python
Type alias over generic parameter in generic class produces an error
Bug Report
Given a class context (eg. class class Foo[T]), mypy fails to infer the generic parameter when used as part of a type alias that is declared in the class context.
Using type aliases within a class can help avoid repetition when complex generics are used throughout the class.
To Reproduce
class X[Y]:
type Z = list[Y] # produces: error: All type parameters should be declared ("Y" not declared) [valid-type]
def __init__(self, val: Y) -> None:
self.val = val
def make_list(self) -> Z:
return [self.val]
reveal_type(X(123).make_list()) # Revealed type *expected* to be "builtins.list[builtins.int]"
# is instead "builtins.list[Any]"
https://mypy-play.net/?mypy=latest&python=3.12&gist=845d9b12620d3febc625406e2a093ca2
Expected Behavior
The generic parameter Y should be inferred from the context of the class where it was specified.
It should be noted that pyright is able to deduce Y and therefore Z. See https://pyright-play.net/?code=MYGwhgzhAEAaDaBNAugLgFDS9ALgTwAcBTaALWgF5oQBLCHJZTbZrAEyIDNoB9HmgHY0cfABQQiITgBpoANzAhU0RAEpoAWgB80AHIB7AUQzZT0CVIB0CkJXmL0raB24BbMAGsiPWvXGTOdW0yEzMsACciHABXcIFoeAtOa0UmdEi5IkUefGJRWFEARgAmAGZVS3cvHzocUVVVIA
Actual Behavior
mytypes.py:2: error: All type parameters should be declared ("Y" not declared) [valid-type]
mytypes.py:10: note: Revealed type is "builtins.list[Any]"
Found 1 error in 1 file (checked 1 source file)
Your Environment
- Mypy version used: 1.12
- Mypy command-line flags:
--strict - Mypy configuration options from
mypy.ini(and other config files): none - Python version used: 3.12
Hm, FWIW such use case is explicitly prohibited for old-style type alias syntax (because in old syntax the intent was ambiguous). With new-style syntax this may be OK. In the meantime you can use an easy workaround:
class Gen[T]:
type Alias[S] = Some[Complex[Type[Gen[S]]]]
var: Alias[T] # OK
cc @JukkaL
@ilevkivskyi Thanks, that was really helpful.
It should be noted that pyright is able to infer the types of Y and therefore Z. See https://pyright-play.net/?code=MYGwhgzhAEAaDaBNAugLgFDS9ALgTwAcBTaALWgF5oQBLCHJZTbZrAEyIDNoB9HmgHY0cfABQQiITgBpoANzAhU0RAEpoAWgB80AHIB7AUQzZT0CVIB0CkJXmL0raB24BbMAGsiPWvXGTOdW0yEzMsACciHABXcIFoeAtOa0UmdEi5IkUefGJRWFEARgAmAGZVS3cvHzocUVVVIA
Am I doing the workaround wrong? It does not work as I understand it.
If I want to do
class A[T]:
type elems = list[T]
which works in e.g. pyright. To make it work in mypy with the workaround it would be
class A[T]:
type helper[S] = list[S]
elems: helper[T]
mylist: A[float].elems = [1]
reveal_type(mylist)
Produces
main.py:5: error: Invalid type comment or annotation [valid-type]
main.py:7: note: Revealed type is "Any" <------ I expect list[float]
Found 1 error in 1 file (checked 1 source file)
Playground: https://mypy-play.net/?mypy=latest&python=3.12&gist=12d802b27914bef54ccb3f7721909d5b
