How can I type hint an attribute in Python 3.5?

Question

I have a class where I want the initial value of an attribute to be None:

class SomeClass:
    def __init__(self):
        self.some_attribute = None

How can I add type hinting, so that the IDE understands that some_attribute is usually of the type AnotherClass?

Answer 1

In Python 3.5, you have to write

self.some_attribute = None  # type: AnotherClass

Since Python 3.6, new type hinting syntax was added for variables (PEP 526):

self.some_attribute: AnotherClass = None

This will probably make every type-checking system complain, because None is in fact not an instance of AnotherClass. Instead, you can use typing.Union[None, AnotherClass], or the shorthand:

from typing import Optional
...
self.some_attribute: Optional[AnotherClass] = None

Answer 2

Python 3.10+ uses pipe sign | which many languages use as the or operator (PEP 604) to denote Typing.Union.

So…

self.some_attribute: AnotherClass|None = None

More concise, no Typing import needed.

FWIW, generally 3.10 also does away with many built containers needing Typing imports: list[int], not List[int] (which still works).

Note: if using forward declarations, Optional["AnotherClass"] is still needed, pipe doesn't work.

Also pipe handles more than None, since it’s a shorthand for Typing.Union:

some_attribute: AnotherClass|YetAnotherClass|None