Decorators Explained: A Powerful Tool That Should Be in Your Python Toolbelt.
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The document provides an in-depth explanation of decorators in Python, highlighting their functionality as wrappers that allow injecting code before or after function calls. It discusses various implementation techniques, including function and class decorators, and presents examples such as counting function calls, caching results with memoization, and enforcing type checks. Additionally, it outlines practical applications of decorators for retries, logging, and authorization.
![Example: memoize
class Memoize(object):
def __init__(self, func):
self.cache = {}
self.func = func
def __call__(self, *args, **kwargs):
cache_key = '%s%s' % (args, kwargs)
if cache_key in self.cache:
return self.cache[cache_key]
else:
result = self.func(*args, **kwargs)
self.cache[cache_key] = result
return result
@Memoize
def function(bound):
sum = 0
for x in range(bound):
sum += x
return sum](https://image.slidesharecdn.com/decorators-2-140907225936-phpapp02/85/Decorators-Explained-A-Powerful-Tool-That-Should-Be-in-Your-Python-Toolbelt-37-320.jpg)
![Example: type checker
def type_check(*types):
def receiver(func):
def wrapper(*args, **kwargs):
for index, type_arg in enumerate(types):
if type(args[index]) != type_arg:
raise TypeError('Expected %s at index %s' %
(type_arg, index))
return func(*args, **kwargs)
return wrapper
return receiver
@type_check(int, int, str)
def function(first, second, third):
print 'success'](https://image.slidesharecdn.com/decorators-2-140907225936-phpapp02/85/Decorators-Explained-A-Powerful-Tool-That-Should-Be-in-Your-Python-Toolbelt-39-320.jpg)
![Example: singleton
instances = {}
def singleton(cls):
def create(*args):
if cls not in instances:
instances[cls] = cls(*args)
return instances[cls]
return create
@singleton
class Toolbelt(object):
def __init__(self):
pass](https://image.slidesharecdn.com/decorators-2-140907225936-phpapp02/85/Decorators-Explained-A-Powerful-Tool-That-Should-Be-in-Your-Python-Toolbelt-41-320.jpg)
Decorators Explained: A Powerful Tool That Should Be in Your Python Toolbelt.
- 1. Decorators Explained A Powerful Tool That Should Be in Your Python Toolbelt
- 2. Samuel Fortier-Galarneau Software Developer @ Digium, Inc @samuelg http://github.com/samuelg
- 4. Agenda • Primer • What is a decorator? • How do they work? • Examples
- 5. Primer Functions • Functions in Python are first class members • They can be passed to other functions and return new functions def add(first, second): return first + second def partial(func, first): def wrapper(second): return func(first, second) return wrapper
- 6. Primer Functions >>> func = partial(add, 1) >>> func(2) 3
- 7. Primer Variable function arguments • Functions can have variable positional and keyword arguments via *args and **kwargs • *args and **kwargs can be packed again and passed along to another function def func(*args, **kwargs): print args print kwargs
- 8. Primer Variable function arguments >>> func(1, 2, my_arg=‘value') (1, 2) {'my_arg': 'value'}
- 9. Primer Variable function arguments def func(*args, **kwargs): other_func(*args, **kwargs) def other_func(arg1, arg2, arg3): print arg1 print arg2 print arg3
- 10. Primer Variable function arguments >>> func(1, 2, arg3=3) 1 2 3
- 11. What is a decorator? • Injects code before/after a function call or object creation • A callable wrapper around a callable resource (object that implements __call__ is callable) • Similar to macros in C/C++ but uses built in Python syntax and language semantics
- 12. What is a decorator? • Can be implemented as a function or a class • Can be applied to a function or a class • Functions have an implicit __call__ method which makes them callable • Classes must implement a __call__ method to qualify as a decorator
- 13. How do they work? Decorator function def decorator(func): def wrapper(): print 'in wrapper()' return func() return wrapper @decorator def my_function(): print 'in my_function()'
- 14. How do they work? Decorator function >>> my_function() in wrapper() in my_function()
- 15. How do they work? Decorator function def decorator(func): def wrapper(): print 'in wrapper()' return func() return wrapper @decorator def my_function(): print 'in my_function()'
- 16. How do they work? Decorator function def decorator(func): def wrapper(): print 'in wrapper()' return func() return wrapper @decorator def my_function(): print 'in my_function()'
- 17. How do they work? Decorator function def decorator(func): def wrapper(): print 'in wrapper()' return func() return wrapper @decorator def my_function(): print 'in my_function()'
- 18. How do they work? Decorator function def decorator(func): def wrapper(): print 'in wrapper()' return func() return wrapper @decorator def my_function(): print 'in my_function()'
- 19. How do they work? Decorator function wrapper is a closure over func def decorator(func): def wrapper(): print 'in wrapper()' return func() return wrapper @decorator def my_function(): print 'in my_function()'
- 20. How do they work? Decorator function def decorator(func): def wrapper(): print 'in wrapper()' return func() return wrapper @decorator def my_function(): print 'in my_function()'
- 21. How do they work? Decorator class class Decorator(object): def __init__(self, func): self.func = func def __call__(self): print 'in __call__()' return self.func() @Decorator def my_function(): print 'in my_function()'
