Consider a simple pair of generic classes:
T = TypeVar("T", str, int)
class Base(Generic[T]):
def __init__(self, v: T):
self.v: T = v
@property
def value(self) -> T:
return self.v
class Child(Base[T]):
def __init__(self, v: T):
super().__init__(v)
x = Child(123)
reveal_type(x.value)
While using T = TypeVar("T")
works as expected. The restricted TypeVar
as shown yields the following errors:
error: Argument 1 to "__init__" of "Base" has incompatible type "str"; expected "T"
error: Argument 1 to "__init__" of "Base" has incompatible type "int"; expected "T"
note: Revealed type is 'builtins.int*'
Note that reveal_type
still works.
Another difference, is that the restricted TypeVar
requires the type annotation for self.v
assignment whereas the unrestricted does not.
In the full use-case, I actually have Callable[[Any], T]
, but the issue is the same.
This is with mypy 0.910
and Python 3.9.7
.
Bounding T
to an Union[int,str]
should do the job:
T = TypeVar("T", bound=Union[str, int])
class Base(Generic[T]):
def __init__(self, v: T):
self.v: T = v
@property
def value(self) -> T:
return self.v
class Child(Base[T]):
def __init__(self, v: T):
super().__init__(v)
x = Child(123)
reveal_type(x.value)
y = Child('a')
reveal_type(y.value)