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April 11, 2014
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Decorators: A Powerful Weapon in Your Python Arsenal by Colton Myers

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PyCon 2014

April 11, 2014
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Transcript

  1. Decorators A Powerful Weapon in your Python Arsenal Colton Myers

    @basepi ! http://bit.ly/dec-pycon-2014
  2. None

  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!”
  19. None

  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 [email protected] ! ! http://bit.ly/dec-pycon-2014 Thanks!