helpers.py

def extend_instance(instance, *bases, **kwargs):
    """
    Apply subclass (mixin) to a class object or its instance

    By default, the mixin is placed at the start of bases
    to ensure its called first as per MRO. If you wish to
    have it injected last, which is useful for monkeypatching,
    then you can specify 'last=True'. See here:
    http://stackoverflow.com/a/10018792/1267398

    :attr cls: Target object
    :type cls: Class instance

    :attr bases: List of new bases to subclass with

    :attr last: Inject new bases after existing bases
    :type last: bool

    >>> class A(object): pass
    >>> class B(object): pass
    >>> a = A()
    >>> b = B()
    >>> isinstance(b, A)
    False
    >>> extend_instance(b, A)
    >>> isinstance(b, A)
    True
    """
    last = kwargs.get('last', False)
    bases = tuple(bases)
    for base in bases:
        assert inspect.isclass(base), "bases must be classes"
    assert not inspect.isclass(instance)
    base_cls = instance.__class__
    base_cls_name = instance.__class__.__name__
    new_bases = (base_cls,)+bases if last else bases+(base_cls,)
    new_cls = type(base_cls_name, tuple(new_bases), {})
    setattr(instance, '__class__', new_cls)


def extend_class(cls, *bases, **kwargs):
    """
    Add bases to class (late subclassing)

    >>> class A(object): pass
    >>> class B(object): pass
    >>> class C(object): pass
    >>> issubclass(B, A)
    False
    >>> D = extend_class(B, A)
    >>> issubclass(D, A)
    True
    >>> issubclass(D, B)
    True
    """
    last = kwargs.get('last', False)
    bases = tuple(bases)
    for base in bases:
        assert inspect.isclass(base), "bases must be classes"

    new_bases = (cls,)+bases if last else bases+(cls,)
    new_cls = type(cls.__name__, tuple(new_bases), {})
    return new_cls