Describe generic classes as base classes

[JukkaL]

The PEP should probably mention what happens if a generic class is used as a base class.

Examples to consider:

class StrList(List[str]): ...  # StrList is not generic

T = TypeVar('T')

class MyList(List[T]): ...   # Mypy requires Generic[T] as a base class, but it's ugly

x = Undefined(MyList[int])

[gvanrossum]

Yes, the allowed forms for base classes and the idioms to use for various
cases are not yet sorted out. I also don't like that mypy has both
Generic[T] and AbstractGeneric[T].

On Wed, Jan 14, 2015 at 11:09 PM, Jukka Lehtosalo notifications@github.com
wrote:

The PEP should probably mention what happens if a generic class is used as
a base class.

Examples to consider:

class StrList(List[str]): ... # StrList is not generic

T = TypeVar('T')

class MyList(List[T]): ... # Mypy requires Generic[T] as a base class, but it's ugly

x = Undefined(MyList[int])

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[vlasovskikh]

See also https://github.com/JukkaL/typing/issues/36 about Generic and AbstractGeneric.

[JukkaL]

Currently this is not valid (due to Python semantics):

class A(metaclass=ABCMeta): pass
class B(Generic[T], A): pass   # metaclass conflict

This is okay, however:

class A(metaclass=ABCMeta): pass
class B(AbstractGeneric[T], A): pass   # good

Maybe we should just replace Generic with AbstractGeneric and remove AbstractGeneric? This way most reasonable inheritance patterns should work okay. With the current mypy approach, inheritance hierarchies are inherently fragile -- if some base class adds ABCMeta as metaclass (due to adding an abstract base class, for example), it can break subclasses that are generic.

[gvanrossum]

So, what shall we do? If at all possible I'd like to only have Generic.

[JukkaL]

I updated mypy and typing to only have Generic, which also supports defining ABCs. It seems to be okay to me, but I need to do more testing.

[JukkaL]

Added section 'Arbitrary generic types as base classes' to the PEP. Not closing this issue this yet until somebody has reviewed it.

[vlasovskikh]

Everything looks good to me, except for #85.