single_entry_set.py

from collections.abc import MutableSet

class SingleEntrySet(MutableSet):
    """\
    I created this class for debugging.  It works like a regular set but it
    raises an error if it tries to add an item that is already present.
    """

    __slots__ = ("__items",)

    def __init__(self, iterable=()):
        MutableSet.__init__(self)
        self.__items = set()
        for item in iterable:
            self.add(item)

    def __contains__(self, item):
        return item in self.__items

    def __iter__(self):
        return (item for item in self.__items)

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

    def add(self, item):
        if item in self:
            raise ValueError("item already in set: %r" % (item,))
        return self.__items.add(item)

    def discard(self, item):
        return self.__items.discard(item)

    def __repr__(self):
        return "%s({%s})" % (self.__class__.__name__, ", ".join(map(repr, self.__items)))