Katie Silverio - Decorators, unwrapped: How do they work?

Decorators are a syntactically-pleasing way of modifying the behavior of functions in Python. However, they can be highly opaque to Python beginners. It took me a while to learn how to write one, and even after I was confident writing my own decorators, felt like they were magical. The goal of this talk is to demystify decorators by methodically stepping through how and why they work. Along the way we'll touch on closures, scopes, and how Python is compiled.

https://us.pycon.org/2017/schedule/presentation/557/

Transcript

1. Decorators, unwrapped Katie Silverio @astrosilverio
2. @astrosilverio I use decorators but they are I can show you how they ⚙!
3. @astrosilverio Decorators, unwrapped ✴ Formal definition of a decorator ✴ Anatomy of a decorator ✴ Why it works ✴ Common tricks and gotchas
4. @astrosilverio What is a decorator? ✨examples✨
5. @astrosilverio What is a decorator? @timer def my_function(): # code goes here > my_function() my_function ran in 0.005s > my_function() my_function ran in 0.008s
6. @astrosilverio What is a decorator? class MyClass(object): @classmethod def no_need_for_an_instance(cls): # some code goes here > MyClass.no_need_for_an_instance() some output
7. @astrosilverio A decorator changes the behavior of a function without the user having to change the function code itself Also a decorator can be used to decorate many functions!
8. @astrosilverio Time this! def my_function(arg): # some code goes here return my_return_value > result = my_function(some_value) 0.11s > result = my_function(some_other_value) 1.01s
9. @astrosilverio Timing, constructed import time def my_function(arg): start_time = time.time() # original body of original function end_time = time.time() print end_time - start_time return original_return_value
10. @astrosilverio Timing, decorated import my_timer # how does it work? a mystery @my_timer def my_function(arg): # original body of function return original_return_value @my_timer def foo(bar): # foo the bar return baz
11. @astrosilverio What is a function? ✨a first-class object✨
12. @astrosilverio What is a first-class object? ✴ FCOs can be assigned to variables ✴ FCOs can be passed as arguments to other functions ✴ FCOs can be returned from other functions ✴ FCOs can be created within functions
13. @astrosilverio Refactoring goals ✴ Decouple timing code from function code ✴ Make timing code reusable
14. @astrosilverio Back to timing import time def timing_wrapper(func, func_arg): start_time = time.time() value_to_return = func(func_arg) end_time = time.time() print end_time - start_time return value_to_return > timing_wrapper(my_function, some_arg)
15. @astrosilverio Refactoring goals ✴ Decouple timing code from function code ✴ Make timing code reusable ✴ Wrap functions with timing code at definition time
16. @astrosilverio Back to timing import time def my_timer(func): # creates a new function # that executes `func` AND # performs timing code return new_function timed_function = my_timer(my_function)
17. @astrosilverio Final refactor! import time def my_timer(func): def new_wrapped_function(func_arg): start_time = time.time() value_to_return = func(func_arg) end_time = time.time() print end_time - start_time return value_to_return return new_wrapped_function
18. @astrosilverio Final refactor! import my_timer def foo(bar): # foo the bar return baz foo = my_timer(foo)
19. @astrosilverio Back to decorators From PEP 318:
20. @astrosilverio Back to decorators import my_timer def foo(bar): # foo the bar foo = my_timer(foo) import my_timer @my_timer def foo(bar): # foo the bar is equivalent to
21. @astrosilverio A decorator replaces a function with the decorator's own return value, when called with the function it decorates
22. @astrosilverio General decorator form def my_decorator(func): def new_function(*args, **kwargs): # body probably contains some # new code and probably a call # to func and probably returns # the return value of func # probably return new_function
23. @astrosilverio "Probably" def no_op(func): return func def sleight_of_hand(func): def answer_is_42(*args, **kwargs): return 42 return answer_is_42 def black_hole(func): return None
24. @astrosilverio Open Questions ✴ How does the inner function have access to the wrapped function? ✴ Why do many people use @wraps? ✴ What order are stacked decorators applied in? ✴ How does the @ syntactic sugar work?
25. @astrosilverio Scope def my_function(some_arg): print some_arg # works
26. @astrosilverio Scope def my_function(some_arg): print some_arg # works print locals() > my_function(1) 1 {'some_arg': 1}
27. @astrosilverio Scope def my_function(some_arg): print some_arg # works def inner_function(): print some_arg # works print locals() return inner_function > my_function(42)() 42 42 {'some_arg': 42}
28. @astrosilverio Open Questions ✴ How does the inner function have access to the wrapped function? ✴ Why do many people use @wraps? ✴ What order are stacked decorators applied in? ✴ How does the @ syntactic sugar work?
29. @astrosilverio Timing decorator import time def my_timer(func): def new_wrapped_function(func_arg): start_time = time.time() value_to_return = func(func_arg) end_time = time.time() print end_time - start_time return value_to_return return new_wrapped_function
30. @astrosilverio Misdirection import my_timer @my_timer def foo(bar): # foo the bar! return baz > foo.__name__ new_wrapped_function # ???
31. @astrosilverio Enter @wraps! ✴ Replaces the wrapper's __name__ with the name of the wrapped function ✴ Replaces the wrapper's __doc__ with the docstring of the wrapped function ✴ Does this through decorating the wrapper
32. @astrosilverio Fix it with decorators import time from functools import wraps def my_timer(func): @wraps def new_wrapped_function(func_arg): start_time = time.time() value_to_return = func(func_arg) end_time = time.time() print end_time - start_time return value_to_return return new_wrapped_function
33. @astrosilverio Open Questions ✴ How does the inner function have access to the wrapped function? ✴ Why do many people use @wraps? ✴ What order are stacked decorators applied in? ✴ How does the @ syntactic sugar work?
34. @astrosilverio Decorators can stack! import my_timer import another_decorator @another_decorator @my_timer def foo(bar): # do something! return baz
35. @astrosilverio Remember @my_timer def foo(bar): # do something! foo = my_timer(foo) is equivalent to
36. @astrosilverio What is happening? foo = another_decorator(my_timer(foo)) foo = my_timer(another_decorator(foo)) or
37. @astrosilverio Always wrap down @another_decorator @my_timer def foo(bar): # do something! foo = another_decorator(my_timer(foo)) is equivalent to
38. @astrosilverio More Questions ✴ How does the @ syntactic sugar work? ✴ How do decorators that take arguments work? ✴ How do class decorators work? ✴ How do you write a decorator that is not a function? ✴ How does @wraps work?
39. @astrosilverio How do decorators work?
40. @astrosilverio Decorators, in brief ✴ @decorator is really syntactic sugar for object = decorator(object) ✴ no other rules!
41. @astrosilverio Further Reading ✴ http://simeonfranklin.com/blog/2012/jul/1/ python-decorators-in-12-steps/ ✴ https://www.thecodeship.com/patterns/guide- to-python-function-decorators/ ✴ https://www.python.org/dev/peps/pep-0318/