Allow subclassing without supertyping

[dmoisset]

This issue comes from python/mypy#1237. I'll try to make a summary of the discussion here

Some one reported an issue with an artificial example (python/mypy#1237 (comment)):

class Foo:
    def factory(self) -> str:
        return 'Hello'

class Bar(Foo):
    def factory(self) -> int:
        return 10

and then with the following stub files (python/mypy#1237 (comment)):

class QPixmap(QPaintDevice):
    def swap(self, other: 'QPixmap') -> None: ...

class QBitmap(QPixmap):
    def swap(self, other: 'QBitmap') -> None: ...

Which mypy currently reports as erroneous: Argument 1 of "swap" incompatible with supertype "QPixmap"

These were initially was argued to be a correct error because they violate Liskov Substitution Principle (the same case by a change of return type, the second is a covariant argument change). The problem in this scenario is that the actual classes are not meant to be substituted (the first example is actually a wrapper for a non-virtual C++ function so they're not substitutable, and the second aren't supposed to be mixed together, just reuse part of the code). (see python/mypy#1237 (comment))

There was also a suggestion that allow covariant args explicitly (in the same way that Eiffel allows it and adds a runtime check) with a decorator

class QBitmap(QPixmap):
    @covariant_args
    def swap(self, other: 'QBitmap') -> None: ...

My proposal instead was add what some dialects of Eiffel call "non-conforming inheritance" ("conforms" is the Eiffel word for what PEP-483 calls "is-consistent-with"). It is essentially a mechanism to use subclassing just as a way to get the implementation from the superclass without creating a subtyping relation between them. See python/mypy#1237 (comment) for a detailed explanation.

The proposal is to haven Implementation in typing so you can write:

class QBitmap(Implementation[QPixmap]):
    def swap(self, other: 'QBitmap') -> None: ...

which just defines a QBitmap class with all the mothods copied from QPixmap, but without making one a subtype of the other. In runtime we can just make Implementation[QPixmap] == QPixmap

[dmoisset]

As a follow-up to the discussion, @gvanrossum suggested using a class decorator. The problem with this approach is that class-level is not the right granularity to decide this, but each individual inheritance relation is. Ina multiple-inheritance context, you could want to inherit the interface of a SuperClassA, and the implementation of SuperClassB. I think that might be quite common with mixins (actually I think most mixins could be used with this form of inheritance)

Looking deeper into what Eiffel does (but this is a bit of a side-comment, not part of the main proposal), there it was also possible to define a superclass in a way that only implementation inheritance from it was allowed, and that could work well with mixins (and implemented as a class decorator). However you still need to be able to say "I just want the implementation of this superclass" when inheriting, because the writer of the superclass may not have foreseen that you wanted that.

[dmoisset]

Also note that this idea makes self types more viable. Things like TotalOrderingMixin where you have

    def __lt__(self: T, other:T): -> bool ...

can be defined more accurately (currently this is in the stdlib), and with no errors (note that the < operator/ __lt__ method is actually covariant in its argument in Python 3). TotalOrderingMixin is something that would create an error in every case with the current mypy implementation if self types are added as is.

[ilevkivskyi]

I think this is a very nice feature. In general, it is good that we separate classes (code reuse) and types (debugging/documentation). If it is added, then I would also mention this feature in PEP483 (theory) as an example of differentiating classes and types.

Between a decorator and a special class I chose the second one, since as @dmoisset mentioned, it is more flexible and suitable fox mixin use cases. Also, it is very in the spirit of gradual typing, one can mark particular bases as Implementation while still typecheck with respect to other bases.

[gvanrossum]

I'd like to explore this further. I expect that specifying semantics that work for every use case will be tricky. E.g. when using implementation inheritance, how much control does the subclass have and how much control does the superclass have? Do you have to repeat the signature for every method? Only describe exceptions? Which use cases should be care about most? I hope that one of you can come up with a concrete, detailed proposal.

[ilevkivskyi]

I think it is better to have a minimalistic solution, rather than try to make a complex proposal that will cover all use cases. Later, more features for most popular use cases could be added.

Here is a detailed proposal with examples:

1. At runtime Implementation[MyClass] is MyClass. So that the separation of control between subclass/superclass is as usual. Also, all normal mechanisms work at runtime:

class Base:
    def __init__(self, number: int) -> None:
        self.number = number

class AnotherBase:
    ...

class Derived(Implementation[Base], AnotherBase):
    def __init__(self) -> None:
        super().__init__(42) # This works since Implementation[Base] is just Base

2. Use cases for Implementation are situations where a user wants to reuse some code in another class without creating excessive "type contracts". Of course one can use Any or #type: ignore, but Implementation will allow more precise type checking.

3. The derived class is not considered a subtype of implementation base classes. An explicit cast might be used under user's responsibility. Examples:

from typing import cast

def func(x: Base) -> None:
    ...
def another_func(x: AnotherBase) -> None:
    ...

func(Base(42))              # this is OK
another_func(Derived())     # this is also OK
func(Derived())             # Marked as error by a typechecker
func(cast(Base, Derived())) # Passes typecheck

4. Methods of an implementation base class could be redefined in an incompatible way:

class Base:
    def method(self, x: int) -> None:
        ...

class AnotherBase:
    def another_method(self, y: int) -> None:
        ...

class Derived(Implementation[Base], AnotherBase):
    def another_method(self, y: str) -> None: ... # This is prohibited by a typechecker
    def method(self, x: str) -> None: ...         # Although, this is OK

5. As @JukkaL proposed, a typechecker checks that the redefined methods do not "spoil" the already checked code by re-type-checking the bodies of all base class methods as if they were written in the subclass (after translating type variables suitably if the base class is generic).

