def __init__(self):
        # Don't call this from a concrete subclass!
        if self.__class__ is BaseSet:
            raise TypeError, ("BaseSet is an abstract class.  "
                              "Use Set or ImmutableSet.")

    # Standard protocols: __len__, __repr__, __str__, __iter__
This is an abstract class.
dataset/ETHPy150Open azoft-dev-team/imagrium/env/Lib/sets.py/BaseSet.__init__