What are some common uses for Python decorators? [closed]

Answer 1

One obvious use is for logging, of course:

import functools

def log(logger, level='info'):
    def log_decorator(fn):
        @functools.wraps(fn)
        def wrapper(*a, **kwa):
            getattr(logger, level)(fn.__name__)
            return fn(*a, **kwa)
        return wrapper
    return log_decorator

# later that day ...
@log(logging.getLogger('main'), level='warning')
def potentially_dangerous_function(times):
    for _ in xrange(times): rockets.get_rocket(NUCLEAR=True).fire()

Answer 2

I use decorators for type checking parameters which are passed to my Python methods via some RMI. So instead of repeating the same parameter counting, exception-raising mumbo-jumbo again and again.

For example, instead of:

def myMethod(ID, name):
    if not (myIsType(ID, 'uint') and myIsType(name, 'utf8string')):
        raise BlaBlaException() ...

I just declare:

@accepts(uint, utf8string)
def myMethod(ID, name):
    ...

and accepts() does all the work for me.

Answer 3

The Twisted library uses decorators combined with generators to give the illusion that an asynchronous function is synchronous. For example:

@inlineCallbacks
def asyncf():
    doStuff()
    yield someAsynchronousCall()
    doStuff()
    yield someAsynchronousCall()
    doStuff()

Using this, code that would have been broken up into a ton of little callback functions can be written quite naturally as a single block, making it a lot easier to understand and maintain.

Answer 4

Decorators are used for anything that you want to transparently "wrap" with additional functionality.

Django uses them for wrapping "login required" functionality on view functions, as well as for registering filter functions.

You can use class decorators for adding named logs to classes.

Any sufficiently generic functionality that you can "tack on" to an existing class or function's behavior is fair game for decoration.

There's also a discussion of use cases on the Python-Dev newsgroup pointed to by PEP 318 -- Decorators for Functions and Methods.

Answer 5

For nosetests, you can write a decorator that supplies a unit test function or method with several sets of parameters:

@parameters(
   (2, 4, 6),
   (5, 6, 11),
)
def test_add(a, b, expected):
    assert a + b == expected

Answer 6

I am using the following decorator for making a function threadsafe. It makes the code more readable. It is almost similar to the one proposed by John Fouhy but the difference is that one work on a single function and that there is no need to create a lock object explicitely.

def threadsafe_function(fn):
    """decorator making sure that the decorated function is thread safe"""
    lock = threading.Lock()
    def new(*args, **kwargs):
        lock.acquire()
        try:
            r = fn(*args, **kwargs)
        except Exception as e:
            raise e
        finally:
            lock.release()
        return r
    return new

class X:
    var = 0

    @threadsafe_function     
    def inc_var(self):
        X.var += 1    
        return X.var