I think the re-check should be recursive, i.e. include bases of bases, etc. Of course, this could slow down the type-check, but not the runtime.

6. A typechecker uses the normal MRO to find the signature of a method. For example, in this code:

class Base:
    def method(self, x: int) -> None: ...

class AnotherBase:
    def another_method(self, y: int) -> None: ...

class Derived(Implementation[Base], AnotherBase):
    def method(self, x: str) -> None: ...

a typechecker infers that Derived().method has signature (str) -> None, and Derived().another_method has signature (int) -> None.

7. Clearly, Implementation could be used only with a proper class. Implementation[Union[t1, t2]], Implementation[Callable[[t1, t2, ...], tr]], etc. rise TypeError.

If we agree on this, then I will submit PRs for typing.py, PEP484, and PEP483.

[JukkaL]

This has the problem that a type checker doesn't see inside the implementations of stubbed classes, and you could break some of the base class methods without a type checker having any way of detecting it.

Maybe implementation inheritance would only be allowed from within your own code somehow. For example, behavior would be undefined if you inherit the implementation of a stdlib or 3rd party library class, unless you include the implementation of the library module in the set of checked files. object would be fine as a base class, of course, but you probably shouldn't override any methods defined in object in an incompatible manner, since this can break a lot of things. This would only be enforced by static checkers, not at runtime.

Not sure if the latter would be too restrictive. To generalize a bit, it would okay if the implementation base class has library classes in the MRO, but methods defined in those classes can't be overridden arbitrarily.

[ilevkivskyi]

@JukkaL This is a good point. We need to prohibit incompatibly overriding methods everywhere except for accessible in the set of checked files. I don't think it is too restrictive. Alternatively, we could allow this with a disclaimer that complete typecheking is not guaranteed in this case.

[gvanrossum]

Yeah, I'm afraid the use case from python/mypy#1237 is not so easily addressed by things that preserve soundness. But I don't have an alternate proposal. :-(

[ilevkivskyi]

@gvanrossum Indeed, there is not much could be done. But I don't think this is bad. As with Any or # type: ignore, people who are going to use this feature know what they do. More important is that Implementation provides more precise typechecking, more "granularity", and probably a bit more safety than Any.

[JukkaL]

The QPixmap example shouldn't be a problem, since it seems to be for a library stub file. We don't verify that stubs are correct anyway, so a stub would be able to use implementation inheritance without restrictions but there are no guarantees that things are safe -- we expect the writer of the stub to reason about whether implementation inheritance is a reasonable thing to use.

For user code, I think that we can provide a reasonable level of safety if we enforce the restrictions that I mentioned.

[JukkaL]

I have a related proposal. What about having a magic class decorator (with no runtime or minimal effect) or a base class that declares a class as a "pure mixin"? A pure mixin could only be used for implementation inheritance, and it wouldn't be type checked by itself at all -- it would only be type checked as part of implementation inheritance. I think that this would allow type checkers to check some mixin use cases that are tricky right now with minimal code changes. The alternative would be to use ABCs to declare the things that the mixin expects to be defined elsewhere, but they are harder to use and arguably add boilerplate.

Example:

@puremixin
class MyMixin:
    def do_stuff(self) -> int:
        self.one_thing()  # ok, even though not defined in this class
        return self.second_thing(1)  # also ok

class Thing(Implementation[MyMixin]):
    def one_thing(self) -> None: pass  # ok
    def second_thing(self, x: int) -> str:    # error: return type not compatible with do_stuff
        return 'x'

[gvanrossum]

But is that the way mixins are typically used? It would be good to look
carefully at some examples of mixins (e.g. one of our internal code bases
uses them frequently).

[ilevkivskyi]

@JukkaL How about the following disclaimer to be added?

"""
Note that implementation inheritance could be used with stubbed extension modules, but there are no guarantees of safety in this case (type checkers could not verify correctness of extension modules). The writer of the stub is expected to reason about whether implementation inheritance is appropriate.

In user code, implementation inheritance is only allowed with classes defined in the set of checked files. Built-in classes could be used as implementation base classes, but their methods could not be redefined in an incompatible way.
"""

I like you proposal of @puremixin decorator (maybe we could choose other name). It looks like it is independent of the initial proposal and could be easily added. I propose @gvanrossum to decide whether we need this.

[JukkaL]

@gvanrossum I've seen mixins like that (that use things that aren't defined anywhere in the mixin class). I'm not sure how common they are.

@ilevkivskyi Looks good. I'd do some small changes (feel free to edit at will):

Note that implementation inheritance could be used with stubbed extension modules, but there are no guarantees of safety in this case (type checkers could not verify correctness of extension modules). The writer of the stub is expected to reason about whether implementation inheritance is appropriate.

->

Note that implementation inheritance could be used with stubbed extension modules, but there are no guarantees of safety in this case (type checkers could not verify correctness of extension modules). As a special case, if a stub uses implementation inheritance within the stub, the writer of the stub is expected to reason about whether implementation inheritance is appropriate. Stubs don't contain implementations, so type checker can't verify the correctness of implementation inheritance in stubs.

Built-in classes could be used ...

->

Built-in and library classes could be used ...