Recursive type aliases tracker

[srittau]

• mypy (Support recursive types mypy#731), supported using a flag
• pytype
• pyright
• pyre
• PyCharm

Test case:

from typing import TypeAlias

Recursive: TypeAlias = str | list["Recursive"]

def foo(r: Recursive) -> None:
    if not isinstance(r, str):
        foo(r[0])
    if not isinstance(r, list):
        r.casefold()

foo("")
foo(list(""))
foo(list(list(""), ""))

[rchen152]

Recursive types should work with pytype. We don't support the X | Y syntax for union types yet, so the test case has to be modified a bit, but pytype accept this:

from typing import TypeAlias, Union

Recursive: TypeAlias = Union[str, list["Recursive"]]

def foo(r: Recursive) -> None:
  if not isinstance(r, str):
    foo(r[0])
  if not isinstance(r, list):
    r.casefold()

foo("")
foo(list(""))
foo(list((list(""), "")))

(I also changed the list[...]s to list(..) in the last two lines; I assume that was a typo.)

[ilevkivskyi]

FWIW mypy version in master supports this under a flag (expect it fails on the last line because of an unrelated bug in special-casing tuple literals, should be a 1-line fix).

[ilevkivskyi]

In general, I guess we will discover a bunch of inconsistencies in type inference in mypy (currently it is a bit arbitrary in some situations, while it should be quite consistent for recursive types, especially if invariant collections are involved, e.g. this test passes if I use Sequence)

[al-yakubovich]

If I want to define nested list type should I use

NestedList = list[Union[str, "NestedList"]]
def f(l: NestedList) -> int:
   …

or

Recursive: TypeAlias = list[str | list["Recursive"]]
def f(l: Recursive) -> int:
   …

?

[AlexWaygood]

Seems like we're all done here! We have our first recursive types successfully added to typeshed: #9134

Thanks @ilevkivskyi for all the incredible work on mypy recently to enable this!

[jleonard-r7]

These are not working for me:

WorkerResult: TypeAlias = Result[str, str] | Result[object, str]
Worker: TypeAlias = Callable[[], WorkerResult | Worker]

gives:

Traceback (most recent call last):
  File "/app/src/app/main.py", line 17, in <module>
    Worker: TypeAlias = Callable[[], WorkerResult | Worker]
                                                    ^^^^^^
NameError: name 'Worker' is not defined

[jleonard-r7]

Python version 3.11.2 (which should include whatever work happened in Nov 2022 on mypy).

[JelleZijlstra]

This issue is about support in type checkers, not at runtime. To prevent the runtime NameError you'll (still) have to enclose the name in quotes.