Mypy shows object for values of large dictionaries

[PythonCoderAS]

Bug Report

I have a large block of decently-nested JSON that I wrote types for using TypedDicts. I would like to verify that my types are correct by assigning a Python representation of one of the JSON blobs using the appropriate type variable. However, I am getting errors like

tests/test.py:15: error: Incompatible types in assignment (expression has type "Dict[str, List[List[object]]]", variable has type "SeriesVolumeCovers")

However, the value I am assigning is

{
    "covers": [
        [
            1,
            "/media/manga/Kaguya-Wants-To-Be-Confessed-To/volume_covers/1/1.png",
            "/media/manga/Kaguya-Wants-To-Be-Confessed-To/volume_covers/1/1.webp",
            "/media/manga/Kaguya-Wants-To-Be-Confessed-To/volume_covers/1/1_blur.png",
        ],
        [
            2,
            "/media/manga/Kaguya-Wants-To-Be-Confessed-To/volume_covers/2/2.png",
            "/media/manga/Kaguya-Wants-To-Be-Confessed-To/volume_covers/2/2.webp",
            "/media/manga/Kaguya-Wants-To-Be-Confessed-To/volume_covers/2/2_blur.png",
        ],
    ]
}

My types look like this:

SeriesVolumeCover = List[Union[str, int]]

class SeriesVolumeCovers(TypedDict):
    covers: List[SeriesVolumeCover]

The two should match, but for some reason mypy is complaining.

To Reproduce

1. Create types like so:

from typing import List, Union, TypedDict

SeriesVolumeCover = List[Union[str, int]]

class SeriesVolumeCovers(TypedDict):
    covers: List[SeriesVolumeCover]

2. Assign variable

data: SeriesVolumeCovers = {
    "covers": [
        [
            1,
            "/media/manga/Kaguya-Wants-To-Be-Confessed-To/volume_covers/1/1.png",
            "/media/manga/Kaguya-Wants-To-Be-Confessed-To/volume_covers/1/1.webp",
            "/media/manga/Kaguya-Wants-To-Be-Confessed-To/volume_covers/1/1_blur.png",
        ],
        [
            2,
            "/media/manga/Kaguya-Wants-To-Be-Confessed-To/volume_covers/2/2.png",
            "/media/manga/Kaguya-Wants-To-Be-Confessed-To/volume_covers/2/2.webp",
            "/media/manga/Kaguya-Wants-To-Be-Confessed-To/volume_covers/2/2_blur.png",
        ],
    ]
}

Expected Behavior

I expected the types to match.

Actual Behavior

The types did not match.

Your Environment

• Mypy version used: 0.950
• Mypy command-line flags: No flags
• Mypy configuration options from mypy.ini (and other config files): No mypy.ini
• Python version used: 3.9
• Operating system and version: MacOS Big Sur/Ubuntu 20 LTS

[Zeckie]

I tried to reproduce the issue on mypy playground, but needed to make a change (added an intermediate variable) to get the same error.

https://mypy-play.net/?mypy=0.950&python=3.9&gist=78d258e683165265cd6dada6dbee741d

This looks to me as though it is a known issue - topic-join-v-union Using join vs. using unions (eg. #12056 )

[PythonCoderAS]

Oh yea in my actual test setup I did have an intermediate variable, as the python structure files were automatically generated

[JelleZijlstra]

Agree, this is another join-vs-union, we have plenty of open issues about that.