Question: List of instances / Callables?

[izn]

I've started testing mypy on an application, but I don't get success when trying to work with instances of class.

ButtonGenerator.__new__ + buttons should accept a List of BaseButton (URLButton) or simply a BaseButton instance.

buttons.py

class BaseButton(object):
    def __init__(self):
        pass    

class ButtonGenerator(BaseButton):
    def __new__(self, buttons: Union[List[BaseButton], BaseButton]) -> Union[List[Dict[str, str]], Dict[str, str]]:
        if (isinstance(buttons, list)):
            if (not all(isinstance(button, BaseButton) for button in buttons)):
                raise ValueError('Invalid Buttons instances.')

            return [button.button_dict for button in buttons]

        return buttons.button_dict


class URLButton(BaseButton):
    def __init__(self, url, title=None):
        self.button_dict = {
            'type': 'external',
            'url': url
        }

        if (title):
            self.button_dict['title'] = title

Errors

buttons.py:17: error: "BaseButton" has no attribute "button_dict"
buttons.py:19: error: "BaseButton" has no attribute "button_dict"

Example: print(ButtonGenerator(URLButton('https://github.com/python/mypy')))

I tried with List of Callable also.

Am I doing it wrong?

[emmatyping]

You need to narrow the type to URLButton it appears. If you tried to pass a BaseButton instance it would cause an attribute error at runtime as BaseButton does not have an attribute button_dict. I believe you want ButtonGenerator to look like:

class ButtonGenerator(BaseButton):
    def __new__(self, buttons: Union[List[URLButton], URLButton]) -> Union[List[Dict[str, str]], Dict[str, str]]:
        if (isinstance(buttons, list)):
            if (not all(isinstance(button, BaseButton) for button in buttons)):
                raise ValueError('Invalid Buttons instances.')

            return [button.button_dict for button in buttons]

        return buttons.button_dict

And that should typecheck.

[izn]

@ethanhs You're right, but URLButton extends from BaseButton - and using pure Python the inheritance (button_dict) works.

Is there some limitation on MyPy about parent/child classes?

(I have a lot of classes extending from BaseButton and all should be accepted on buttons argument in ButtonGenerator)

[emmatyping]

@izn This isn't really a problem with mypy, more of a constraint of nominal static typing. Since you annotated __new__ to take BaseButton or a list thereof, the argument buttons has one of those two types. Since BaseButton type has no attribute button_dict, it is a type error. You can fix this by either adding button_dict to BaseButton if all of your inheriting classes have button_dict or, if there are some that do not have the button_dict attribute, then you need to either create an intermediate class or define a protocol that has that attribute. #1424 may help as well.