Code review request: support for asdict returning TypedDict

[syastrov]

I'm sorry for abusing the issue tracker, but I'm not sure how else to notify someone who could perform a code-review (I did try pinging/mentions). And no worries of course :)

I have brought this PR up-to-date and passing tests again and would like to request a re-review: #8583

@JukkaL Perhaps you have the time at some point to take a look or could ping someone who does?

Although the feature is already mentioned in #5152, I'll try to pitch it.

The pitch:

I have a dataclass that happens to have the same (or a subset of the same) fields as another class.
The list is potentially long, and each one has a different type.
I would like to construct the class out of the dataclass, without having to explicitly list each field as an argument to the class's constructor. It's easy to do this at run-time, but I would like to ensure that it is also statically checked.

To avoid listing each field manually, I can take advantage of splatting the TypedDict.
This should allow mypy to type-check that I do in fact pass the correct arguments to the class constructor.

@dataclass
class FooEntity:
  bar: str
  # long list of fields...

class Foo:
  def __init__(bar: str, ...): pass

def create_foo_from_entity(foo: FooEntity) -> Foo:
  return Foo(**asdict(foo))

I could think of a similar example with a function instead of a class constructor.

You could also have a case where a function is expected to return a certain TypedDict (let's say it's part of an existing API), and you have decided to use dataclasses as your internal representation, among other reasons, because they can have methods, are mutable, and possible to build-up incrementally. To avoid having to explicitly construct the TypedDict from the dataclass (which may include nested dataclasses), you could just call asdict on it and still be assured that the types match what is expected.

I can't otherwise see many use cases for this, as I would assume that many people just directly serialize the return value of asdict, and so don't use it in a precisely-typed manner.

Finally, as a future idea, it would be interesting to be able to use a type annotation AsDict[MyDataclass] to represent the TypedDict returned by calling asdict(instance_of_my_dataclass). This would allow the information about which TypedDict was returned to flow across function boundaries.

[tony]

Does anyone know a workaround to get asdict + TypedDict working w/ mypy?

[erictraut]

The problem I see with this proposal is that asdict is documented to return a dict. A TypedDict is not type compatible with dict; it's a subclass of Mapping. If asdict were changed to return a TypedDict, it would break backward compatibility for code that (correctly) assumes that the return type of asdict is a dict.

For this idea to work, I think there would need to be a new function such as asmapping. At runtime, it would do the same thing as asdict, but its type definition would be different so it could be used in situations where a TypedDict is desired.