Python 类型提示：函数返回类型，作为参数给出，但类型是通用别名，如 Union

Question

We can specify easily return type of function, even if type we expect is provided as one of it's argument:

from typing import TypeVar, Type, Union

T = TypeVar('T')


def to(t: Type[T], v) -> T:
    return t(v)


s: str = to(str, 1)
i: int = to(str, 1)  # Ok: Expected type 'int', got 'str' instead

But it does not work if we provide Union[str, int] as first argument (do not see at function itself, we can do something more complex with this Union, ie construct pydantic model based on Union provided)

G = Union[str, int]
g: G = to(G, 1) # Expected type 'Type[T]', got 'object' instead 

So how to specify that return type of function should be the one provided as first argument? Even if we provide not pure type like int or str, but also Union?

UPDATE

To be more precise there is the function i'd like to decorate

from typing import Type, Union

from pydantic import validate_arguments, BaseModel


def py_model(t, v: dict):
    def fabric():
        def f(x):
            return x

        f.__annotations__['x'] = t
        return validate_arguments(f)

    f = fabric()
    return f(v)

class Model1(BaseModel): foo: int
class Model2(BaseModel): bar: str

print(repr(py_model(Union[Model1, Model2], {'foo':1}))) # Model1(foo=1)
print(repr(py_model(Union[Model1, Model2], {'bar':1}))) # Model2(bar='1')

So when I call py_model(Union[Model1, Model2], ...) I expect it will return one of Model1 or Model2 so Union[Model1, Model2]

Answer 1

How about using a "Callable" for the to function, instead of the operator to construct the type?

You could then have a constructor for Union type as argument.

An example of such implementation would be:

from typing import Any, Callable, TypeVar, Type, Union

T = TypeVar('T')
V = TypeVar('V')


def to(t: Callable[[V], T], v: V) -> T:
    return t(v)

G = Union[str, int]

def build_g(v:G) -> G:
    return v if isinstance(v, int) else str(v)


s: str = to(str, 1)
i1: int = to(int, 1)
i2: int = to(str, 1)  # Ok: Expected type 'int', got 'str' instead

g1: G = to(build_g, 1) # No error raised
g2: G = to(build_g, "2") # No error raised

The code above only raises an error in mypy for i2