python decorators

An easy introduction to python decorators (this deck is mostly code examples). Presented to the Memphis Python User group, December 2014.

1. decorators
2. Use python 3!
3. What is a Decorator?
4. # An example from Flask @app.route("/hello") def some_view(): return "Hello World!"
5. # An example from Flask @app.route("/hello") def some_view(): return "Hello World!"
6. # An example from Django @login_required def some_view(request): # ... return HttpResponse(content)
7. # An example from Django @login_required def some_view(request): # ... return HttpResponse(content)
8. class MyThing: @property def value(self): return self.calculate_value() def calculate_value(self): # do some calculations here >>> thing = MyThing() >>> thing.value 42
9. class MyThing: @property def value(self): return self.calculate_value() def calculate_value(self): # do some calculations here >>> thing = MyThing() >>> thing.value 42
10. class MyThing: @property def value(self): return self.calculate_value() def calculate_value(self): # do some calculations here >>> thing = MyThing() >>> thing.value 42
11. But First!
 Did you know…
12. # Generalized arguments def foo(*args, **kwargs): print("args: {}".format(args)) print("kwargs: {}".format(kwargs))
13. # Generalized arguments def foo(*args, **kwargs): print("args: {}".format(args)) print("kwargs: {}".format(kwargs))
14. # Generalized arguments def foo(*args, **kwargs): print("args: {}".format(args)) print("kwargs: {}".format(kwargs)) >>> foo('a single arg') # args: ('a single arg',) # kwargs: {}
15. # Generalized arguments def foo(*args, **kwargs): print("args: {}".format(args)) print("kwargs: {}".format(kwargs)) >>> foo('a single arg') # args: ('a single arg',) # kwargs: {} >>> foo('arg1', 'arg2', kw1='keyword1', kw2='keyword2') # args: ('arg1', 'arg2') # kwargs: {'kw1': ‘keyword1', ‘kw2': 'keyword2'}
16. explicit > implicit
17. # You can put functions inside of functions? def outside(): name = "i'm outside!" def inside(): print(name) inside()
18. # You can put functions inside of functions? def outside(): name = "i'm outside!" def inside(): print(name) inside() >>> outside() # i'm outside!
19. # Functions are fist-class citizens? def citizen_func(): pass >>> citizen_func.__class__ # '<class 'function'>' >>> issubclass(citizen_func.__class__, object) # True
20. # Functions are fist-class citizens? def citizen_func(): pass >>> citizen_func.__class__ # '<class 'function'>' >>> issubclass(citizen_func.__class__, object) # True
21. # Functions are fist-class citizens? def citizen_func(): pass >>> citizen_func.__class__ # '<class 'function'>' >>> issubclass(citizen_func.__class__, object) # True
22. # You can pass functions as # arguments to functions: def callme(msg): print(msg) def carly(f, y): f(y) carly(callme, "maybe") # maybe
23. # You can pass functions as # arguments to functions: def callme(msg): print(msg) def carly(f, y): f(y) carly(callme, "maybe") # maybe
24. # You can pass functions as # arguments to functions: def callme(msg): print(msg) def carly(f, y): f(y) carly(callme, "maybe") # maybe
25. # You can pass functions as # arguments to functions: def callme(msg): print(msg) def carly(f, y): f(y) carly(callme, "maybe") # maybe
26. # A Holiday decorator! def holiday(f): def wrapper(*args, **kwargs): result = f(*args, **kwargs) return "‚ {0} ‚".format(result) return wrapper
27. # A Holiday decorator! def holiday(f): def wrapper(*args, **kwargs): result = f(*args, **kwargs) return "‚ {0} ‚".format(result) return wrapper @holiday def happy(): return "Happy Holidays!” # >>> happy() # “‚ Happy Holidays! ‚”
28. # A Holiday decorator! def holiday(f): def wrapper(*args, **kwargs): result = f(*args, **kwargs) return "‚ {0} ‚".format(result) return wrapper @holiday def happy(): return "Happy Holidays!” # >>> happy() # “‚ Happy Holidays! ‚”
29. # A simple, DIY timing decorator import time def timed(func): """Decorator that prints time info.""" def wrapper(*args, **kwargs): start = time.time() result = func(*args, **kwargs) end = time.time() print(func.__name__, end - start) return result return wrapper
30. # A simple, DIY timing decorator import time def timed(func): """Decorator that prints time info.""" def wrapper(*args, **kwargs): start = time.time() result = func(*args, **kwargs) end = time.time() print(func.__name__, end - start) return result return wrapper
31. # A simple, DIY timing decorator import time def timed(func): """Decorator that prints time info.""" def wrapper(*args, **kwargs): start = time.time() result = func(*args, **kwargs) end = time.time() print(func.__name__, end - start) return result return wrapper
32. # A simple, DIY timing decorator import time def timed(func): """Decorator that prints time info.""" def wrapper(*args, **kwargs): start = time.time() result = func(*args, **kwargs) end = time.time() print(func.__name__, end - start) return result return wrapper
33. # apply the decorator @timed def foo(): """A Foo function.""" print("Foo.")
