A set that can be iterated and knows its size

more_complete_set.py

class ListSet:
    def __init__(self):
        self._values = []
        self.__index = 0

    def add(self, value):
        if value in self._values:
            return

        self._values.append(value)

    def __iter__(self):
        return self

    def __next__(self):
        if self.__index >= len(self._values):
            raise StopIteration()

        val = self._values[self.__index]
        self.__index += 1

        return val

    def __len__(self):
        return len(self._values)