Problems with wrapper classes

Let's say I have a Person class with a bunch of attributes; it comes from a library I don't control but for which type information exists. Now I have a PersonWrapper class which provides for defaults so that accessing missing attributes never raises an exception (think, use of instances in f-strings).

class PersonWrapper:
    def __init__(self, person: Person, default: Any = '') -> None:
        self.person = person
        self.default = default

    def __getattr__(self, attribute: str) -> Any:
        return getattr(self.person, attribute, self.default)

In general, any place I want to use a Person I might also want to use a PersonWrapper, so I have variables that can be assigned either a Person or a PersonWrapper. mypy seems not to be able to handle this, and I don't really know of any way to tell the type checker that it should treat Person and PersonWrapper as equivalent. For example, if I have this code:

def print_person(original_person: Person) -> None:
    person = PersonWrapper(original_person)
    # This attribute might be missing from the original person object.
    print(f'  Username: {person.username}')
    person = original_person
    # Only print this if the attribute exists.
    phone = getattr(person, 'phoneNumber', None)
    if phone:
        print(f'    Phone: {phone}')

mypy complains about the assignment of original_person to person:

error: Incompatible types in assignment (expression has type "Person", variable has type "PersonWrapper")

Because Person has some dynamic attribute access as well, I can't change the original assignment to:

    person: Person = cast(Person, PersonWrapper(original_person))

because then I get errors such as:

error: "Person" has no attribute "office"

The way I fix this is by changing the second assignment to use cast():

    person = cast(PersonWrapper, original_person)

This feels wrong, but maybe it's okay. What I think i'd like to do is to create an equivalence between PersonWrapper and Person, but then I'd probably still have to handle the dynamic parts of Person in some way. FWIW I didn't have much luck using Union[Person, PersonWrapper] either.

Is there a better way to handle wrapper classes?

Commencez par l’exemple Person/PersonWrapper signalé et par l’interaction entre l’inférence d’affectation, Union[Person, PersonWrapper], cast() et getattr dynamique ; aucun fichier source ni test de mypy n’est indiqué. Définissez le comportement de typage souhaité et ajoutez un test de régression ciblé une fois l’emplacement de l’implémentation identifié.