1.1.402 detects NewTypes as FunctionTypes

[hynek]

Hi,

I've got some confusing new Pyright errors on my svcs project as of 1.1.402.

These lines

S1 = NewType("S1", str)
S2 = NewType("S2", str)

reg.register_value(S1, "foo")
reg.register_value(S2, "bar")

s1: str = con.get(S1)
s2: str = con.get(S2)

with these definitions:

class Registry:
    def register_value(
        self,
        svc_type: type,
        value: object,
        *,
        enter: bool = False,
        ping: Callable | None = None,
        on_registry_close: Callable | Awaitable | None = None,
    ) -> None:
        ...

and

from typing import Any, TypeVar, overload

T1 = TypeVar("T1")

class Container:
    ...
    @overload
    def get(self, svc_type: type[T1], /) -> T1: ...

Suddenly started throwing:

/Users/hynek/FOSS/svcs/tests/typing/core.py
  /Users/hynek/FOSS/svcs/tests/typing/core.py:143:20 - error: Argument of type "type[S1]" cannot be assigned to parameter "svc_type" of type "type" in function "register_value"
    "FunctionType" is not assignable to "type" (reportArgumentType)
  /Users/hynek/FOSS/svcs/tests/typing/core.py:144:20 - error: Argument of type "type[S2]" cannot be assigned to parameter "svc_type" of type "type" in function "register_value"
    "FunctionType" is not assignable to "type" (reportArgumentType)
  /Users/hynek/FOSS/svcs/tests/typing/core.py:146:19 - error: Argument of type "type[S1]" cannot be assigned to parameter "svc_type" of type "type[T1@get]" in function "get"
    Type "FunctionType" is not assignable to type "type[T1@get]" (reportArgumentType)
  /Users/hynek/FOSS/svcs/tests/typing/core.py:147:19 - error: Argument of type "type[S2]" cannot be assigned to parameter "svc_type" of type "type[T1@get]" in function "get"
    Type "FunctionType" is not assignable to type "type[T1@get]" (reportArgumentType)
4 errors, 0 warnings, 0 informations

I've seen FunctionType mentioned in the changelog, but it's unclear to me how any of that is supposed to be one (or why I shouldn't be able to assign a function to a type, but that's a separate issue).

Here's a minimal repro pyright playground:

from typing import NewType, TypeVar


NT = NewType("NT", str)
TV = TypeVar("TV")


def f(t: type[TV], v: TV) -> TV:
    return v


f(NT, NT("foo"))

that also fails with:

Argument of type "type[NT]" cannot be assigned to parameter "t" of type "type[TV@f]" in function "f"
  Type "FunctionType" is not assignable to type "type[NT]"  (reportArgumentType)

[erictraut]

This was an intentional change. See this issue for details.

A value created via a call to NewType is not a type object. While it acts like a subclass of type in many ways — and is modeled as a type internally to pyright and other type checkers, it's actually a function object at runtime. So if you attempt to use it as a type object, you could see failures.

I don't think pyright's previous behavior was causing problems for anyone, and the new behavior is unlikely to catch any bugs in real code. Unfortunately, someone pushed for the addition of a test for this case in the type checker conformance test suite, and I felt compelled to make pyright conform with the test. I tried to push back on the change, but the typing spec contains language that documents the fact NewType returns a function. Sorry for the inconvenience.

Since TV is a legal type form, TypeForm[TV] would work fine here. However, that relies on functionality in PEP 747 which is still in draft form. Pyright has experimental support for TypeForm if you want to try it out.

[hynek]

Hmmm, thanks for having my back on this one; this is really unfortunate since this just flat-out broke the really useful feature of defining specific types for specific use-cases as it is also mentioned in its stdlib docs. :(

Could this be somehow hidden behind a specific setting or error code until PEP 747 is usable in, say, Mypy and Pyright? Because silencing reportArgumentType is a really broad move and I can't break Mypy by using a feature that they haven't implemented yet too – wouldn't you say?