Python Metaclasses
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A talk i did at PyConZA 2013
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Python Metaclasses
- 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)
- 9. Foo = type(‘Foo’, (), {})
- 11. type() is actually not a function. It is a META CLASS.
- 12. A special kind of class that creates classes.
- 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): def _ _init_ _(cls, name, bases, dict): pass def _ _new_ _(meta, name, bases, dct): pass def _ _call_ _(cls, *args, **kwargs): pass
- 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
- 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 ???
Notes de l'éditeur
- Special objects that create classes.
- A little bit of live code to define a function called foo and assign it to a variable and pass it around as an argument.
- Explain new style classes in python 2 and how python 3 classes inherit from object implicitly
- We call type to determine the type(data type) of an object
- Explain how we can use type to create new classes
- Python 2 new style classes
- Python 3 syntax
- Explain how we can use type to determine the data type or class of a python object for example call type on the sys module
- Type is the default meta class used to create our classes
- When u create a class, that class is an instance of type at runtime.Class Foo: passFoo.__class__
- This is the signature of the type meta class constructor.
- Calling type on the Foo class will return type
- __new__() should be overidden when you want to control the creation of the object which happens to be the class__init__() should be overidden when you want to control initiailization of the object after it has been created.__call__() is called after class creation, when the already created class is called to create instances
- _ _new_ _ is called to create the class while _ _init_ _ is called after the class has been created to initialize the class
- Demonstrate inheriting from the bool class. Create a meta class to create a final
- Class type on class Foo and Foo.__class__
- I think essentially the rule is that the metaclass of the derived class must a subclass of all metaclasses of the bases.