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sharmaeklavya2 opened this issue Jun 18, 2016 · 3 comments · Fixed by #2446
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type of Union containing Any is incorrectly inferred after an isinstance check #1720

sharmaeklavya2 opened this issue Jun 18, 2016 · 3 comments · Fixed by #2446
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bug mypy got something wrong

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@sharmaeklavya2
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union_any.py:

from typing import Any, Union

def func(v):
    # type: (Union[int, Any]) -> str
    if isinstance(v, int):
        s = hex(v)
    else:
        s = str(v).lower()
    return "key:" + s

mypy union_any.py:

union_any.py: note: In function "func":
union_any.py:8: error: No overload variant of "str" matches argument types [Union[<ERROR>, void]]

mypy --py2 union_any.py:

union_any.py: note: In function "func":
union_any.py:8: error: Argument 1 to "str" has incompatible type "Union[object, None]"; expected "object"

I discovered this when I used a class which was defined in some library which did not have stubs and I was using --silent-imports.
@sharmaeklavya2 sharmaeklavya2 changed the title isinstance with Any fails type of Union containing Any is incorrectly inferred after an isinstance check Jun 18, 2016
@gvanrossum gvanrossum added the bug mypy got something wrong label Jun 23, 2016
@gvanrossum gvanrossum added this to the 0.4.x milestone Jun 23, 2016
@ddfisher
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In addition to being a Union[Any, ...] -> Any problem, I think this is at least vaguely related to #1691.

@sharmaeklavya2
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Another case where I found this error is when 2 classes inherit from Any:

file1.py:

from typing import Any, Union

class B(Any): name = 'Bob'
class C(Any): name = 'Charlie'

def func(x):
    # type: (Union[B, C]) -> None
    reveal_type(x)
    if isinstance(x, B):
        reveal_type(x)
        print(x.name)
    else:
        reveal_type(x)
        print(x.name)

gives this output:

file1.py: note: In function "func":
file1.py:8: error: Revealed type is 'Union[file1.B, file1.C]'
file1.py:10: error: Revealed type is 'file1.B'
file1.py:13: error: Revealed type is 'Union[<ERROR>, <ERROR>]'
file1.py:14: error: Some element of union has no attribute "name"

It works fine when I replace Any by object.

@gvanrossum
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On that second case: inheriting from Any is illegal (though it can happen legally when you inherit from a class whose import is suppressed by --silent-imports).

@JukkaL JukkaL mentioned this issue Oct 4, 2016
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@ilevkivskyi ilevkivskyi mentioned this issue Nov 13, 2016
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Keep Any when restricting Union in isinstance ilevkivskyi/mypy
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@ddfisher @gvanrossum @sharmaeklavya2