Pyright fails to recognize valid calls when ParamSpec, Concatenate, and factories are used

[lexicalunit]

Describe the bug
Contrived code but in the real version this makes sense as we're working with pytest and using the factories-as-fixtures pattern.

See the inline comments on the three calls to bulk_maker() for the pyright errors:

from typing import Any, Callable, List

# For python 3.8:
from typing_extensions import Concatenate, ParamSpec, TypeAlias

# For python 3.10:
# from typing import Concatenate, ParamSpec, TypeAlias


class Company:
    id: int

    def __init__(self, id: int):
        self.id = id
        print(self)

    def __repr__(self) -> str:
        return f"<company {self.id}>"


Factory: TypeAlias = Callable[..., Company]


def make_company(*args: Any, **kwargs: Any) -> Company:
    return Company(*args, **kwargs)


P = ParamSpec("P")
BulkFactory: TypeAlias = Callable[Concatenate[int, P], List[Company]]


def make_n_companies() -> BulkFactory[P]:
    def factory(n: int, *args: P.args, **kwargs: P.kwargs) -> List[Company]:
        return [make_company(*args, **kwargs) for _ in range(n)]

    return factory


bulk_maker = make_n_companies()

print()
bulk_maker(5, 10)  # Expected 1 positional argument

print()
bulk_maker(5, id=10)  # No parameter named "id"

print()
bulk_maker(5)  # Arguments for ParamSpec "P@make_n_companies" are missing

The first two calls to bulk_maker() are fine. The last one is incorrect and will fail at runtime:

± python example.py

<company 10>
<company 10>
<company 10>
<company 10>
<company 10>

<company 10>
<company 10>
<company 10>
<company 10>
<company 10>

Traceback (most recent call last):
  File "foo.py", line 47, in <module>
    bulk_maker(5)  # Arguments for ParamSpec "P@make_n_companies" are missing
  File "foo.py", line 33, in factory
    return [make_company(*args, **kwargs) for _ in range(n)]
  File "foo.py", line 33, in <listcomp>
    return [make_company(*args, **kwargs) for _ in range(n)]
  File "foo.py", line 24, in make_company
    return Company(*args, **kwargs)
TypeError: __init__() missing 1 required positional argument: 'id'

Expected behavior
There should not be any pyright errors for the first two calls to bulk_maker().

VS Code extension or command-line

• Are you running pyright as a VS Code extension or a command-line tool? Both
• Which version? pyright 1.1.253 and **pylance v2022.6.10`
• Error is seen when using python 3.8 and 3.10. No difference.

[erictraut]

This is an incorrect usage of ParamSpec. The problem is P is scoped to the function make_n_companies, but P appears only in the return type annotation, so there's no way for P to be solved in the context of a call to make_n_companies.

If your intent is to support any callable whose first parameter is an int, then you should parameterize the ParamSpec with ..., as in:

def make_n_companies() -> BulkFactory[...]:
   ...

The ... is the equivalent of Any for a ParamSpec.

[lexicalunit]

def make_n_companies() -> BulkFactory[...]:
   ...

This doesn't actually work at all though. If I make this change I get:

± python3.10 example.py
Traceback (most recent call last):
  File "exmaple.py", line 32, in <module>
    def make_n_companies() -> BulkFactory[...]:
  File ".../Versions/3.10/lib/python3.10/typing.py", line 312, in inner
    return func(*args, **kwds)
  File ".../Versions/3.10/lib/python3.10/typing.py", line 1075, in __getitem__
    arg = arg[subargs]
  File ".../Versions/3.10/lib/python3.10/typing.py", line 312, in inner
    return func(*args, **kwds)
  File ".../Versions/3.10/lib/python3.10/typing.py", line 1081, in __getitem__
    return self.copy_with(tuple(new_args))
  File ".../Versions/3.10/lib/python3.10/typing.py", line 1294, in copy_with
    raise TypeError("The last parameter to Concatenate should be a "
TypeError: The last parameter to Concatenate should be a ParamSpec variable.

