Decorators: A Powerful Weapon in Your Python Arsenal by Colton Myers

Transcript

1. Decorators A Powerful Weapon in your Python Arsenal Colton Myers @basepi ! http://bit.ly/dec-pycon-2014
3. Come visit our booth!
4. Open space tonight Room 523A 17:00-19:00
5. Salt Sprint on Monday
6. The Plan • What is a decorator? • How are decorators constructed? • How do they work? • The “right” way to make decorators • Examples
7. What is a decorator?
8. @my_decorator def my_awesome_function(): pass
9. Decorators wrap functions
10. Decorators wrap functions • Add functionality • Modify behavior • Perform setup/teardown • Diagnostics (timing, etc)
11. But first…
12. What is a function?
13. Everything in Python is an object.
14. Functions are objects
15. def myfunc(): print(‘hello!’)
16. myfunc() #prints “hello!” f = myfunc f() #prints “hello!” def myfunc(): print(‘hello!’)
17. Functions can create other functions
18. def make_printer(word): def inner(): print(word) return inner ! ! p = make_printer(‘such wow!’) p() #prints “such wow!”
20. This is called a closure
21. def make_printer(word): def inner(): print(word) return inner
22. def make_printer(word): def inner(): print(word) return inner
23. Decorators are closures
24. Decorators are closures …usually.
25. (Decorators can also be created with classes)
26. def make_printer(word): def inner(): print(word) return inner
27. def my_decorator(wrapped): def inner(*args, **kwargs): return wrapped(*args, **kwargs) return inner
28. def my_decorator(wrapped): def inner(*args, **kwargs): return wrapped(*args, **kwargs) return inner @my_decorator def myfunc(): pass
29. def myfunc(): pass def my_decorator(wrapped): def inner(*args, **kwargs): return wrapped(*args, **kwargs) return inner myfunc = my_decorator(myfunc)
30. def my_decorator(wrapped): def inner(*args, **kwargs): return wrapped(*args, **kwargs) return inner @my_decorator def myfunc(): pass
31. def shout(wrapped): def inner(*args, **kwargs): print(‘BEFORE!’) ret = wrapped(*args, **kwargs) print(‘AFTER!’) return ret return inner @shout def myfunc(): print(‘such wow!’)
32. def shout(wrapped): def inner(*args, **kwargs): print(‘BEFORE!’) ret = wrapped(*args, **kwargs) print(‘AFTER!’) return ret return inner @shout def myfunc(): print(‘such wow!’)
33. def shout(wrapped): def inner(*args, **kwargs): print(‘BEFORE!’) ret = wrapped(*args, **kwargs) print(‘AFTER!’) return ret return inner @shout def myfunc(): print(‘such wow!’)
34. >>> myfunc() BEFORE! such wow! AFTER!
35. How does this work?
36. def shout(wrapped): def inner(*args, **kwargs): print(‘BEFORE!’) ret = wrapped(*args, **kwargs) print(‘AFTER!’) return ret return inner @shout def myfunc(): print(‘such wow!’)
37. def shout(wrapped): def inner(*args, **kwargs): print(‘BEFORE!’) ret = wrapped(*args, **kwargs) print(‘AFTER!’) return ret return inner myfunc = shout(myfunc) def myfunc(): print(‘such wow!’)
38. def shout(wrapped): def inner(*args, **kwargs): print(‘BEFORE!’) ret = wrapped(*args, **kwargs) print(‘AFTER!’) return ret return inner myfunc = shout(myfunc) def myfunc(): print(‘such wow!’)
39. def shout(wrapped): def inner(*args, **kwargs): print(‘BEFORE!’) ret = wrapped(*args, **kwargs) print(‘AFTER!’) return ret return inner myfunc = shout(myfunc) def myfunc(): print(‘such wow!’)
40. def shout(wrapped): def inner(*args, **kwargs): print(‘BEFORE!’) ret = wrapped(*args, **kwargs) print(‘AFTER!’) return ret return inner myfunc = shout(myfunc) def myfunc(): print(‘such wow!’)
