Allow adding custom methods to TypedDict

[mitar]

I would like to extend TypedDict with extra abstract methods into, for example, class MyParams, which would then allow that those who will be extending MyParams, will be able to both specify types of values they want to list as params, but also have to implement abstract methods.

Currently, if I do:

class MyParams(TypedDict):
    @abstractmethod
    def compute(self):
        pass

A subclass like Foo = MyParams('Foo', {'a': int}) fails because it seems MyParams lost its special power to initialize it in this way. If I use class-based syntax, it works, but method compute is still not available on the instance (nor required to be defined, but this is clear problem because metaclass is not abstract metaclass).

[ilevkivskyi]

The initial design has a strong requirement for TypedDicts to be very fast and memory-efficient. This is why at runtime TypedDict instances are just normal dicts:

>>> from mypy_extensions import TypedDict
>>> class TD(TypedDict):
...     x: int
...     y: int
...
>>> td = TD({'x': 1, 'y': 2})
>>> type(td)
<class 'dict'>

therefore, you just can't call any additional methods. I therefore propose to close this, unless we want to re-design it. Maybe we can open an issue at typing tracker to give an informative error if someone tries to add methods to TypedDict.

[JukkaL]

Yeah, this is out of scope for TypedDict. TypedDict has been designed for a very specific use case where the runtime representation is just a regular dict object. If you want to define methods, you have to use a real class, unfortunately.

[mitar]

Hm, but my own class will not have a nature that mypy will know which types are stored under which keys, and to check for totality? Does this mean I will also have to roll out my own mypy plugin to be able to type check such class?

[mitar]

I mean, ideally, I should be able to specify to TypedDict which base class (extending dict) it is used at runtime, instead of dict. But dict should be the default for sure.

[ilevkivskyi]

@mitar

Hm, but my own class will not have a nature that mypy will know which types are stored under which keys, and to check for totality? Does this mean I will also have to roll out my own mypy plugin to be able to type check such class?

This is just another request for Intersection and/or something like @apply_protocol discussed in #2087