Fix typing on session_and_runs.

[JoeZiminski]

sessions_and_runs is really a Dict[str, Union[str, List[str]]] but typing it is difficult. See:

However, Mapping does not allow item assigment, so MultableMapping must be used. But, then I was running into
this issue.

As a workaround, I have typed everything to Dict[str, List[str]] which is what it should be. The type Dict[str, Union[str, List[str]]] was in case user-input a string we do not want to crash. However, Dict[str, Union[str, List[str]]] is now cast to Dict[str, Union[str, List[str]] to be safe but all calls made in the package are Dict[str, List[str]] .

Discussed further here:

dmr
JoeZiminski (Joe Ziminski)
Thanks for providing such a great support system. Could somone explain the below behaviour (that mypy is failing when the dictionary is passed as a variable, but not passed directly). Is this a bug or am I missing something? thanks!

from typing import Dict, List, Union

def func(a: Dict[str, Union[str, List[str]]]) -> None:
    pass

passed_as_var = {"test_1": ["list_str"]}
func(passed_as_var)
# error: Argument 1 to "func" has incompatible type "dict[str, list[str]]"; expected "dict[str, str | list[str]]"  [arg-type]
# note: "Dict" is invariant -- see https://mypy.readthedocs.io/en/stable/common_issues.html#variance
# note: Consider using "Mapping" instead, which is covariant in the value type
# Found 1 error in 1 file (checked 1 source file)

func({"test_2": ["list_str"]})
# passes
I think what's happened here is

mypy has deduced that the type of passed_as_var is Dict[str, List[str]]. You can check this using reveal_type.
For all it knows, func might modify a so that its values become str instead of List[str].
Doing so would break the type deduced in (1), which would be bad.
I think mypy should be happy if you change either a: Mapping[str, Union[str, List[str]]] or if you annotate passed_as_var: Dict[str, Union[str, List[str]]]. (Disclaimer: untested!) My vote would be for the former.
JoeZiminski (Joe Ziminski)
Message deleted
JoeZiminski (Joe Ziminski)
Thanks dmr! that makes sense and the suggestions resolve the problem. Indeed reveal_type shows passed_as_var as Dict[str, List[str]]. I am trying to wrap my head around invariance, covariance, contravariance. In this case, is it because List[str] is a subtype of Union[str, List[str]]. 'Dict' values are invariant and so will not accept a subtype, while Mapping is covariant so will accept this subtype?
Today
TeamSpen210 (Spencer Brown)
Yes, that is correct.