Sequence of types

[DouweM]

The following works in pyright, but not in pyrefly, ty, or (which is why I'm here!) mypy:

from __future__ import annotations

from dataclasses import dataclass

from typing_extensions import (
    Generic,
    Sequence,
    TypeVar,
    assert_type,
)

T = TypeVar("T")


@dataclass
class Agent(Generic[T]):
    output_type: Sequence[type[T]]


class Foo:
    pass


class Bar:
    pass


# pyright - works
# mypy - error: Expression is of type "Agent[object]", not "Agent[Foo | Bar]"  [assert-type]
# pyrefly - assert_type(Agent[Foo], Agent[Bar | Foo]) failed + Argument `list[type[Bar] | type[Foo]]` is not assignable to parameter `output_type` with type `Sequence[type[Foo]]` in function `Agent.__init__`
# ty - `Agent[Foo | Bar]` and `Agent[Unknown]` are not equivalent types
assert_type(Agent([Foo, Bar]), Agent[Foo | Bar])

# pyright - works
# mypy - error: Expression is of type "Agent[Never]", not "Agent[int | str]"  [assert-type]
# pyrefly - assert_type(Agent[int], Agent[str | int]) failed + Argument `list[type[str] | type[int]]` is not assignable to parameter `output_type` with type `Sequence[type[int]]` in function `Agent.__init__`
# ty - `Agent[int | str]` and `Agent[Unknown]` are not equivalent types
assert_type(Agent([int, str]), Agent[int | str])

# works
assert_type(Agent[Foo | Bar]([Foo, Bar]), Agent[Foo | Bar])

# works
assert_type(Agent[int | str]([int, str]), Agent[int | str])

It would be great to see this work in mypy, but if there's a good reason the other 2 out of 3 typecheckers also don't support this I'd love to understand why!

• This is related to a new PydanticAI feature, if you're curious check out Support functions as output_type, as well as lists of functions and other types pydantic/pydantic-ai#1785 (comment)
• Issues for other type checkers:
  • Sequence of types facebook/pyrefly#345
  • Handle type[T] for type variables astral-sh/ty#501

[Viicos]

• I think this is pretty much already tracked by Use union instead of join for unifying types #12056.