Writing Readable Python Syntax

I am trying to clean up some code that contains some rather long and ugly if/else statements and I wanted to get the opinon of the community on the what is considered to be the most elegant approach.

Would the following be acccpetable or might there be a more Pythonic, or succint syntax? Any and all opinions on the matter is greatly appreciated!

Here's a snippet:

def __add__(self, other):

    conditions = [
        index in self.items, 
        self.output[i] not in self.values
        ]

    if isinstance(other, (list, tuple, set)):
        for index in other:
            if all(conditions):
                self.__add_items__(index)
                self.values.add(self.output[i])

     conditions = [
        other in self.items, 
        self.output not in self.values
        ]

    if all(conditions):
        self.__add_items__(other)
        self.values.add(self.output[other])