russell's paradox in python

naive_sets.py

class Set(object):
    """Sets can contain anything, including themselves. This leads to
    paradoxical behavior: given R, the set of all sets that don't contain
    themselves, does R contain R? Here this becomes an infinite recursion.
    """
    def __init__(self, predicate):
        self.predicate = predicate

    def __contains__(self, obj):
        return self.predicate(obj)

# We hit the recursion limit with this:
R = Set(lambda s: s not in s)
print R in R

sets.py

class Set(object):
    """If we add another stipulation -- no sets contain themselves -- we
    exorcise this particular paradox.
    """
    def __init__(self, predicate):
        self.predicate = predicate

    def __contains__(self, obj):
        return obj is not self and self.predicate(obj)

# no sets contain themselves, so this is always False.
R = Set(lambda s: s not in s)
print R in R