[Python-Dev] PEP 544 (Protocols): adding a protocol to a class post-hoc (original) (raw)

Ivan Levkivskyi levkivskyi at gmail.com
Sun Jul 1 08:06:13 EDT 2018

On 30 June 2018 at 23:54, Tin Tvrtković <tinchester at gmail.com> wrote:

[...]

An attrs class has a special class-level field, attrsattrs, which holds the attribute definitions. So maybe we can define a protocol: class AttrsClass(Protocol): attrsattrs: ClassVar[Tuple[Attribute, ...]] then we could define asdict as (simplified): def asdict(inst: AttrsClass) -> Dict[str, Any]: ... and it should work out. My question is how to actually add this protocol to attrs classes. Now, we currently have an attrs plugin in mypy so maybe some magic in there could make it happen in this particular case.

Just add a Var with an appropriate name and type to the TypeInfo. This is literary a dozen lines of code, you can ask on mypy tracker or typing Gitter chat for more help.

My second use case is a small library I've developed for work, which basically wraps attrs and generates and sticks methods on a class for serialization/deserialization. Consider the following short program, which does not typecheck on the current mypy.

class Serializable(Protocol): def serialize(self) -> int: ... def makeserializable(cl: Type) -> Type: cl = attr.s(cl) cl.serialize = lambda self: 1 return cl

@makeserializable class A: a: int = attr.ib() def serialize(inst: Serializable) -> int: return inst.serialize() serialize(A(1)) error: Argument 1 to "serialize" has incompatible type "A"; expected "Serializable" error: Too many arguments for "A" I have no desire to write a mypy plugin for this library. So I guess what is needed is a way to annotate the class decorator, telling mypy it's adding a protocol to a class. It seems to have trouble getting to this conclusion by itself.

A proper solution for this would be to introduce intersection types, and type your decorator as following:

T = TypeVar('T') def make_serializable(cls: Type[T]) -> Type[Intersection[T, Serializable]]: ...

However, intersection types are unlikely to appear in mypy this year. In best case they could appear around mid-2019, so you are better with writing a plugin for now.

(The second error implies the attrs plugin doesn't handle wrapping attr.s, which is unfortunate but a different issue.)

Your decorator is typed as (Type) -> Type, thats it, the function is a black box for mypy (with few special exceptions), if some effect of a function is not declared in its signature, then it is lost forever.

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