...

try:
    from collections.abc import MutableSet
except ImportError:
    from collections import MutableSet

class SetMixin(MutableSet):

"""
    Mix-in for sets.  You must define __iter__, add, remove
    """

def __len__(self):
        length = 0
        for item in self:
            length += 1
        return length

def __contains__(self, item):
        for has_item in self:
            if item == has_item:
                return True
        return False

issubset = MutableSet.__le__
    issuperset = MutableSet.__ge__

union = MutableSet.__or__
    intersection = MutableSet.__and__
    difference = MutableSet.__sub__
    symmetric_difference = MutableSet.__xor__

def copy(self):
        return set(self)

def update(self, other):
        self |= other

def intersection_update(self, other):
        self &= other

def difference_update(self, other):
        self -= other

def symmetric_difference_update(self, other):
        self ^= other

def discard(self, item):
        try:
            self.remove(item)
        except KeyError:
            pass

@classmethod
    def _from_iterable(cls, it):
        return set(it)