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