AnyOf - Union for return types

Sometimes a function or method can return one of several types, depending on the passed in arguments or external factors. Best example is probably open(), but there are other examples in the standard library, like shutil.copy()[1]. In many of those cases, the caller knows what return type to expect.

Currently there are several options, but none of them is really satisfactory:

• Use @overload. This is the best solution if it can be used. But that is often not the case, like in the examples above.
• Use a mypy plugin. This solution does not scale to outside the standard library (and arguably not even inside it) and is mypy-specific.
• Use Union as the return type. This is usually not recommended, since it means that the caller needs to use isinstance() to use the return type.
• Use Any as the return type. This is currently best practice in those cases, but of course provides no type safety at all.

Therefore, I propose to add another type, for example AnyOf[...] that acts like Union, but can be used everywhere any of its type arguments could be used.

from datetime import date
x: AnyOf[str, date] = ...
s: str = x  # ok
dt: date = x  # ok
i: int = x  # type error
u: Union[str, bytes] = x  # ok
x = u  # type error (although the type checker could do something smart here and infer that u can only be str here)

I also think that the documentation should make it clear that using AnyOf is a code smell.

[1] Currently the type behaviour in shutil is broken in my opinion, but that does not change the fact that currently it is as it is.

Leggi la proposta AnyOf e confronta i suoi esempi—open(), shutil.copy(), Union e @overload—con il comportamento attuale di typing. Esamina la discussione dell’issue per verificare se c’è accordo sulla semantica e sulla documentazione; il lavoro è completo quando esiste una specifica definita che copre le assegnazioni a ciascun tipo membro, la compatibilità con Union e i tipi non validi.