Jack's practical definition of a decorator is:
- A function that takes one argument
- Returns something useful
In many cases, a function decorator can be described more specifically:
- A function that takes one argument (the function being decorated)
- Returns the same function or a function with a similar signature
As Jack states in his talk, a decorator is merely syntactic sugar. The same functionality can be achieved without using the decorator syntax. This code snippet:
@mydecorator def myfunc(): pass
is equivalent to:
def myfunc(): pass myfunc = mydecorator(myfunc)
Here are two of the simplest examples from Jack's talk:
This is the simplest decorator. It does nothing. It takes the decorated function as an argument and returns the same function without doing anything.
def identity(ob): return ob @identity def myfunc(): print "my function" myfunc() print myfunc
my function <function myfunc at 0xb76db17c>
Hello world decorator
I am dumb. This one doesn't do what it's supposed to.
This decorator prints "Hello world" before returning the decorated function.
def helloworld(ob): print "Hello world" return ob @helloworld def myfunc(): print "my function" myfunc() print myfunc
Hello world my function <function myfunc at 0xb78360d4>
A simple decorator that actually does something (and is not broken like the Hello world decorator above)
This decorator is used to print some text before and after calling the decorated function. Most of the time the decorated function is wrapped by a function which calls the decorated function and returns what it returns. ?When is a wrapper not needed?
from functools import wraps def mydecorator(f): @wraps(f) def wrapped(*args, **kwargs): print "Before decorated function" r = f(*args, **kwargs) print "After decorated function" return r return wrapped @mydecorator def myfunc(myarg): print "my function", myarg return "return value" r = myfunc('asdf') print r
Before decorated function my function asdf After decorated function return value
What if I want to pass arguments to the decorator itself (not the decorated function)?
A decorator takes exactly one argument so you will need a factory to create the decorator. Unlike the previous example, notice how the factory function is called with parentheses,
@mydecorator_not_actually(count=5), to produce the real decorator.
from functools import wraps def mydecorator_not_actually(count): def true_decorator(f): @wraps(f) def wrapped(*args, **kwargs): for i in range(count): print "Before decorated function" r = f(*args, **kwargs) for i in range(count): print "After decorated function" return r return wrapped return true_decorator @mydecorator_not_actually(count=5) def myfunc(myarg): print "my function", myarg return "return value" r = myfunc('asdf') print r
Before decorated function Before decorated function Before decorated function Before decorated function Before decorated function my function asdf After decorated function After decorated function After decorated function After decorated function After decorated function return value
References / See also
- Fabric post-run processing Python decorator — posted 2010-11-06
- Using a Python timeout decorator for uploading to S3 — posted 2010-04-27
- Trying out a Retry decorator in Python — posted 2009-11-17
Thats also a very good tutorial in case you are anyone else is interested...
PATX: That is a great article. Thanks!
Try this link also
Its Very usefull
I love and like decorators Chelmsford
Thanks for the examples. Unlike a lot of other Python documentation, I am not finding clear explanations in the online Python documation. So, again, thank you.
Thank you for your decorator related posts (and for many others). Thanks to commenters for additional links to which I'd add the following:
Just wished the second one had been available freely available and, at least, in a less crappy format...
Loved how you start with an ultra simple example in actual Python. Hugely helpful.
Crestfallen how your first example has been crossed out. The next first example is so complicated. Nested decoration! My brain is mush. Suggest a hello world decorator is feasible and deserves a home here. Anyway, here's a much simpler working decorator: https://stackoverflow.com/a...