- 22. How do they work? Decorator class >>> my_function() in __call__() in my_function()
- 23. How do they work? Decorator function with arguments def decorator(*args, **kwargs): def receiver(func): def wrapper(): print 'in wrapper()' print args print kwargs return func() return wrapper return receiver @decorator(1, config='value') def my_function(): print 'in my_function()'
- 24. How do they work? Decorator function with arguments >>> my_function() in wrapper() (1,) {'config': 'value'} in my_function()
- 25. How do they work? Decorator function with arguments def decorator(*args, **kwargs): def receiver(func): def wrapper(): print 'in wrapper()' print args print kwargs return func() return wrapper return receiver @decorator(1, config='value') def my_function(): print 'in my_function()'
- 26. How do they work? Decorator function with arguments def decorator(*args, **kwargs): def receiver(func): def wrapper(): print 'in wrapper()' print args print kwargs return func() return wrapper return receiver @decorator(1, config='value') def my_function(): print 'in my_function()'
- 27. How do they work? Decorator function with arguments def decorator(*args, **kwargs): def receiver(func): def wrapper(): print 'in wrapper()' print args print kwargs return func() return wrapper return receiver @decorator(1, config='value') def my_function(): print 'in my_function()'
- 28. How do they work? Decorator function with arguments def decorator(*args, **kwargs): def receiver(func): def wrapper(): print 'in wrapper()' print args print kwargs return func() return wrapper return receiver @decorator(1, config='value') def my_function(): print 'in my_function()'
- 29. How do they work? Decorator function with arguments def decorator(*args, **kwargs): def receiver(func): def wrapper(): print 'in wrapper()' print args print kwargs return func() return wrapper return receiver @decorator(1, config='value') def my_function(): print 'in my_function()'
- 30. How do they work? Decorator function with arguments def decorator(*args, **kwargs): def receiver(func): def wrapper(): print 'in wrapper()' print args print kwargs return func() return wrapper return receiver @decorator(1, config='value') def my_function(): print 'in my_function()'
- 31. How do they work? Decorator function with arguments def decorator(*args, **kwargs): def receiver(func): def wrapper(): print 'in wrapper()' print args print kwargs return func() return wrapper return receiver @decorator(1, config='value') def my_function(): print 'in my_function()'
- 32. How do they work? Decorator function with arguments def decorator(*args, **kwargs): def receiver(func): def wrapper(): print 'in wrapper()' print args print kwargs return func() return wrapper return receiver @decorator(1, config='value') def my_function(): print 'in my_function()'
- 33. How do they work? Decorator class with arguments class Decorator(object): def __init__(self, *args, **kwargs): self.args = args self.kwargs = kwargs def __call__(self, func): def wrapper(): print 'in __call__()' print self.args print self.kwargs return func() return wrapper @Decorator(1, config='value') def my_function(): print 'in my_function()'
- 34. How do they work? Decorator class with arguments >>> my_function() in __call__() (1,) {'config': 'value'} in my_function()
- 35. Example: counting function calls class Counter(object): def __init__(self, func): self.count = 0 self.func = func def __call__(self): self.count += 1 return self.func() def getCount(self): return self.count @Counter def function(): pass
- 36. Example: counting function calls >>> function() >>> function() >>> function() >>> function.getCount() 3
- 37. Example: memoize class Memoize(object): def __init__(self, func): self.cache = {} self.func = func def __call__(self, *args, **kwargs): cache_key = '%s%s' % (args, kwargs) if cache_key in self.cache: return self.cache[cache_key] else: result = self.func(*args, **kwargs) self.cache[cache_key] = result return result @Memoize def function(bound): sum = 0 for x in range(bound): sum += x return sum
- 38. Example: memoize >>> function(100000000) ~ 11 seconds >>> function(100000000) ~ 10 milliseconds
- 39. Example: type checker def type_check(*types): def receiver(func): def wrapper(*args, **kwargs): for index, type_arg in enumerate(types): if type(args[index]) != type_arg: raise TypeError('Expected %s at index %s' % (type_arg, index)) return func(*args, **kwargs) return wrapper return receiver @type_check(int, int, str) def function(first, second, third): print 'success'
- 40. Example: type checker >>> function(1, 2, ‘works') success >>> function('will', 'not', ‘work') Traceback (most recent call last): File "type.py", line 17, in <module> function('will', 'not', 'work') File "type.py", line 7, in wrapper raise TypeError('Expected %s at index %s' % (type_arg, index)) TypeError: Expected <type 'int'> at index 0
- 41. Example: singleton instances = {} def singleton(cls): def create(*args): if cls not in instances: instances[cls] = cls(*args) return instances[cls] return create @singleton class Toolbelt(object): def __init__(self): pass
- 42. Example: singleton >>> toolbelt = Toolbelt() >>> another = Toolbelt() >>> another == toolbelt True
- 43. Other uses • Retries • Pre/Post conditions • Function input/output transforms • Performance logging • Authorization
- 44. Thanks! Samuel Fortier-Galarneau @samuelg http://github.com/samuelg http://www.slideshare.net/sgalarne/decorators-2