在 Python 中使用 object 作为类型有什么问题？

Question

The following program works, but gives a MyPy error:

from typing import Type, TypeVar, Any, Optional


T = TypeVar('T')


def check(element: Any, types: Type[T] = object) -> Optional[T]:
    if not isinstance(element, types):
        return None
    return element


print(check(123, int))
print(check(123, object))

MyPy complains:

main.py:7: error: Incompatible default for argument "types" (default has type "Type[object]", argument has type "Type[T]")
Found 1 error in 1 file (checked 1 source file)

What am I doing wrong?

Replacing object with Type[object] mysteriously works.

Answer 1

You're using the type variable in the wrong place, it should be used with element not types .

from typing import Optional, Type, TypeVar

T = TypeVar('T')

def check(element: T, types: Type = object) -> Optional[T]:
    if not isinstance(element, types):
        return None
    return element

Answer 2

The problem was that the default value has to fit for every possible substitution for T . Since it doesn't the right way to solve this is to define overloads, one with the Type[T] producing Optional[T] and one with Literal[object] and producing Any . Then in the combined declaration, the default can be provided.

This was addressed by Guido here .