Type hint for attribute that might not exist

[cjw296]

Cross posting from Stack Overflow:

How do I do a type hint for attribute that might not exist on the object in Python?

For example:

class X:
    __slots__ = ('attr',)
    attr: int  # either int or doesn't exist - what do I put here?

So, concretely:

>>> x = X()
>>> x.attr
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'X' object has no attribute 'attr'
>>> x.attr = 1
>>> x.attr
1

Slots may not always be involved, but I can see wanting to do something similar when specifying a Protocol.

[sobolevn]

Currently this is not supported. There was a discussion about potentially adding Missing[] special form, but it was declined.

Right now the most correct way is to have some attribute defined as such:

class MyClass:
     def __init__(self) -> None:
         self.attr: int | None = None

     def method(self) -> None:
         self.attr = 1  # or whatever

[cjw296]

Well no, this isn't correct. attr not being present is indication to a walker looking for attributes to look further up in the tree, None is a valid value, so would stop that traversal and cannot be used.

[carljm]

I'm curious what your desired behavior would be for such an attribute? Always emit an error/warning when accessing it, but also infer the resulting type as int?

[cjw296]

In an ideal world, it would be an indication that mypy should report an error when the attribute is accessed other than through getattr(obj, 'attr', None).