Godley/Music-Library

class hashdict(dict):
    """
    hashable dict implementation, suitable for use as a key into
    other dicts.

    based on answers from
       http://stackoverflow.com/questions/1151658/python-hashable-dicts

    """

    def __key(self):
        return tuple(sorted(self.items()))

    def __repr__(self):
        return "{0}({1})".format(self.__class__.__name__, ", ".join(
            "{0}={1}".format(str(i[0]), repr(i[1])) for i in self.__key()))

    def __hash__(self):
        return hash(self.__key())

    def __setitem__(self, key, value):
        raise TypeError("{0} does not support item assignment"
                        .format(self.__class__.__name__))

    def __delitem__(self, key):
        raise TypeError("{0} does not support item assignment"
                        .format(self.__class__.__name__))

    def clear(self):
        raise TypeError("{0} does not support item assignment"
                        .format(self.__class__.__name__))

    def pop(self, *args, **kwargs):
        raise TypeError("{0} does not support item assignment"
                        .format(self.__class__.__name__))

    def popitem(self, *args, **kwargs):
        raise TypeError("{0} does not support item assignment"
                        .format(self.__class__.__name__))

    def setdefault(self, *args, **kwargs):
        raise TypeError("{0} does not support item assignment"
                        .format(self.__class__.__name__))

    def update(self, *args, **kwargs):
        raise TypeError("{0} does not support item assignment"
                        .format(self.__class__.__name__))
    # update is not ok because it mutates the object
    # __add__ is ok because it creates a new object
    # while the new object is under construction, it's ok to mutate it

    def __add__(self, right):
        result = hashdict(self)
        dict.update(result, right)
        return result