A metaclass adds a new instance of a mutable class variable to each subclass of MutableBase without the need to write this attribute into the subclass' definition, i.e., users of MutableBase need not know about it.

gistfile1.py

class MetaBase(type):
    """
    Metaclass for MutableBase.

    This metaclass is one of two possible solutions to having a per-class
    mutable class attribute. The per-class aspect is tricky because the
    attribute is a mutable object. It is thus shared among MutableBase and all
    its subclasses.

    Shared state of the mutable class attribute is avoided with this metaclass
    by creating each class with a separate mutable instance.
    """
    def __new__(mcls, cls_name, cls_bases, cls_dct):
        """
        Adds a unique `dict` instance to each (sub)class.
        """
        cls_dct["mutable_attr"] = dict()
        return super(MetaBase, mcls).__new__(mcls, cls_name, cls_bases, cls_dct)