Dynamically mixin a base class to an instance in Python

Question

Is it possible to add a base class to an object instance (not a class!) at runtime? Something along the lines of how Object#extend works in Ruby:

class Gentleman(object):
  def introduce_self(self):
    return "Hello, my name is %s" % self.name

class Person(object):
  def __init__(self, name):
    self.name = name

p = Person("John")
# how to implement this method?
extend(p, Gentleman)
p.introduce_self() # => "Hello, my name is John"

Answer 1

This dynamically defines a new class GentlePerson , and reassigns p 's class to it:

class Gentleman(object):
  def introduce_self(self):
    return "Hello, my name is %s" % self.name

class Person(object):
  def __init__(self, name):
    self.name = name

p = Person("John")
p.__class__ = type('GentlePerson',(Person,Gentleman),{})
print(p.introduce_self())
# "Hello, my name is John"

Per your request, this modifies p 's bases, but does not alter p 's original class Person . Thus, other instances of Person are unaffected (and would raise an AttributeError if introduce_self were called).

---

Although it was not directly asked in the question, I'll add for googlers and curiosity seekers, that it is also possible to dynamically change a class's bases but (AFAIK) only if the class does not inherit directly from object :

class Gentleman(object):
  def introduce_self(self):
    return "Hello, my name is %s" % self.name

class Base(object):pass
class Person(Base):
  def __init__(self, name):
    self.name = name

p = Person("John")
Person.__bases__=(Gentleman,object,)
print(p.introduce_self())
# "Hello, my name is John"

q = Person("Pete")
print(q.introduce_self())
# Hello, my name is Pete

Answer 2

Slightly cleaner version:

def extend_instance(obj, cls):
    """Apply mixins to a class instance after creation"""
    base_cls = obj.__class__
    base_cls_name = obj.__class__.__name__
    obj.__class__ = type(base_cls_name, (base_cls, cls),{})

Answer 3

Although it's already answered, here is a function:

def extend(instance, new_class):
    instance.__class__ = type(
          '%s_extended_with_%s' % (instance.__class__.__name__, new_class.__name__), 
          (instance.__class__, new_class), 
          {},
          )