prototypes.py

# There are a dozen ways to implement the Prototype/Properties Pattern in Python.
#
# You could use copy.copy(), or program with classes and classmethods only.
# for example.
#
# But if you want to use an existing library in this style, and don't want to loose
# Python's flexibility (multiple inheritance, etc) I believe this solution works best.
#
# Prototypes, in Pythonic terms, are objects that behave like instances and classes/types,
# unifying instantiation and inheritance. So the most obvious solution, even if wasteful,
# is to have a thin layer over a type and it's single instance.

# TODOs:
#  - A way to add/remove prototypes from objects


class BaseObject:
    """ Could be any class that you want to adapt to the prototyped style
    """
    pass


class PObject:
    __slots__ = ['ptype', 'pinstance', 'pname', 'pbases']

    def __init__(self, pname='Object', ptype=None, pinstance=None,
                 pbases=(BaseObject,), **attribs):
        self.pname = pname
        self.ptype = ptype
        self.pinstance = pinstance
        self.pbases = pbases
        if pinstance is not None:
            self.pinstance.__dict__.update(attribs)

    def clone(self, name=None, *args, **kw):
        name = name or self.pname
        bases = self.pbases if self.ptype is None else (self.ptype,)
        new_ptype = type(name, bases, {})
        new_pinstance = new_ptype(*args, **kw)
        attrs = self.pinstance.__dict__ if self.pinstance is not None else {}
        return PObject(name, new_ptype, new_pinstance, **attrs)

    def add_method(self, func):
        setattr(self.ptype, func.__name__, func)

    def __getattr__(self, name):
        return getattr(self.pinstance, name)

    def __setattr__(self, name, value):
        if name in self.__slots__:
            super().__setattr__(name, value)
        else:
            self.pinstance.__setattr__(name, value)

    def __repr__(self):
        return '<Proto: {}>'.format(self.pname)

# Root object
Object = PObject().clone()

### Example
person = Object.clone('Person')
person.name = 'John Doe'
person.arms = 2
@person.add_method
def say_name(self):
    print(self.name)

# Anna retains the number of arms of its parent
person1 = person.clone()
person1.name = 'Anna Doe'
person1.say_name()