How to type hint variable that is initially None but is guaranteed to get a value

Question

I have a class variable as shown below:

class MyClass:
    def __init__(self):
        self.value: MyOtherClass | None = None
        self.initialize_value()

    def initialize_value(self):
        self.value = MyOtherClass()

    def use_value(self):
        return self.value.used

self.value is guaranteed to be initialized as an instance of MyOtherClass before being used in self.use_value(); however, it must remain as None in the constructor. The dilemma here is how to properly typehint this situation.

• If I choose to typehint it as shown above, I get an error: Item "None" of "Optional[MyOtherClass]" has no attribute "used" [union-attr]
• If I instead remove the variable definition from the constructor and move the typehint to self.initialize_value(), I get an Instance attribute value defined outside __init__ error from Pylint

What is the best way to go about this? Thanks a lot!

Answer 1

To avoid repetition, when such an attribute is used throughout the class in multiple methods, a common pattern for me is protecting it and defining a property that raises an error, if the attribute behind it is not set:

class MyClass:
    def __init__(self):
        self._value: MyOtherClass | None = None
        self.initialize_value()

    @property
    def value(self) -> MyOtherClass:
        if self._value is None:
            raise AttributeError("...")
        return self._value

    def initialize_value(self):
        self._value = MyOtherClass()

    def use_value(self):
        return self.value.used

This also removes ambiguity for the type checker since the return type of the property is always MyOtherClass.