Make Context.obj generic

[Goldziher]

Hi there,

I suggest updating the typing of the Context class to have a bound generic T, e.g.

T = TypeVar("T")

class Context(Generic[T]):
    def __init__(
        self,
        command: "Command",
        parent: t.Optional["Context"] = None,
        info_name: t.Optional[str] = None,
        obj: t.Optional[T] = None, # <-- updated from t.Any to the generic T type
        auto_envvar_prefix: t.Optional[str] = None,
        default_map: t.Optional[t.MutableMapping[str, t.Any]] = None,
        terminal_width: t.Optional[int] = None,
        max_content_width: t.Optional[int] = None,
        resilient_parsing: bool = False,
        allow_extra_args: t.Optional[bool] = None,
        allow_interspersed_args: t.Optional[bool] = None,
        ignore_unknown_options: t.Optional[bool] = None,
        help_option_names: t.Optional[t.List[str]] = None,
        token_normalize_func: t.Optional[t.Callable[[str], str]] = None,
        color: t.Optional[bool] = None,
        show_default: t.Optional[bool] = None,
    ) -> None:
    ...

This will allow using context.obj in a typed setting:

class State(TypedDict):
    value: str


@click.command()
@click.pass_context
def my_command(ctx: Context[State]) -> None:
    click.echo(ctx.obj["value"])

As it stands now, I will have to cast ctx.obj to the State type to get type checking working:

@click.command()
@click.pass_context
def my_command(ctx: Context) -> None:
    state = cast(State, ctx.obj)
    click.echo(ctx.obj["value"])

I will be happy to contribute this change if there is a go ahead.