Mypy complains about incompatible type when variable type is subset of expected type

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Trying to build an xarray Dataset, where we build the input to the arguments "coords" and "attrs" before passing it to the constructor:

coordinates = {"time": ("time", time_coordinates)}
attributes = {"some_flag": False}
...
ds = xr.Dataset(data_vars=variables, coords=coordinates, attrs=attributes)

What puzzles me is the output of mypy run against this code:

error: Argument "coords" to "Dataset" has incompatible type "Dict[str, Tuple[str, Any]]"; expected "Optional[Mapping[Hashable, Any]]"
error: Argument "attrs" to "Dataset" has incompatible type "Dict[str, bool]"; expected "Optional[Mapping[Hashable, Any]]"

Isn't a dict a Mapping? And isn't str also Hashable? And aren't Tuples and bools of type Any, in any case? What do I not understand about mypy and/or Python type hints here?

python-typing mypy python-3.x
1 Answers

Using the information from Selcuk, I found this somewhat verbose solution, as detailed in the mypy docs: As the keys of Mappings are invariant, one needs to hint explicitly that the str there is of type Hashable. (While strings are a subtype of Hashable, Mappings keys are not covariant, disallowing subtypes.). Or, as Selcuk puts it in his comment:

str is a Hashable, but since dict is a mutable data type you must be passing the exact same type for the keys, not a subtype. There is a possibility that the called function might add another Hashable key to the passed argument, breaking the source.

coordinates: Dict[Hashable, Tuple[str, Any]] = {
    "time": ("time", time_coordinates)
}
attributes: Dict[Hashable, Any] = {"some_flag": False}
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