[erictraut]

Hmm, this looks like a bug in the runtime handling of Concatenate. You can work around it by enclosing the BulkFactory[...] in quotes or using from __future__ import annotations.

If I understand the problem you're trying to solve here, a ParamSpec isn't the right approach in the first place. A callback protocol would be much more appropriate.

class BulkFactory(Protocol):
    def __call__(
        self, count: int, /, id: int, *args: Any, **kwargs: Any
    ) -> List[Company]:
        ...

[erictraut]

The runtime exception appears to be a bug in the typing_extensions library. I've filed this issue to track this.

[lexicalunit]

@erictraut I tried using the Protocol approach and that also has one issue that I can not resolve.

Example Code

from __future__ import annotations

from typing import Any, Callable, Generic, List, Optional, Protocol, TypeVar

from typing_extensions import ParamSpec, TypeAlias


def i_default() -> int:
    return 4


def s_default() -> str:
    return "foo"


class Company:
    id: int

    def __init__(
        self,
        id: Optional[int] = None,
        # ... dozens more optional properties go here
    ):
        self.id = id or i_default()  # each property has its own default
        print(self)

    def __repr__(self) -> str:
        return f"<company {self.id}>"


class User:
    id: int

    def __init__(
        self,
        uuid: Optional[str] = None,
        # ... dozens more optional properties go here
    ):
        self.uuid = uuid or s_default()  # each property has its own default
        print(self)

    def __repr__(self) -> str:
        return f"<user {self.uuid}>"


def make_company(*args: Any, **kwargs: Any) -> Company:
    return Company(*args, **kwargs)


def make_user(*args: Any, **kwargs: Any) -> User:
    return User(*args, **kwargs)


T = TypeVar("T")
P = ParamSpec("P")


class BulkFactory(Protocol, Generic[T]):
    def __call__(
        self,
        n: int,
        /,
        *args: Any,
        **kwargs: Any,
    ) -> List[T]:
        ...


def make_n(maker: Callable[P, T]) -> BulkFactory[T]:
    def inner(n: int, /, *args: P.args, **kwargs: P.kwargs) -> List[T]:
        return [maker(*args, **kwargs) for _ in range(n)]

    return inner


# create our bulk factories for user and company:
make_n_users = make_n(make_user)
make_n_companies = make_n(make_company)

# create a bunch of users and companies:
make_n_users(5)  # this should work, giving us all default values
make_n_users(5, uuid="whatever")  # this should work, giving us some
                                  # default values while others are set

make_n_companies(5)
make_n_companies(5, id=5)

Pylance Issue

On the line return inner we get:

(function) inner: (n: int, /, *args: P.args, **kwargs: P.kwargs) -> List[T@make_n]
Expression of type "(n: int, /, *args: P.args, **kwargs: P.kwargs) -> List[T@make_n]" cannot be assigned to return type "BulkFactory[T@make_n]"
  Type "(n: int, /, *args: P.args, **kwargs: P.kwargs) -> List[T@make_n]" cannot be assigned to type "(n: int, /, *args: Any, **kwargs: Any) -> List[T@make_n]"
    Parameter "**kwargs" has no corresponding parameter

EDIT:
I got this to work by adding P to the generic:

T = TypeVar("T")
P = ParamSpec("P")


class BulkFactory(Protocol, Generic[T, P]):
    def __call__(
        self,
        n: int,
        /,
        *args: P.args,
        **kwargs: P.kwargs,
    ) -> List[T]:
        ...


def make_n(maker: Callable[P, T]) -> BulkFactory[T, P]:
    def inner(n: int, /, *args: P.args, **kwargs: P.kwargs) -> List[T]:
        return [maker(*args, **kwargs) for _ in range(n)]

    return inner

Thank you!

[erictraut]

Yeah, that looks like a bug. Could you please open a new issue in the pyright issue tracker so this doesn't get lost? Thanks!

[lexicalunit]

Oh it does? I figured I was just doing something wrong 😅 -- I'll open an issue now.