mypy (v.1.15.0) complains with the following message Access to generic instance variables via class is ambiguous for the following code:

from typing import Self

class A:
    B: Self
    
A.B = A()

If I remove B: Self, then is says "type[A]" has no attribute "B".

How make mypy happy? You can play with this here: mypy playground

mypy (v.1.15.0) complains with the following message Access to generic instance variables via class is ambiguous for the following code:

from typing import Self

class A:
    B: Self
    
A.B = A()

If I remove B: Self, then is says "type[A]" has no attribute "B".

How make mypy happy? You can play with this here: mypy playground

• python
• python-typing
• mypy

Share Improve this question Follow edited Mar 26 at 17:12 InSync 11.1k44 gold badges1818 silver badges5656 bronze badges asked Mar 26 at 16:59 PinkFloydPinkFloyd 2,21544 gold badges2929 silver badges4848 bronze badges 3

• 2 It should be B: typing.ClassVar[Self]. I'll find a dupe. – InSync Commented Mar 26 at 17:12
• Potential dupe targets: Python class members type hinting, Type Hints Convention for Instance Variables Python. Neither of them are particularly good targets, but I can't find a better one. – InSync Commented Mar 26 at 17:25
• @InSync If you post your comment as an answer, I'll accept it... thx – PinkFloyd Commented Mar 26 at 17:56

Add a comment  | 

1 Answer 1

Sorted by: Reset to default 3

In addition to InSync's solution, you can also do this:

class A:
    B: 'A'
    
A.B = A()

Or better yet:

from typing import ClassVar

class A:
    B: ClassVar['A']
    
A.B = A()

This is an example of a forward reference, and mypy handles those by putting the type name in quotes. Using ClassVar just tells the type checker to disallow setting B through an instance, since it's supposed to be static.
I prefer this over Self because the concept of self in programming usually refers to a specific instance, which doesn't make sense in the context of a static variable.