Overeager type narrowing

I'm implementing type narrowing and thinking about intersection types for Python in general and I noticed this behavior in mypy:

Point = tuple[int, int]

class Shape: pass

def scale(x: float, o: int | Point | Shape):
  if isinstance(o, tuple):
    reveal_type(o)

Will show Tuple[builtins.int, builtins.int] which might seem reasonable but what if there's

class ColorShape(Shape, tuple[Shape, str]): pass

We should say that the intersection of Point and tuple (= tuple[Any, ...]) is Point (= tuple[int, int]). The intersection of Shape and tuple is not known to be empty.

The intersection of int and tuple is known to be empty for CPython because:

>>> class A(int, tuple): pass
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: multiple bases have instance lay-out conflict

Inizia eseguendo l’esempio inline di mypy e controllando il tipo rivelato per il ramo isinstance(o, tuple). Traccia il comportamento del restringimento dei tipi per i membri dell’unione Point, Shape e int; il lavoro è completato quando il restringimento preserva le possibili intersezioni di tuple senza trattare i tipi non correlati o potenzialmente compatibili come impossibili.