How to get all the data in an object which inherits from an in-built class

Question

I am trying to code a set, inheriting from a list just for practice. I have the following code:

class MyList(list):
    def __init__(self):
        list.__init__(self)
        self.idx = 0

    def append(self, *args):
        for i in args:
            try:
                self.__getitem__(i)
            except AttributeError:
                self.__setitem__(self.idx, i)
    
    def __repr__(self):
        # print all items
        pass
    

What I am trying to do with the try and except statements is to check if an item is already in the list, however I know that that won't work as all self.__getitem__() uses the index to look for an item. However, what I really wanted to do was to check if something passed in already exists inside the list. In short, how does one access all the data of an inherited class?