MixIn

MiscUtils.MixIn.MixIn(pyClass, mixInClass, makeAncestor=False, mixInSuperMethods=False)

Mixes in the attributes of the mixInClass into the pyClass.

These attributes are typically methods (but don’t have to be). Note that private attributes, denoted by a double underscore, are not mixed in. Collisions are resolved by the mixInClass’ attribute overwriting the pyClass’. This gives mix-ins the power to override the behavior of the pyClass.

After using MixIn(), instances of the pyClass will respond to the messages of the mixInClass.

An assertion fails if you try to mix in a class with itself.

The pyClass will be given a new attribute mixInsForCLASSNAME which is a list of all mixInClass’ that have ever been installed, in the order they were installed. You may find this useful for inspection and debugging.

You are advised to install your mix-ins at the start up of your program, prior to the creation of any objects. This approach will result in less headaches. But like most things in Python, you’re free to do whatever you’re willing to live with. :-)

There is a bitchin’ article in the Linux Journal, April 2001, “Using Mix-ins with Python” by Chuck Esterbrook, which gives a thorough treatment of this topic.

An example, that resides in the Webware MiddleKit plug-in, is MiddleKit.Core.ModelUser.py, which install mix-ins for SQL interfaces. Search for “MixIn(“.

If makeAncestor is True, then a different technique is employed: a new class is created and returned that is the same as the given pyClass, but will have the mixInClass added as its first base class. Note that this is different from the behavior in legacy Webware versions, where the __bases__ attribute of the pyClass was changed. You probably don’t need to use this and if you do, be aware that your mix-in can no longer override attributes/methods in pyClass.

If mixInSuperMethods is True, then support will be enabled for you to be able to call the original or “parent” method from the mixed-in method. This is done like so:

class MyMixInClass:
def foo(self):
    MyMixInClass.mixInSuperFoo(self)  # call the original method
    # now do whatever you want