34. # apply the decorator @timed def foo(): """A Foo function.""" print("Foo.")
35. # Or using the old-style syntax. def foo(): """A Foo function.""" print("Foo.") old_foo = timed(old_foo)
36. # Or using the old-style syntax. def foo(): """A Foo function.""" print("Foo.") foo = timed(foo)
37. # Problems! >>> foo.__name__ # Expected: 'foo' >>> foo.__doc__ # Should show docstring
38. # Problems! >>> foo.__name__ # Expected: 'foo' >>> foo.__doc__ # Should show docstring wrapper None
39. # Use functools.wraps to keep that meta-data! from functools import wraps def timed(func): """Better timing decorator.""" @wraps(func) def wrapper(*args, **kwargs): start = time.time() result = func(*args, **kwargs) end = time.time() print(func.__name__, end - start) return result return wrapper
40. # Use functools.wraps to keep that meta-data! from functools import wraps def timed(func): """Better timing decorator.""" @wraps(func) def wrapper(*args, **kwargs): start = time.time() result = func(*args, **kwargs) end = time.time() print(func.__name__, end - start) return result return wrapper
41. # Better! >>> foo.__name__ # Expected: 'foo' >>> foo.__doc__ # Should show docstring # AND! The original function is # in __wrapped__ >>> foo.__wrapped__()
42. # A Practical Example. Logging function Calls. import time def logged(func): @wraps(func) def wrapper(*args, **kwargs): result = func(*args, **kwargs) msg = "[{}] {} called with args: {}, kwargs: {}" print(msg.format( time.ctime(), func.__name__, args, kwargs )) return result return wrapper
43. @logged def super_funk(*args, **kwargs): pass >>> super_funk('Blammo', kap='pow') # [Mon Nov 17 16:38:44 2014] super_funk called # with args: ('Blammo',), kwargs: {'kap': 'pow'}
44. from decimal import Decimal def currency(func): @wraps(func) def wrapper(*args, **kwargs): result = func(*args, **kwargs) result = Decimal(str(result)) result = result.quantize(Decimal(".01")) return "$ {0}".format(result) return wrapper
45. @currency def calculate_tax(value, rate=0.1): return value * rate >>> calculate_tax(100))) # “$ 10.00”
46. # Decorators with arguments def currency(symbol='$'): def decorate(func): @wraps(func) def wrapper(*args, **kwargs): result = func(*args, **kwargs) result = Decimal(str(result)) result = result.quantize(Decimal(".01")) return "{} {}".format(symbol, result) return wrapper return decorate
47. # Decorators with arguments def currency(symbol='$'): def decorate(func): @wraps(func) def wrapper(*args, **kwargs): result = func(*args, **kwargs) result = Decimal(str(result)) result = result.quantize(Decimal(".01")) return "{} {}".format(symbol, result) return wrapper return decorate
48. # Decorators with arguments def currency(symbol='$'): def decorate(func): @wraps(func) def wrapper(*args, **kwargs): result = func(*args, **kwargs) result = Decimal(str(result)) result = result.quantize(Decimal(".01")) return "{} {}".format(symbol, result) return wrapper return decorate Take it another level!
49. # Decorators with arguments def currency(symbol='$'): def decorate(func): @wraps(func) def wrapper(*args, **kwargs): result = func(*args, **kwargs) result = Decimal(str(result)) result = result.quantize(Decimal(".01")) return "{} {}".format(symbol, result) return wrapper return decorate
50. @currency(symbol="£") def calculate_tax(value, rate=0.1): return value * rate >>> calculate_tax(100) # ‘£ 10.00’
51. Resources # Python Cookbook, 3rd ed. # Chapter 9, Metaprogramming http://www.jeffknupp.com/blog/2013/11/29/improve-your-python-decorators-explained/ http://simeonfranklin.com/blog/2012/jul/1/python-decorators-in-12-steps/ https://realpython.com/blog/python/primer-on-python-decorators/ http://python-3-patterns-idioms-test.readthedocs.org/en/latest/PythonDecorators.html