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),{})