u = anystr_tuple_func(s)  # type: Tuple[Union[bytes, Text]]
        reveal_type(self.u)

• What is the actual behavior/output?

foo.py:15: error: Revealed type is 'builtins.str*'
foo.py:15: error: Revealed type is 'builtins.unicode*'
foo.py:17: error: Revealed type is 'builtins.str'
foo.py:17: error: Revealed type is 'builtins.unicode'
foo.py:19: error: Revealed type is 'builtins.str'
foo.py:19: error: Revealed type is 'builtins.unicode'
foo.py:21: error: Revealed type is 'Tuple[builtins.unicode]'

• What is the behavior/output you expect?

foo.py:15: error: Revealed type is 'builtins.str*'
foo.py:15: error: Revealed type is 'builtins.unicode*'
foo.py:17: error: Revealed type is 'builtins.str'
foo.py:17: error: Revealed type is 'builtins.unicode'
foo.py:19: error: Revealed type is 'builtins.str'
foo.py:19: error: Revealed type is 'builtins.unicode'
foo.py:21: error: Revealed type is 'Tuple[builtins.str]'
foo.py:21: error: Revealed type is 'Tuple[builtins.unicode]'

• What are the versions of mypy and Python you are using?
  Do you see the same issue after installing mypy from Git master?

Python 3.7.3, mypy 0.701

Yes (as of b29a433)

• What are the mypy flags you are using? (For example --strict-optional)

--py2. Note I get the expected output with both str and bytes without --py2.

---

To be slightly more concrete, this came about trying to figure out what was going on with code calling os.path.split (which is (AnyStr) -> Tuple[AnyStr, AnyStr]), and not getting the results I was expecting.

[gsnedders]

Oh, so the problem here actually is passing a Union[bytes, Text] to the AnyStr:

def anystr_func(s):
    # type: (AnyStr) -> AnyStr
    return s

def foo(s):
    # type: (Union[bytes, Text]) -> None
    reveal_type(s)
    reveal_type(anystr_func(s))

gives:

foo2.py:35: error: Revealed type is 'Union[builtins.str, builtins.unicode]'
foo2.py:36: error: Revealed type is 'builtins.unicode*'

[ilevkivskyi]

For technical reasons, on Python 2 str is considered a subtype of unicode (I know this is bad) and therefore the union gets simplified. It is unlikely this will be ever fixed. On Python 3 however mypy says:

Value of type variable "AnyStr" of "anystr_func" cannot be "Union[bytes, str]"

Maybe we should allow union math for this case the same way as we do for overloads?

[JukkaL]

Closing due to Python 2 feature freeze (#12237).