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<nothing> is inferred when mimicking GADT #8252
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In the example below of the same bug, after the
|
I also ran into this recently. From some more playing around with it, it seems like there's an issue when:
My minimal examples to show this: from typing import Any, Generic, TypeVar
T = TypeVar('T')
class Foo(Generic[T]):
def __init__(self, value: T):
self.value = value
def int_through_superclass(self) -> int:
if isinstance(self, IntFoo):
# error: <nothing> has no attribute "value"
return self.value
return -1
class IntFoo(Foo[int]):
def int_through_subclass(self) -> int:
# no issue
return self.value
def int_through_subclass_with_isinstance_check(self) -> int:
if isinstance(self, IntFoo):
# no issue
return self.value
return -1
def t_func(foo: Foo[T]) -> T:
if isinstance(foo, IntFoo):
# error: <nothing> has no attribute "value"
return foo.value
return foo.value
def int_func(foo: Foo) -> int:
if isinstance(foo, IntFoo):
# no issue
return foo.value
return -1
def int_t_func(foo: Foo[T]) -> int:
if isinstance(foo, IntFoo):
# error: <nothing> has no attribute "value"
return foo.value
return -1 There may be other cases (like methods in subclasses), but I didn't check them. |
Still opened and still an issue, I'd love being able to mimick GADTs in python |
While this issue came before, I'm closing this in favour of #12949 which describes the same bug in a much simpler way. Thanks for reporting! |
As mypy understands
isinstance
checks, I wondered if I can use this to mimic GADT (Generalized algebraic data type) in functional languages such as Haskell and OCaml.I tried to mimic following Haskell code
with the following Python code:
But in the
elif isinstance(e, Add)
branch the type ofe
is inferred as'<nothing>'
causing type errors.As
'<nothing>'
is not documented (c.f. #3030), I'm not sure if this is intended behavior. So I report it here just in case, though the attempt is just out of curiosity.The text was updated successfully, but these errors were encountered: