--strict changes return type to Any

[erzoe]

Bug Report

Running mypy with --strict does not find an error which is found without --strict because the return type of a method is changed to Any.

To Reproduce

import cantools

ffn = 'test.dbc'
dbc = cantools.database.load_file(ffn)
assert isinstance(dbc, cantools.database.can.database.Database)

reveal_type(dbc.decode_message)

rawdata = b'ABCDEABC'
data = dbc.decode_message(0x123, rawdata)
reveal_type(data)
print(data['voltage'])

Running mypy without --strict: The return type of dbc.decode_message is recognized correctly and an expected error message is printed for the last line.

$ mypy test.py 
test.py:9: note: Revealed type is "def (frame_id_or_name: Union[builtins.int, builtins.str], data: builtins.bytes, decode_choices: builtins.bool =, scaling: builtins.bool =, decode_containers: builtins.bool =, allow_truncated: builtins.bool =) -> Union[builtins.dict[builtins.str, Union[builtins.float, builtins.str, cantools.database.can.signal.NamedSignalValue]], typing.Sequence[Union[Tuple[cantools.database.can.message.Message, builtins.dict[builtins.str, Union[builtins.float, builtins.str, cantools.database.can.signal.NamedSignalValue]]], Tuple[builtins.int, builtins.bytes]]]]"
test.py:13: note: Revealed type is "Union[builtins.dict[builtins.str, Union[builtins.float, builtins.str, cantools.database.can.signal.NamedSignalValue]], typing.Sequence[Union[Tuple[cantools.database.can.message.Message, builtins.dict[builtins.str, Union[builtins.float, builtins.str, cantools.database.can.signal.NamedSignalValue]]], Tuple[builtins.int, builtins.bytes]]]]"
test.py:14: error: No overload variant of "__getitem__" of "Sequence" matches argument type "str"
test.py:14: note: Possible overload variants:
test.py:14: note:     def __getitem__(self, int) -> Union[Tuple[Message, Dict[str, Union[float, str, NamedSignalValue]]], Tuple[int, bytes]]
test.py:14: note:     def __getitem__(self, slice) -> Sequence[Union[Tuple[Message, Dict[str, Union[float, str, NamedSignalValue]]], Tuple[int, bytes]]]

Running mypy with --strict: The return type of dbc.decode_message is considered to be Any and because of that the error is not recognized.

$ mypy --strict test.py
test.py:9: note: Revealed type is "def (frame_id_or_name: Union[builtins.int, builtins.str], data: builtins.bytes, decode_choices: builtins.bool =, scaling: builtins.bool =, decode_containers: builtins.bool =, allow_truncated: builtins.bool =) -> Any"
test.py:13: note: Revealed type is "Any"
Success: no issues found in 1 source file

Expected Behavior

mypy should detect the return type even when using --strict and print the same error message like without --strict.

Actual Behavior

mypy does not find the error.

Your Environment

• Mypy version used: 0.982 (compiled: yes)
• Mypy command-line flags: --strict
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.9.2
• cantools version: 37.2.0

References

• dbc.decode_message is defined here: https://github.com/cantools/cantools/blob/master/cantools/database/can/database.py#L451
• It's return type DecodeResultType is defined here: https://github.com/cantools/cantools/blob/master/cantools/typechecking.py#L61

[erzoe]

The problem seems to be --no-implicit-reexport. mypy --strict --implicit-reexport test.py shows the expected error message.

EDIT: If I see things correctly the return type DecodeResultType relies on implicit reexport which is disabled by --strict. So I guess mypy treats it as Any because it does not know it. The problem is that this is happening silently. I think mypy should give an error that the return type is not defined.

Implicit reexport is explained here: https://mypy.readthedocs.io/en/stable/command_line.html#cmdoption-mypy-no-implicit-reexport

[hauntsaninja]

Thanks, closing as a duplicate of the (partially fixed) #13965