akvilas akvilas - 1 month ago 8
Python Question

How can I type hint an attribute in Python 3.5?

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

In Python 3.5, you have to write

self.some_attribute = None  # type: AnotherClass

In Python 3.6, new type hinting syntax will be 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
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