TypedDict update() does not accept TypedDict with compatible subset keys

[yuyangguo42]

from mypy_extensions import TypedDict

A = TypedDict(
    'A', {
        'foo': int,
        'bar': int,
    }
)

B = TypedDict(
    'B', {
        'foo': int,
    }
)

a = A({'foo': 1, 'bar': 2})
b = B({'foo': 2})
a.update({'foo': 2})
a.update(b)

Expect this to pass type checking but got this error:

test.py:19: error: Argument 1 to "update" of "TypedDict" has incompatible type "B"; expected "TypedDict({'foo'?: int, 'bar'?: int})"

[JukkaL]

This is a pretty subtle issue. The update call is arguably not type safe. In a more complex program, the argument could be a value of a subtype of B that has the key 'bar' with a list value, for example. This would break type safety, as a['bar'] would be a list instead of an integer.

However, the same reasoning could be applied to update in general, and it turns out that the way update is treated is generally unsafe. There could be two values of different subtypes of A with incompatible values for some key, and if we are looking at both through type A, first.update(second) could again break type safety.

Not sure what should be done. If we want to be safe, we could require that the argument to update is a dict literal. If we want to be flexible, we should probably allow the original example above.

An explicit cast to A is perhaps the most reasonable workaround: a.update(cast(A, b))

[callmesangio]

Hi,

I get the same mypy error with the following code:

from mypy_extensions import TypedDict


class Movie(TypedDict):
    name: str
    year: int


m1 = Movie(name='Blade Runner', year=1982)
m2 = Movie(name='Star Wars', year=1977)
m1.update(m2)

error: Argument 1 to "update" of "TypedDict" has incompatible type "Movie"; expected "TypedDict({'name'?: str, 'year'?: int})"

Is this something related to the described issue?

Thanks!

[ilevkivskyi]

Is this something related to the described issue?

Yes, I think this is an aspect of the same problem. In this case however it looks to me as a bug, mypy is overly strict in your case, since we know dict.update() never modifies its argument.

[mr-c]

Hello, the bug in #6462 (comment) just bit me and is blocking my adoption of TypedDict.

Cheers,

[kratsg]

I've tried two ways of doing this. One is to iterate over the keys and assign (doesn't work), but this version seems to be the only thing that mypy is happy with

from mypy_extensions import TypedDict


class Movie(TypedDict):
    name: str
    year: int


m1 = Movie(name='Blade Runner', year=1982)
m2 = Movie(name='Star Wars', year=1977)

m1['name'] = m2['name']
m1['year'] = m2['year']