Expert Python Programming - Third Edition

New Python 3 syntax for metaclasses

Metaclasses are not a new feature and have been available in Python since version 2.2. Anyway, the syntax of this changed significantly and this change is neither backward nor forward compatible. The new syntax is as follows:

class ClassWithAMetaclass(metaclass=type): 
    pass 

In Python 2, this must be written as follows:

class ClassWithAMetaclass(object): 
    __metaclass__ = type 

Class statements in Python 2 do not accept keyword arguments, so Python 3 syntax for defining metaclasses will raise the SyntaxError exception on import. It is still possible to write code using metaclasses that will run on both Python versions, but it requires some extra work. Fortunately, compatibility-related packages such as six ...