Recipe for MixIn''''''s in PythonLanguage which allows in-place runtime mixing and unmixing. As a bonus, there is a 'functional-style' mixin function, which preserves the base class.

def mixIn (base, addition):
    """Mixes in place, i.e. the base class is modified.
    Tags the class with a list of names of mixed members.
    """
    assert not hasattr(base, '_mixed_')
    mixed = []
    for item, val in addition.__dict__.items():
        if not hasattr(base, item):
            setattr(base, item, val)
            mixed.append (item)
    base._mixed_ = mixed


def unMix (cla):
    """Undoes the effect of a mixin on a class. Removes all attributes that
    were mixed in -- so even if they have been redefined, they will be
    removed.
    """
    for m in cla._mixed_: #_mixed_ must exist, or there was no mixin
        delattr(cla, m)
    del cla._mixed_
            

def mixedIn (base, addition):
    """Same as mixIn, but returns a new class instead of modifying
    the base.
    """
    class newClass: pass
    newClass.__dict__ = base.__dict__.copy()
    mixIn (newClass, addition)
    return newClass

----
Old recipe

 def mixin (existingClass, mixinClass):
 	for item, val in mixinClass.__dict__.items():
 		if not hasattr(existingclass, item):
 			setattr(existingclass, item, val)

This copies not just functions, but any class members.

Example usage:

 class addSubMixin:
  	def add(self, value):
 		return self.number + value

  	def subtract(self, value):
  		return self.number - value


 class myClass:
  	def __init__(self, number):
  		self.number = number

Then, at runtime, you can mix any class into any other with:

 mixin(myClass, addSubMixin)
 myInstance = myClass(4)
 myInstance.add(2)
 myInstance.subtract(2)


----
why not do it like this:

 class myClass(myClass, myMixin)
   pass

----
because it is not at runtime.

Person.__bases__ assignment can be made at runtime, for e.g. driver selection.

''CAUTION'' the assignment is to the class.

----

Good examples of MixIn''''''s in the PythonLanguage are in Socket''''''Server module from standard library.

such as:

	class ForkingUDPServer(F''''''orkingMixIn, UDPServer): pass
	class ForkingTCPServer(F''''''orkingMixIn, TCPServer): pass

	class ThreadingUDPServer(T''''''hreadingMixIn, UDPServer): pass
	class ThreadingTCPServer(T''''''hreadingMixIn, TCPServer): pass

You just choose the flavors and add them in. Wants forking? add Forking''''''Mix''''''In.

see also Twisted Python and wxPython for a lot of MixIn uses.

--JuneKim

----
This does not work for new-style classes, right?

     



----
See MixIn.
----
CategoryPython