Reasons behind reportInvalidTypeVarUse

[ethframe]

I have the following code:

from typing import Generic, Optional, TypeVar

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


class A(Generic[T]):
    def __init__(self, a: int, b: Optional[T]):
        self.a = a
        self.b = b

    @classmethod
    def from_a(cls, other: "A[V]") -> "A[T]":
        return A(other.a, None)

It's intended use is as follows:

a = A(1, 'a')  # Type of "a" is "A[str]"
b = A[int].from_a(a)  # Type of "b" is "A[int]"

However, pyright complains about a type variable V in a classmethod:

TypeVar "V" appears only once in generic function signature

I don't understand why there is such a restriction on the use of type variables.

[jakebailey]

The V is only in one place, so it doesn't actually do anything. Your code could be written as:

from typing import Generic, Optional, TypeVar

T = TypeVar('T')

class A(Generic[T]):
    def __init__(self, a: int, b: Optional[T]):
        self.a = a
        self.b = b

    @classmethod
    def from_a(cls, other: "A[Any]") -> "A[T]":
        return A(other.a, None)

Because V was never used to actually constrain any other parts of from_a.

[erictraut]

@ethframe, many people misunderstand how to use TypeVars. We added these errors to help them avoid the most common misuses. Mypy provides the same checks and errors. If you don't like how the reportInvalidTypeVarUse diagnostic check works, you can disable it globally or on a per-file basis.

@JelleZijlstra, you would need to write A[Any] if the type param is invariant. A[object] is acceptable if it's covariant.

[ethframe]

@erictraut, I still don't understand why using a type variable once in a signature is considered as a misuse. Mypy succesfully typecheck such code, and even PEP 484 contains examples when type variable appears once in a signature.

[erictraut]

This is a debatable issue, and I can (and have) argued both sides. For additional context, see this thread in the typing sig: https://mail.python.org/archives/list/[email protected]/thread/NRFNHGXHXPGBR6FP3TIOZZ6VS4XJZX6K/#NRFNHGXHXPGBR6FP3TIOZZ6VS4XJZX6K. We laded on our current solution because of a steady stream of bug reports based on a misunderstanding of how TypeVars are supposed to be used.

[ethframe]

Well, I ran into this problem again, with signature like def foo(a: A[T, V], b: A[T, U]) -> A[T, V]. Guess I'd better turn this diagnostic off.

However, I found a workaround, that actually looks like a bug:

from typing import Generic, TypeVar

T = TypeVar('T')


class A(Generic[T]):
    ...


def foo(x: T) -> None: ...  # TypeVar "T" appears only once in generic function signature
def bar(x: A[T]) -> None: ...  # TypeVar "T" appears only once in generic function signature


B = TypeVar('B', bound=object)


def baz(x: B) -> None: ...  # TypeVar "B" appears only once in generic function signature
def qux(x: A[B]) -> None: ...

That is, if the type variable's bound is explicitly set to object, pyright doesn't emit error if the variable is used as a parameter of some generic type. @erictraut, is this behavior expected?

[erictraut]

Yes, this is intended behavior. If the TypeVar is explicitly bound or constrained, it is allowed to be used only once in a signature.