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provide more verbosity for mismatched type types #13878

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@altendky altendky commented Oct 12, 2022 •
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When assigning to a protocol hinted name or attribute invalid values are clearly described as
to how they do not satisfy the hinted protocol. This is presently not the case for Type[]
hints. This PR enables the verbosity and clarity for the Type[] hints as well.

Draft for:

  • Consideration of other implications
  • Consideration of handling more cases

Example source

from typing import Generic, Protocol, Type, TypeVar

T = TypeVar("T")


class AProtocol(Protocol):
    def m(self, x: int) -> str:
        ...


def check(o: AProtocol) -> None:
    ...


class C:
    def m(self, x: bytes) -> str:
        ...


check(C())
check(o=C())
o: AProtocol = C()


class ProtocolChecker(Generic[T]):
    """Instances of this class can be used as decorators that will result in type hint
    checks to verifying that other classes implement a given protocol.
    from typing import Protocol
    class MyProtocol(Protocol):
       def a_method(self) -> int:
           ...
    @ProtocolChecker[MyProtocol]()
    class AnotherClass:
       def a_method(self) -> int:
           return 53
    """

    def __call__(self, cls: Type[T]) -> Type[T]:
        return cls

check_aprotocol = ProtocolChecker[AProtocol]()

check_aprotocol(cls=C)

@check_aprotocol
class D:
    def m(self, x: bytes) -> str:
        ...

Output from mypy main branch showing the minimal messaging has incompatible type "Type[C]"; expected "Type[AProtocol]" for Type[AProtocol] errors.

$ venv/bin/mypy ../x.py
../x.py:16: error: Missing return statement  [empty-body]
../x.py:20: error: Argument 1 to "check" has incompatible type "C"; expected "AProtocol"  [arg-type]
../x.py:20: note: Following member(s) of "C" have conflicts:
../x.py:20: note:     Expected:
../x.py:20: note:         def m(self, x: int) -> str
../x.py:20: note:     Got:
../x.py:20: note:         def m(self, x: bytes) -> str
../x.py:21: error: Argument "o" to "check" has incompatible type "C"; expected "AProtocol"  [arg-type]
../x.py:21: note: Following member(s) of "C" have conflicts:
../x.py:21: note:     Expected:
../x.py:21: note:         def m(self, x: int) -> str
../x.py:21: note:     Got:
../x.py:21: note:         def m(self, x: bytes) -> str
../x.py:22: error: Incompatible types in assignment (expression has type "C", variable has type "AProtocol")  [assignment]
../x.py:22: note: Following member(s) of "C" have conflicts:
../x.py:22: note:     Expected:
../x.py:22: note:         def m(self, x: int) -> str
../x.py:22: note:     Got:
../x.py:22: note:         def m(self, x: bytes) -> str
../x.py:43: error: Argument "cls" to "__call__" of "ProtocolChecker" has incompatible type "Type[C]"; expected "Type[AProtocol]"  [arg-type]
../x.py:45: error: Argument 1 to "__call__" of "ProtocolChecker" has incompatible type "Type[D]"; expected "Type[AProtocol]"  [arg-type]
../x.py:47: error: Missing return statement  [empty-body]
Found 7 errors in 1 file (checked 1 source file)

Output from this branch showing the desired verbosity when checking the Type[AProtocol] hints.

$ venv/bin/mypy ../x.py
../x.py:16: error: Missing return statement  [empty-body]
../x.py:20: error: Argument 1 to "check" has incompatible type "C"; expected "AProtocol"  [arg-type]
../x.py:20: note: Following member(s) of "C" have conflicts:
../x.py:20: note:     Expected:
../x.py:20: note:         def m(self, x: int) -> str
../x.py:20: note:     Got:
../x.py:20: note:         def m(self, x: bytes) -> str
../x.py:21: error: Argument "o" to "check" has incompatible type "C"; expected "AProtocol"  [arg-type]
../x.py:21: note: Following member(s) of "C" have conflicts:
../x.py:21: note:     Expected:
../x.py:21: note:         def m(self, x: int) -> str
../x.py:21: note:     Got:
../x.py:21: note:         def m(self, x: bytes) -> str
../x.py:22: error: Incompatible types in assignment (expression has type "C", variable has type "AProtocol")  [assignment]
../x.py:22: note: Following member(s) of "C" have conflicts:
../x.py:22: note:     Expected:
../x.py:22: note:         def m(self, x: int) -> str
../x.py:22: note:     Got:
../x.py:22: note:         def m(self, x: bytes) -> str
../x.py:43: error: Argument "cls" to "__call__" of "ProtocolChecker" has incompatible type "Type[C]"; expected "Type[AProtocol]"  [arg-type]
../x.py:43: note: Following member(s) of "C" have conflicts:
../x.py:43: note:     Expected:
../x.py:43: note:         def m(self, x: int) -> str
../x.py:43: note:     Got:
../x.py:43: note:         def m(self, x: bytes) -> str
../x.py:45: error: Argument 1 to "__call__" of "ProtocolChecker" has incompatible type "Type[D]"; expected "Type[AProtocol]"  [arg-type]
../x.py:45: note: Following member(s) of "D" have conflicts:
../x.py:45: note:     Expected:
../x.py:45: note:         def m(self, x: int) -> str
../x.py:45: note:     Got:
../x.py:45: note:         def m(self, x: bytes) -> str
../x.py:47: error: Missing return statement  [empty-body]
Found 7 errors in 1 file (checked 1 source file)

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@altendky
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Welp, I managed to get to green CI over at altendky#157. I'll try to take another pass at some point to add test coverage for this of some form as well as to consider if there's a more general solution to this. I feel like the conditions are a bit specific to the case I want covered. I presume other devs experienced with the mypy code would know both of these points a lot better offhand and I would appreciate any pointers.

@altendky altendky mentioned this pull request Apr 6, 2023
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According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

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