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Fix TypedDict subtyping #11670

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Fix TypedDict subtyping #11670

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15 changes: 9 additions & 6 deletions mypy/subtypes.py
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Original file line number Diff line number Diff line change
Expand Up @@ -250,8 +250,6 @@ def visit_instance(self, left: Instance) -> bool:
return False
return True
right = self.right
if isinstance(right, TupleType) and mypy.typeops.tuple_fallback(right).type.is_enum:
return self._is_subtype(left, mypy.typeops.tuple_fallback(right))
if isinstance(right, Instance):
if TypeState.is_cached_subtype_check(self._subtype_kind, left, right):
return True
Expand Down Expand Up @@ -297,16 +295,21 @@ def visit_instance(self, left: Instance) -> bool:
return True
if isinstance(item, Instance):
return is_named_instance(item, 'builtins.object')
if isinstance(right, LiteralType) and left.last_known_value is not None:
return self._is_subtype(left.last_known_value, right)
if isinstance(right, CallableType):
# Special case: Instance can be a subtype of Callable.
call = find_member('__call__', left, left, is_operator=True)
if call:
return self._is_subtype(call, right)
return False
else:
return False
# Special cases
if isinstance(right, LiteralType) and left.last_known_value is not None:
return self._is_subtype(left.last_known_value, right)
right_fallback = mypy.typeops.try_getting_instance_fallback(right)
if (right_fallback is not None
and ((isinstance(right, TupleType) and right_fallback.type.is_enum)
or isinstance(right, TypedDictType))):
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IIRC the fallback of a TypedDict type is actually not a "real" type, but a sort of implementation artifact. I believe that the actual bug is that we leak the fallback as an instance type. In particular, it shouldn't be possible to bind a TypedDict type to Type[T], since a TypedDict type is not a real class, and doesn't support isinstance, among other things. I suspect that is where things go wrong, and this change would be just papering over the root cause. I'm not 100% sure what is going on here, though.

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the actual bug is that we leak the fallback as an instance type

Yes you're right. I am confused by Type[T] before and I just saw #9773 and other related discussions. If my understanding is correct, these issues should be fiexd by two steps: 1. throw errows when binding a TypedDict to Type[T]. 2. implement TypeForm[t] to support the original design pattern described in those issues.

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oops, I still need some extra investigation to catch up disscussions on Type[T] and TypeForm. It'd be great if you could provide some suggestions on implementation since it's a quite common issue.

return self._is_subtype(left, right_fallback)
return False

def visit_type_var(self, left: TypeVarType) -> bool:
right = self.right
Expand Down
20 changes: 20 additions & 0 deletions test-data/unit/check-typeddict.test
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Expand Up @@ -493,6 +493,26 @@ reveal_type(fun(b)) # N: Revealed type is "builtins.object*"
[builtins fixtures/dict.pyi]
[typing fixtures/typing-typeddict.pyi]

[case testTypedDictTempTestName]
from typing import Any, Type, TypeVar
from typing_extensions import TypedDict

T = TypeVar('T')
class C(TypedDict):
x: str

def gen() -> Any:
return {'x': 1}
def foo(cls: Type[T]) -> T:
return gen()
def fun(c: C):
pass

fun(foo(C))
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The call foo(C) should actually generate an error (see my other comment). For example, if the call would make sense, we should handle cases like these, which probably won't work correctly right now:

d = foo(C)
d['x'] = 1  # Should be ok
d['y'] = 1  # Should be an error
d['x'] = 'x'  # Should be an error

reveal_type(foo(C)) # N: Revealed type is "__main__.C*"
[builtins fixtures/dict.pyi]
[typing fixtures/typing-full.pyi]

-- Join

[case testJoinOfTypedDictHasOnlyCommonKeysAndNewFallback]
Expand Down