Blog

One of the features added back in Python 2.2 was class methods. These differ from traditional methods in the following ways:

1. They can be called on both the class itself and instances of the class.
2. Rather than binding to an instance, they bind to the class. This means that the first argument passed to the method is a class object rather than an instance.

For most intents and purposes, class methods are written the same way as normal instance methods. One place that things differ is overriding a class method in a subclass. The following simple example demonstrates the problem:

class SubClass(ParentClass):
    @classmethod
    def create(cls, arg):
        ret = ParentClass.create(cls, arg)
        ret.dosomethingelse()
        return ret

This code is broken because the ParentClass.create() call is calling the version of create() method in the context of ParentClass, rather than calling an unbound method like it would with a normal instance method. The most likely outcome will be a TypeError due to the method receiving too many arguments.

So how do you chain up to the parent class implementation? You use the super() object, which was also added in Python 2.2 as an alternative way to chain to the parent implementation of a method. The above code rewritten as follows:

class SubClass(ParentClass):
    @classmethod
    def create(cls, arg):
        ret = super(SubClass, cls).create(arg)
        ret.dosomethingelse()
        return ret

If you haven’t ever used the super() object, this is what it is doing in the above example:

1. SubClass is looked up in the list cls.__mro__ (a linearised list of ancestor classes in the order used for method resolution).
2. The class dict for each ancestor class coming after SubClass in cls.__mro__ is checked to see if it contains “create“.
3. The super() object returns a version of “create” in the context of cls using the __get__(cls) “descriptor get” method.
4. When this bound method gets called, cls will be passed in instead of the parent class.

Previously I’d ignored super() for the most part, since I could use the old chaining syntax. This shows a place where the old-style syntax can’t be applied.