41. def shout(wrapped): def inner(*args, **kwargs): print(‘BEFORE!’) ret = wrapped(*args, **kwargs) print(‘AFTER!’) return ret return inner myfunc = shout(myfunc) def myfunc(): print(‘such wow!’)
42. def shout(wrapped): def inner(*args, **kwargs): print(‘BEFORE!’) ret = wrapped(*args, **kwargs) print(‘AFTER!’) return ret return inner myfunc = shout(myfunc) def myfunc(): print(‘such wow!’)
43. def shout(wrapped): def inner(*args, **kwargs): print(‘BEFORE!’) ret = wrapped(*args, **kwargs) print(‘AFTER!’) return ret return inner myfunc = shout(myfunc) def myfunc(): print(‘such wow!’)
44. Good decorators are versatile
45. def shout(wrapped): def inner(*args, **kwargs): print(‘BEFORE!’) ret = wrapped(*args, **kwargs) print(‘AFTER!’) return ret return inner myfunc = shout(myfunc) def myfunc(): print(‘such wow!’)
46. def shout(wrapped): def inner(*args, **kwargs): print(‘BEFORE!’) ret = wrapped(*args, **kwargs) print(‘AFTER!’) return ret return inner myfunc = shout(myfunc) def myfunc(): print(‘such wow!’)
47. *args and **kwargs together take any number of positional and/or keyword arguments
48. *args is a list **kwargs is a dictionary
49. def shout(wrapped): def inner(*args, **kwargs): print(‘BEFORE!’) ret = wrapped(*args, **kwargs) print(‘AFTER!’) return ret return inner myfunc = shout(myfunc) def myfunc(): print(‘such wow!’)
50. def shout(wrapped): def inner(*args, **kwargs): print(‘BEFORE!’) ret = wrapped(*args, **kwargs) print(‘AFTER!’) return ret return inner myfunc = shout(myfunc) def myfunc(): print(‘such wow!’)
51. *args becomes args[0], args[1], args[2], …
52. **kwargs becomes key0=kwargs[key0], key1=kwargs[key1], …
53. def shout(wrapped): def inner(*args, **kwargs): print(‘BEFORE!’) ret = wrapped(*args, **kwargs) print(‘AFTER!’) return ret return inner myfunc = shout(myfunc) def myfunc(): print(‘such wow!’)
54. This decorator is still wrong.
55. >>> myfunc.__name__ ‘inner’
56. def shout(wrapped): def inner(*args, **kwargs): print(‘BEFORE!’) ret = wrapped(*args, **kwargs) print(‘AFTER!’) return ret inner.__name__ = wrapped.__name__ return inner myfunc = shout(myfunc) def myfunc(): print(‘such wow!’)
57. >>> import inspect >>> inspect.getargspec(myfunc) ArgSpec(args=[], varargs='args', keywords='kwargs', defaults=None)
58. Graham Dumpleton created wrapt http://bit.ly/decorators2014
59. import wrapt ! @wrapt.decorator def pass_through(wrapped, instance, args, kwargs): return wrapped(*args, **kwargs) ! @pass_through def function(): pass
60. import wrapt ! @wrapt.decorator def pass_through(wrapped, instance, args, kwargs): return wrapped(*args, **kwargs) ! @pass_through def function(): pass
