Learn what Python decorators are, how they are implemented, and when you should turn to them to simplify your solutions.

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