try:
    from collections.abc import MutableSet
except ImportError:
    from collections.abc import MutableSet
[docs]
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__
[docs]
    def copy(self):
        return set(self) 
[docs]
    def update(self, other):
        self |= other 
[docs]
    def intersection_update(self, other):
        self &= other 
[docs]
    def difference_update(self, other):
        self -= other 
[docs]
    def symmetric_difference_update(self, other):
        self ^= other 
[docs]
    def discard(self, item):
        try:
            self.remove(item)
        except KeyError:
            pass 
[docs]
    @classmethod
    def _from_iterable(cls, it):
        return set(it)