Meta-programming in Python

A lightning talk I did for the Amsterdam Python Meetup group about meta-programming in Python.

Transcript

1. Meta-programming in Python Daniël Franke <daniel@ams-sec.org> @ainmosni - http://ams-sec.org
2. What?
3. What? • Objects are instances of classes
4. What? • Objects are instances of classes • Classes are Objects as well
5. What? • Objects are instances of classes • Classes are Objects as well • What are Classes instances of?
6. Classes are objects In [1]: class Spam(object): ...: pass ...: In [2]: type(Spam)
7. Classes are objects In [1]: class Spam(object): ...: pass ...: In [2]: type(Spam) Out[2]: type
8. type: Creating the creators In [1]: Spam = type('Spam', (object,), {'canned': True})
9. type: Creating the creators In [1]: Spam = type('Spam', (object,), {'canned': True}) In [2]: type(Spam) Out[2]: type
10. type: Creating the creators In [1]: Spam = type('Spam', (object,), {'canned': True}) In [2]: type(Spam) Out[2]: type In [3]: canned_spam = Spam() In [4]: type(canned_spam) Out[4]: __main__.Spam
11. type: Creating the creators In [1]: Spam = type('Spam', (object,), {'canned': True}) In [2]: type(Spam) Out[2]: type In [3]: canned_spam = Spam() In [4]: type(canned_spam) Out[4]: __main__.Spam In [5]: canned_spam.canned Out[5]: True In [6]: Spam.canned Out[6]: True
12. A simple metaclass In [1]: class ismeta(type): ...: def __new__(metaclass, name, bases, cls_dict): ...: cls_dict['canned'] = ‘Metacan’ ...: return type.__new__(metaclass, name, bases, cls_dict) ...:
13. A simple metaclass In [1]: class ismeta(type): ...: def __new__(metaclass, name, bases, cls_dict): ...: cls_dict['canned'] = ‘Metacan’ ...: return type.__new__(metaclass, name, bases, cls_dict) ...: In [2]: class Spam(object): ...: __metaclass__ = ismeta ...: pass ...:
14. A simple metaclass In [1]: class ismeta(type): ...: def __new__(metaclass, name, bases, cls_dict): ...: cls_dict['canned'] = ‘Metacan’ ...: return type.__new__(metaclass, name, bases, cls_dict) ...: In [2]: class Spam(object): ...: __metaclass__ = ismeta ...: pass ...: In [3]: Spam.canned Out[3]: 'Metacan'
15. Reading class properties In [1]: class ismeta(type): ...: def __new__(metaclass, name, bases, cls_dict): ...: for key, value in cls_dict.items(): ...: print('{}: {}'.format(key, value)) ...: return type.__new__(metaclass, name, bases, cls_dict) ...:
16. Reading class properties In [1]: class ismeta(type): ...: def __new__(metaclass, name, bases, cls_dict): ...: for key, value in cls_dict.items(): ...: print('{}: {}'.format(key, value)) ...: return type.__new__(metaclass, name, bases, cls_dict) ...: In [2]: class Spam(object): ...: __metaclass__ = ismeta ...: def hello(self): ...: print('Hello byte') ...:
17. Reading class properties In [1]: class ismeta(type): ...: def __new__(metaclass, name, bases, cls_dict): ...: for key, value in cls_dict.items(): ...: print('{}: {}'.format(key, value)) ...: return type.__new__(metaclass, name, bases, cls_dict) ...: In [2]: class Spam(object): ...: __metaclass__ = ismeta ...: def hello(self): ...: print('Hello byte') ...: __module__: __main__ __metaclass__: <class '__main__.ismeta'> hello: <function hello at 0x2acf140>
18. Adding methods via the metaclass In [1]: def hello(self): ...: print('Hello byte!') ...:
19. Adding methods via the metaclass In [1]: def hello(self): ...: print('Hello byte!') ...: In [2]: class ismeta(type): ...: def __new__(metaclass, name, bases, cls_dict): ...: cls_dict['hi'] = hello ...: return type.__new__(metaclass, name, bases, cls_dict) ...:
20. Adding methods via the metaclass In [1]: def hello(self): ...: print('Hello byte!') ...: In [2]: class ismeta(type): ...: def __new__(metaclass, name, bases, cls_dict): ...: cls_dict['hi'] = hello ...: return type.__new__(metaclass, name, bases, cls_dict) ...: In [3]: class Spam(object): ...: __metaclass__ = ismeta ...: pass ...:
21. Adding methods via the metaclass In [1]: def hello(self): ...: print('Hello byte!') ...: In [2]: class ismeta(type): ...: def __new__(metaclass, name, bases, cls_dict): ...: cls_dict['hi'] = hello ...: return type.__new__(metaclass, name, bases, cls_dict) ...: In [3]: class Spam(object): ...: __metaclass__ = ismeta ...: pass ...: In [4]: Spam().hi() Hello byte!
22. Why?
23. Why? • “Self aware” class methods
24. Why? • “Self aware” class methods • Bookkeeping
25. Why? • “Self aware” class methods • Bookkeeping • Slots
26. Why? • “Self aware” class methods • Bookkeeping • Slots • Syntactic sugar
27. Why? • “Self aware” class methods • Bookkeeping • Slots • Syntactic sugar • Obfuscation
28. A “practical” example In [1]: spam_types = {} In [2]: class ismeta(type): ...: def __new__(metaclass, name, bases, cls_dict): ...: new_class = type.__new__(metaclass, name, ...: bases, cls_dict) ...: if not attr_dict.get('abstract', False): ...: spam_types[name] = new_class ...: return new_class In [3]: class Spam(object): ...: __metaclass__ = ismeta ...: abstract = True In [4]: class CannedSpam(Spam): ...: pass
29. A “practical” example In [5]: class EmailSpam(Spam): ...: pass ...: In [6]: spam_types Out[6]: {'CannedSpam': __main__.CannedSpam, 'EmailSpam': __main__. EmailSpam}
30. Thank you Any questions?