Implicit TypeVar expansion of a Generic via another generic method

I really struggled to come up with a title for this, so I apologize for the ambiguity. I also considered "Spooky TypeVar action at a distance".

I'm investigating strategies for static type checking of apache beam, which is a streaming pipeline API.

Basically, I have a generic Transform with an In and Out type, and another generic Collection that wants to apply its type to the input type of Transform in order to produce a new Collection with the Transform's output type. These operations are repeated in a chain to form a pipeline:

Collection[None] -> Transform[None, A] -> Collection[A] -> Transform[A, B] -> Collection[B]

The catch is that a Transform's output type can be related to its input type via a TypeVar, and a Collection contains the concrete type of that input TypeVar. So when we call Collection.apply(transform) we want to resolve that chain of TypeVar dependencies to produce an expanded/concrete output type:

from typing import *

T = TypeVar('T')
InT = TypeVar('InT')
OutT = TypeVar('OutT')


class Transform(Generic[InT, OutT]):
    "Takes an input Collection and produces an output Collection"

    def __init__(self, fn: Callable[[InT], OutT]):
        self.fn = fn

    def call(self, arg: InT) -> OutT:
        return self.fn(arg)


def get_op(f: Callable[[InT], OutT]) -> Transform[InT, OutT]:
    "Get a Transform from a callable"
    return Transform(f)


class Collection(Generic[T]):
    "Collection of elements"

    def apply(self, op: Transform[T, OutT]) -> 'Collection[OutT]':
        """
        Apply the this Collection to a Transform and produce a new 
        output Collection
        """


# -- test:

def make_string(x: None) -> str:
    return 'foo'


def make_ones(x: T) -> Tuple[T, int]:
    return (x, 1)


c1: Collection = Collection()

str_op = get_op(make_string)
reveal_type(str_op)  # revealed: Transform[None, builtins.str*]

c2 = p1.apply(str_op)
reveal_type(c2)  # revealed: Collection[builtins.str*]

tuple_op = get_op(make_ones)
reveal_type(tuple_op)  # revealed: Transform[T`-1, Tuple[T`-1, builtins.int]]

c3 = c2.apply(tuple_op)
# Desired type is Collection[Tuple[builtins.str, builtins.int]]
reveal_type(c3)  # revealed: Collection[Tuple[T`-1, builtins.int]]

The crux of the issue is here:

class Collection(Generic[T]):
    "Collection of elements"

    def apply(self, op: Transform[T, OutT]) -> 'Collection[OutT]':
        """
        Apply the this Collection to a Transform and produce a new 
        output Collection
        """

My naive desire was that my Collection[builtins.str*] would apply its str type to T-1 of Transform[T-1, Tuple[T-1, builtins.int]] to produce Collection[Tuple[builtins.str, builtins.int]].

Instead I get:

error: Argument 1 to "apply" of "Collection" has incompatible type "Transform[T, Tuple[T, int]]"; expected "Transform[str, Tuple[T, int]]"

Is there any future universe where this is possible in mypy, or is it just too ambiguous?

Is it possible to write a mypy plugin to produce the desired result for Collection.apply?

• edit: clarified relationship between Transform and Collection TypeVars

Bắt đầu bằng cách tái hiện ví dụ Transform/Collection tổng quát được cung cấp trong mypy, tập trung vào Collection.apply và các kết quả reveal_type của nó. Theo dõi cách checker xử lý mối quan hệ giữa các TypeVar và đánh giá liệu một điểm vào plugin có thể hỗ trợ điều này hay không; được xem là hoàn tất khi xác lập được một hướng triển khai có phạm vi rõ ràng hoặc ghi lại lý do vì sao suy luận được yêu cầu không được hỗ trợ.