build a typeddict from another

[dfroger]

I'm looking for a way to build a typeddict from another:

given:

class Foo(TypedDict):
    a: int
    b: int


class Bar(Foo):
    c: int


foo = Foo(a=1, b=2)

I would like to do bar = Bar(**foo, c=5), because we perform this operation many times with many dict and many keys, and bar = Bar(a=foo["a"], b=foo["b"], c=3) would be really too verbose.

I fear this is not possible yet. Any chance that such a functionality could be implemented?

I don't see a better workaround than bar = Bar(**foo, c=5) # type: ignore, maybe I'm missing something?

Thanks!

[sameershaik-tech]

can i work on this?

[wimax-grapl]

(Regardless of the TypedDict stuff, I think mypy just currently doesn't have any support for inferring the keys contained in a **kwargs splat.)

[dfroger]

So I think I will use a cast. The scope of the dangerous code (ie, not type checked) is limited, it's flexible and not verbose, so it's not so bad.

foo = Foo(a=1, b=2)

# dangerous code (not type checked): developper must ensure the missing keys are added
foo = cast(Bar, foo)
foo["c"] = 1
# end of dangerous code

[dfroger]

Adding this kind of code to the above snippet could be useful to remember to update the dangerous code when Foo or Bar is changed:

assert Bar.__annotations__ == {**Foo.__annotations__, "c": int}

Wonder if this assertion could be done statically?

[AlexWaygood]

Duplicate of #9408