Narrow Union by shape

Question

How to narrow Union by shape? I don't want to check actual types with isinstance or manual casting (there are a lot of types). Also I can't modify type definitions.

class X:
    title = "1"
class Y:
    name = "2"
class Z:
    name = "3"

for (i, r) in enumerate([X(), Y(), Z()]): # type of r: X | Y | Z

    if hasattr(r, "title"):
        print(r.title) # error
    else:
        print(r.name)  # error

Type check error says:

(variable) title: str | Unknown
Cannot access member "title" for type "Y"
  Member "title" is unknown
Cannot access member "title" for type "Z"
  Member "title" is unknown

(variable) name: Unknown | str
Cannot access member "name" for type "X"
  Member "name" is unknown

Answer 1

The issue

In a way Pylance checker is correct.

The types are unrelated. There would be no check errors for related types:

class Base:
    title: str
    name: str

class X(Base):
    title = "1"
class Y(Base):
    name = "2"
class Z(Base):
    name = "3"

for (i, r) in enumerate([X(), Y(), Z()]): # type of r: X | Y | Z

    if hasattr(r, "title"):
        print(r.title) # no error
    else:
        print(r.name) # no error

Also, duck typing is not applicable here since the set of members is different. There would be no errors if types were considered equivalent:

class X:
    name: str
    title = "1"
class Y:
    name = "2"
    title: str
class Z:
    name = "3"
    title: str

for (i, r) in enumerate([X(), Y(), Z()]): # type of r: X | Y | Z

    if hasattr(r, "title"):
        print(r.title) # no error
    else:
        print(r.name) # no error

Workaround

Although you can trick the Pylance checker so it won't complain about the code, there are a couple of options:

Option 1. Suppress type checks

class X:
    title = "1"
class Y:
    name = "2"
class Z:
    name = "3"

for (i, r) in enumerate([X(), Y(), Z()]): # type of r: X | Y | Z

    if hasattr(r, "title"):
        print(r.title) # type: ignore
    else:
        print(r.name) # type: ignore

Option 2. Use dynamic access with getattr instead of direct member access

class X:
    title = "1"
class Y:
    name = "2"
class Z:
    name = "3"

for (i, r) in enumerate([X(), Y(), Z()]): # type of r: X | Y | Z

    if hasattr(r, "title"):
        print(getattr(r, "title"))
    else:
        print(getattr(r, "name"))