El vie, 19 mar 2021 a las 12:42, Greg Werbin () escribió:

Note: this was originally posted on the Discourse forum ( https://discuss.python.org/t/7721), but it was recommended that I post here instead.

In PEP-589: “Inheritance”, it is stated:

...

Changing a field type of a parent TypedDict class in a subclass is not allowed.

However, this restriction seems perhaps too restrictive in the presence of typing.Literal and PEP-586.

Consider a situation like the following, which might describe some polymorphic data returned from a web API:

from typing import Literal, TypedDict

# Assume these all have several other fields

class BaseDatum(TypedDict): id: str datumType: str

class Thing(BaseDatum): datumType: Literal["thing"]

class Widget(BaseDatum): datumType: Literal["widget"]

This seems like a reasonable thing that someone might want to do, especially in a context where you can’t use something like Marshmallow for runtime validation & deserialization (don’t want performance overhead, restricted from using 3rd-party dependencies, etc).

However, in the above example, MyPy emits:

...

tmp1.py:10: error: Overwriting TypedDict field "datumType" while extending tmp1.py:13: error: Overwriting TypedDict field "datumType" while extending Found 2 errors in 1 file (checked 1 source file)

Typically, Literal["widget"] is considered a subclass of str, as in:

from typing import Literal x: Literal["hello"] = "hello" y: str = x

It appears that the MyPy devs faithfully honored the restriction on changing type that is stated in PEP-589, but I think in this specific case the restriction is excessive.

Should there be a revision to the TypedDict specification to allow this particular case? For that matter, why shouldn’t TypedDict members be covariant in general? Or is there a good reason why the restriction is what it is?

Changing the type in a child is unsafe because TypedDicts are mutable: def f(datum: BaseDatum): datum["datumType"] = "some string" w: Widget f(w) assert w["datumType"] == "widget" # boom We could get around that if we had a notion of an immutable TypedDict, but that's not currently a part of the type system. We could add something like `class BaseDatum(TypedDict, immutable=True)` to disallow modifications; this would make it safe to allow covariance in child classes.

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