Python Metaclasses

A talk i did at PyConZA 2013

1. Python Meta Classes Kisitu Augustine Software Developer at ThoughtWorks Twitter: @austiine04 Github : austiine04

2. SOME BASICS

3. Everything is an object in python.

4. Classes create instances. Class Foo( object): def _ _ init _ _(self, bar ) : self.bar = bar f = Foo(‘Alex Bar’ )

5. type( f )

6. Creating new types

7. Class Foo( object): pass

8. Class Foo: pass

9. Foo = type( ‘Foo’ , (), {})

10. type( cls ,* args ,* * kwargs )

11. type() is actually not a function. It is a META CLASS.

12. A special kind of class that creates classes.

13. type(name, bases, cls_dct )

14. Class Foo ( object): def _ _ init _ _(self, bar ) : self.bar = bar At runtime class Foo is an instance of type

15. Defining a meta class class Meta(type): d ef _ _ init _ _( cls , name, bases, dict ): pass def _ _new_ _(meta, name, bases, dct ): pass def _ _call_ _ ( cls , * args , ** kwargs ): pass *remember to call super in each of method you override

16. _ _new_ _() vs _ _ init _ _()

17. class Foo(object): _ _ metaclass _ _ = Meta def _ _ init _ _(self): pass

18. class Foo( metaclass = Meta): def _ _ init _ _(self): pass

19. Show us the code

20. Example #1 Making a class final

21. Example #2 Decorating class methods

22. def log( function): def wrapper_function ( * args , ** kwargs ): print “Calling ……….”, function .__name __ return function(* args , ** kwargs ) return wrapper_function

23. Some advanced basics

24. A class is an instance of its metaclass at runtime.

25. Metaclasses go down the inheritance chain.

26. Things can get quite ugly if you are inheriting from multiple classes each with its own meta class.

27. With great power comes great responsibility

28. Questions ???