mypy types issue - Possible overload variant

Question

I seem to get errors with the following piece of code using mypy

payload = {
    'flag': 'good'
}
payload = payload['flag']
scheme: Dict[str, Dict[str, str]] = {
    'a_string': {
        'status': 'abc_string'
    }
}

scheme.get(payload).get('status')

or

payload = {
    'flag': 'good'
}
payload = payload['flag']
scheme = {
    'a_string': {
        'status': 'abc_string'
    }
}

scheme.get(payload).get('status')

I get the following error:

error: No overload variant of "get" of "Mapping" matches argument type "Dict[str, Any]" scheme.get(payload).get('status')

But when the types are this:

payload = {
    'flag': 'good'
}
payload = payload['flag']
scheme: Dict = {
    'a_string': {
        'status': 'abc_string'
    }
}

scheme.get(payload).get('status')

It works fine.

Why does this behaviour happen?

Answer 1

You should check all the errors raised by mypy.

src/test.py:5: error: Incompatible types in assignment (expression has type "str", variable has type "Dict[str, str]")
src/test.py:12: error: No overload variant of "get" of "Mapping" matches argument type "Dict[str, str]"
src/test.py:12: note: Possible overload variant:
src/test.py:12: note:     def get(self, key: str) -> Optional[Dict[str, str]]
src/test.py:12: note:     <1 more non-matching overload not shown>

Note how it complains on line 5 with Incompatible types in assignment.

You should change your code to something like:

flag = payload['flag']
...
scheme.get(flag).get('status')

but then mypy will complain with:

src/test.py:12: error: Item "None" of "Optional[Dict[str, str]]" has no attribute "get"

indicating that you may be calling get on None, which happens because scheme.get(flag) returns an Optional.