import collections

class MySimpleSet(collections.Set):
    def __init__(self, iterable=()):
        self.inner = set(iterable)
    def __contains__(self, item):
        return item in self.inner
    def __iter__(self):
        return iter(self.inner)
    def __len__(self):
        return len(self.inner)

MySimpleSet() | set()  # This works
set() | MySimpleSet()  # This doesn't