What is the correct (or best) way to subclass the Python set class, adding a new instance variable?

Question

I\'m implementing an object that is almost identical to a set, but requires an extra instance variable, so I am subclassing the built-in set object. What is the best way to

Answer 1

I think that the recommended way to do this is not to subclass directly from the built-in set, but rather to make use of the Abstract Base Class Set available in collections.abc.

Using the ABC Set gives you some methods for free as a mix-in so you can have a minimal Set class by defining only __contains__(), __len__() and __iter__(). If you want some of the nicer set methods like intersection() and difference(), you probably do have to wrap them.

Here's my attempt (this one happens to be a frozenset-like, but you can inherit from MutableSet to get a mutable version):

from collections.abc import Set, Hashable

class CustomSet(Set, Hashable):
    """An example of a custom frozenset-like object using
    Abstract Base Classes.
    """
    __hash__ = Set._hash

    wrapped_methods = ('difference',
                       'intersection',
                       'symetric_difference',
                       'union',
                       'copy')

    def __repr__(self):
        return "CustomSet({0})".format(list(self._set))

    def __new__(cls, iterable=None):
        selfobj = super(CustomSet, cls).__new__(CustomSet)
        selfobj._set = frozenset() if iterable is None else frozenset(iterable)
        for method_name in cls.wrapped_methods:
            setattr(selfobj, method_name, cls._wrap_method(method_name, selfobj))
        return selfobj

    @classmethod
    def _wrap_method(cls, method_name, obj):
        def method(*args, **kwargs):
            result = getattr(obj._set, method_name)(*args, **kwargs)
            return CustomSet(result)
        return method

    def __getattr__(self, attr):
        """Make sure that we get things like issuperset() that aren't provided
        by the mix-in, but don't need to return a new set."""
        return getattr(self._set, attr)

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

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

    def __iter__(self):
        return iter(self._set)