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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