61. What about decorators with arguments?
62. @unittest.skipIf() (simplified)
63. def skipIf(conditional, message): def dec(wrapped): def inner(*args, **kwargs): if not conditional: return wrapped(*args, **kwargs) else #skipping print(message) return inner return dec @skipIf(True, ‘I hate doge’) def myfunc(): print(‘very print’)
64. def skipIf(conditional, message): def dec(wrapped): def inner(*args, **kwargs): if not conditional: return wrapped(*args, **kwargs) else #skipping print(message) return inner return dec myfunc = skipIf(True, ‘I hate doge’)(myfunc) def myfunc(): print(‘very print’)
65. def skipIf(conditional, message): def dec(wrapped): def inner(*args, **kwargs): if not conditional: return wrapped(*args, **kwargs) else #skipping print(message) return inner return dec myfunc = skipIf(True, ‘I hate doge’)(myfunc) def myfunc(): print(‘very print’)
66. def skipIf(conditional, message): def dec(wrapped): def inner(*args, **kwargs): if not conditional: return wrapped(*args, **kwargs) else #skipping print(message) return inner return dec myfunc = skipIf(True, ‘I hate doge’)(myfunc) def myfunc(): print(‘very print’)
67. def skipIf(conditional, message): def dec(wrapped): def inner(*args, **kwargs): if not conditional: return wrapped(*args, **kwargs) else #skipping print(message) return inner return dec myfunc = skipIf(True, ‘I hate doge’)(myfunc) def myfunc(): print(‘very print’)
68. def skipIf(conditional, message): def dec(wrapped): def inner(*args, **kwargs): if not conditional: return wrapped(*args, **kwargs) else #skipping print(message) return inner return dec myfunc = skipIf(True, ‘I hate doge’)(myfunc) def myfunc(): print(‘very print’)
69. def skipIf(conditional, message): def dec(wrapped): def inner(*args, **kwargs): if not conditional: return wrapped(*args, **kwargs) else #skipping print(message) return inner return dec myfunc = skipIf(True, ‘I hate doge’)(myfunc) def myfunc(): print(‘very print’)
70. def skipIf(conditional, message): def dec(wrapped): def inner(*args, **kwargs): if not conditional: return wrapped(*args, **kwargs) else #skipping print(message) return inner return dec myfunc = skipIf(True, ‘I hate doge’)(myfunc) def myfunc(): print(‘very print’)
71. def skipIf(conditional, message): def dec(wrapped): def inner(*args, **kwargs): if not conditional: return wrapped(*args, **kwargs) else #skipping print(message) return inner return dec myfunc = skipIf(True, ‘I hate doge’)(myfunc) def myfunc(): print(‘very print’)
72. def skipIf(conditional, message): def dec(wrapped): def inner(*args, **kwargs): if not conditional: return wrapped(*args, **kwargs) else #skipping print(message) return inner return dec @skipIf(True, ‘I hate doge’) def myfunc(): print(‘very print’)
73. What about with wrapt?
74. import wrapt ! def with_arguments(myarg1, myarg2): @wrapt.decorator def wrapper(wrapped, instance, args, kwargs): return wrapped(*args, **kwargs) return wrapper ! @with_arguments(1, 2) def function(): pass
75. import wrapt ! def with_arguments(myarg1, myarg2): @wrapt.decorator def wrapper(wrapped, instance, args, kwargs): return wrapped(*args, **kwargs) return wrapper ! @with_arguments(1, 2) def function(): pass
76. import wrapt ! def with_arguments(myarg1, myarg2): @wrapt.decorator def wrapper(wrapped, instance, args, kwargs): return wrapped(*args, **kwargs) return wrapper ! @with_arguments(1, 2) def function(): pass
77. A few last examples
78. Counting function calls
79. def count(wrapped): def inner(*args, **kwargs): inner.counter += 1 return wrapped(*args, **kwargs) inner.counter = 0 return inner @count def myfunc(): pass
80. def count(wrapped): def inner(*args, **kwargs): inner.counter += 1 return wrapped(*args, **kwargs) inner.counter = 0 return inner @count def myfunc(): pass
81. >>> myfunc() >>> myfunc() >>> myfunc() >>> myfunc.counter 3
82. import time def timer(wrapped): def inner(*args, **kwargs): t = time.time() ret = wrapped(*args, **kwargs) print(time.time()-t) return ret return inner @timer def myfunc(): print(‘so example!’)
83. import time def timer(wrapped): def inner(*args, **kwargs): t = time.time() ret = wrapped(*args, **kwargs) print(time.time()-t) return ret return inner @timer def myfunc(): print(‘so example!’)
84. >>> myfunc() so example! 4.0531158447265625e-06
85. Colton Myers @basepi colton.myers@gmail.com ! ! http://bit.ly/dec-pycon-2014 Thanks!