pandera.api.dataframe.model.DataFrameModel has unexpected generic subclass behaviour

[adzcai]

Describe the bug

Typically in Python, when making a generic subclass of a generic type, you can provide TypeVars as the type arguments to the base type. (See mypy docs for details.) However, creating a generic subclass of pandera.api.dataframe.model.DataFrameModel doesn't work as expected, and trying to instantiate the generic subclass with concrete type arguments raises a ValueError.

• I have checked that this issue has not already been reported.
• I have confirmed this bug exists on the latest version of pandera.
• (optional) I have confirmed this bug exists on the main branch of pandera.

Code Sample, a copy-pastable example

I've provided the expected Python behaviour alongside the current pandera behaviour.

from pandera.api.dataframe.model import DataFrameModel, TDataFrame, TSchema
from typing import Generic, TypeVar

class Foo(Generic[TDataFrame, TSchema]): ...

class Bar(Foo[TDataFrame, TSchema]): ...

class Schema(DataFrameModel[TDataFrame, TSchema]): ...

print(Bar.__parameters__)  # (~TDataFrame, ~TSchema)
print(Schema.__parameters__)  # ()

import pandas as pd
from pandera.api.pandas.container import DataFrameSchema

x: Bar[pd.DataFrame, DataFrameSchema]  # no error
y: Schema[pd.DataFrame, DataFrameSchema]  # ValueError: Expected 0 generic arguments but found 2

Expected behavior

I'd expect the generic subclass (Schema above) to behave like a generic type.

Desktop (please complete the following information):

• OS: macOS
• Browser: Safari
• Version: 0.20.3

Additional context

For now, this can be resolved by explicitly adding Generic to the subclass's bases:

class Schema(DataFrameModel[TDataFrame, TSchema], Generic[TDataFrame, TSchema]): ...

But with typical Python generics, this shouldn't be necessary. My guess is that overriding __class_getitem__ source and returning a new type is different from what Python does. We should probably be returning a typing._GenericAlias:

type(Bar[pd.DataFrame, DataFrameSchema])  # typing._GenericAlias