contribute_to_class.py

    def contribute_to_class(self, cls, name, private_only=False):
        """
        Register the field with the model class it belongs to.

        If private_only is True, create a separate instance of this field
        for every subclass of cls, even if cls is not an abstract model.
        """
        self.set_attributes_from_name(name)
        self.model = cls
        cls._meta.add_field(self, private=private_only)
        if self.column:
            # Don't override classmethods with the descriptor. This means that
            # if you have a classmethod and a field with the same name, then
            # such fields can't be deferred (we don't have a check for this).
            print(f"getattr(cls={cls}, self={self}), column={self.column}")
            #print(f"cls.__dict__={cls.__dict__}, {self.attname}")
            if not getattr(cls, self.attname, None): # <-- __get__ called invoked when self.attname is used!
                setattr(cls, self.attname, self.descriptor_class(self))