CODEWITHSUNDEEP
pythonpython-2.7oopinheritancemultiple-inheritance
billy_chapters
billy_chapters

Reputation: 113

In python, how do I call the correct constructor from the method of a mixin class?

Say I have two container classes that behave slightly differently, but have a lot of common methods. I use a mixin class to add these methods to both containers.

For example:

class SpamContainer(Mixin, BaseClass):
    def __init__(self, spam):
        super(SpamContainer, self).__init__(spam)

class DifferentSpamContainer(Mixin, DifferentBaseClass):
    def __init__(self, spam):
        super(DifferentSpamContainer, self).__init__(spam)

class Mixin(object):
    # has no __init__()

    def useful_method(self):
        for s in self.spam:
            # do something

Now I want to add a new method to Mixin that creates and returns a new container object - if self is a SpamContainer, I want to create one of those, whereas if it's a DifferentSpamContainer, I want to create one of those. How can I call the correct constructor from within a method of Mixin?

I thought maybe I could use something like this:

class Mixin(object):
    def method(self, spam):
        return self.__init__(spam)

or 

class Mixin(object):
    def another_method(self, spam):
        return self.__class__(spam)

but I can't get it to work.

I'm using python 2.7.

Upvotes: 1

Views: 1650

Answers (1)

mgilson
mgilson

Reputation: 309929

You use a classmethod:

class Mixin(object):
    @classmethod
    def alternate_constructor(cls, spam):
        return cls(spam)

Note that your use of mixins here seems to be a little off. Frequently, when using inheritance (single or multiple), you'll want to call up to the method on the base-class. e.g. in your case, Mixin and the various classes that it is being mixed with can't both have their constructors called. In fact, you aren't even calling __init__ in BaseClass for SpamContainer for example. This is a bit strange. If you're going to be doing any work with multiple inheritance, I strongly suggest you read Super Considered Super! and companion article Super considered harmful. This will help you to know the pitfalls of multiple inheritance (and how to avoid them).

Upvotes: 3

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