How can I type hint an attribute in Python 3.5?

问题:

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?

回答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