The Visitor Pattern in Python
Running nunit-console Under mono on OSX

Runtime Class Mixins with Python

Thursday, January 5th 2012

Say you want to mix in a method into an existing class but can’t get to the class’ definition. In Python, you can use the following decorator to do that.

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# mixin.py

import inspect

def mixin_to(cls):
  def f(fn):
  	if inspect.isfunction(fn):
	  setattr(cls, fn.func_name, fn)
	elif inspect.isclass(fn):
	  for name in dir(fn):
	    attr = getattr(fn, name)
	    if inspect.ismethod(attr):
	      setattr(cls, name, attr.im_func)
	return fn
  return f

This method handles mixing in a single method as well as another class’ methods. You can even mix methods into a class after instantiating it.

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class Unadorned(object):
  pass

# Instantiate an Unadorned
u = Unadorned()

# Mixin in methods from a class
@mixin_to(Unadorned)
class MixinClass(object):
  def mixin_method(self):
    return "mixin_method calls " + self.mixin_function()

# Mixin a function
@mixin_to(Unadorned)
def mixin_function(o):
  return "mixin_function!"

# Use the method mixed into the class
print u.mixin_method() # PRINTS "mixin_method calls mixin_function!"

Wow! That’s nice. You can download mixin.py for your use.