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"

Asked By: Niklas B.

||

Source

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

Answered By: unutbu

Answer 2

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), 
          {},
          )

Answered By: reclosedev

Answer 3

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),{})

Answered By: sleepycal

Dynamically mixin a base class to an instance in Python

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