JsonValue is contravariant, should be covariant

[tamird]

JsonValue is contravariant, should be covariant

[sydney-runkle]

@tamird,

Thanks for reporting this - definitely looks like a typing bug. The concern I have here is that we can't use Sequence as the type hint, as not all sequence types are JSON serializable (consider sets, for example).

Here's one possible alternative:

if TYPE_CHECKING:
    # This seems to only be necessary for mypy
    J = TypeVar('J', bound='JsonValue')
    JsonValue: TypeAlias = Union[
        List[J],
        Dict[str, J],
        str,
        bool,
        int,
        float,
        None,
    ]

Let me know your thoughts on that. I can open a PR to fix this issue if you approve of said approach as well :).

[sydney-runkle]

One drawback is that this requires homogeneity in terms of the subtype of JsonValue used within a list or dict, which we might not want...

[tamird]

I'm pretty sure sets are not Sequences: https://mypy-play.net/?mypy=latest&python=3.12&gist=f71b466422216f85473dbb9880dd5f5c&flags=warn-unreachable.

What would be the problem with

    JsonValue: TypeAlias = Union[
        Sequence['JsonValue'],
        Mapping[str, 'JsonValue'],
        str,
        bool,
        int,
        float,
        None,
    ]

? Other than this discriminator having to be made more general.

[sydney-runkle]

@tamird,

Good point about sets. What about deque types? Deques are sequences, but aren't json serializable...

[tamird]

Can you give a runnable example? I don't understand why it isn't possible to serialize any sequence as a JSON array and to deserialize any JSON array as a list.