Factory function to create a class with an unknown number of mixins

Question

I am wanting to make a function that creates a new class from a variable amount of so-called mixin classes. My first instinct is to use *args ...

>>> def mixins(*args):
        class Foo(*args):

SyntaxError: invalid syntax

... but didn't get very far ... Since that didn't work, I tried this, which seems to work:

>>> def mixins(*args):
        class Foo(args[0]):
            pass
        for arg in args[1:]:
            class Foo(Foo, arg):
                pass
        return Foo

Question

Are there other approaches to solving this problem?

Motivation

I have created an Abstract Base Class that has many abstract methods. I have several types of subclasses, which each implement some of the needed abstract methods but not all of them. Sometimes I can create a working instance by mixing in 2 subclasses, sometimes it takes more to implement all of the abstract methods.

My instinct tells me there must be a better approach. Can you explain exactly what the motivation of doing that? Perhaps using class-decorators would be more suitable. Does your problem require an inheritance-based solution? — shx2, Mar 12 '13 at 06:10
@shx2 I have added text about the motivation for the question. — Wesley Baugh, Mar 12 '13 at 06:15

grc · Accepted Answer · 2013-03-12T06:20:24.510

2

You can do this using the type function:

def mixins(*args):
    return type('Foo', args, {})

You might also want to make sure that the class always inherits from object:

def mixins(*args):
    bases = args if object in args else args + (object,)
    return type('Foo', bases, {})

edited Mar 12 '13 at 06:20

answered Mar 12 '13 at 06:15

grc

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Excellent! I remember reading about the `type` function recently in this excellent (and long) answer for [What is a metaclass in Python?](http://stackoverflow.com/a/6581949/1988505). – Wesley Baugh Mar 12 '13 at 06:21

GodMan · Answer 2 · 2013-03-12T06:19:21.550

Yes. Why not?

Here is how you can make class objects on the fly:

type('a',(object,),{})

This will create a class object of type - type and which derives from the object base class and whose attributes are an empty dictionary.

So, in your case,the call would be:

type('Foo',args,{})

Here, args will be contain all the base classes you want to inherit Foo from. and the resultant would be a clas object which you can use to return from mixins.

Factory function to create a class with an unknown number of mixins

Question

Motivation

2 Answers2