While I like to think of myself as a reasonably competent Python coder, one aspect of the language I’ve never been able to grok is decorators.
I know what they are (superficially), I’ve read tutorials, examples, questions on Stack Overflow, and I understand the syntax, can write my own, occasionally use @classmethod and @staticmethod, but it never occurs to me to use a decorator to solve a problem in my own Python code. I never encounter a problem where I think, “Hmm…this looks like a job for a decorator!”
So, I’m wondering if you guys might offer some examples of where you’ve used decorators in your own programs, and hopefully I’ll have an “A-ha!” moment and get them.
回答 0
我将装饰器主要用于计时目的
def time_dec(func):def wrapper(*arg):
t = time.clock()
res = func(*arg)print func.func_name, time.clock()-t
return res
return wrapper
@time_decdef myFunction(n):...
As pointed out in the comments, since Python 2.5 you can use a with statement in conjunction with a threading.Lock (or multiprocessing.Lock since version 2.6) object to simplify the decorator’s implementation to just:
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() ...
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.
回答 6
当然,一种明显的用途是用于日志记录:
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()
I use them mainly for debugging (wrapper around a function that prints its arguments and result) and verification (e.g. to check if an argument is of correct type or, in the case of web application, if the user has sufficient privileges to call a particular method).
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)exceptExceptionas e:raise e
finally:
lock.release()return r
return new
class X:
var =0@threadsafe_functiondef inc_var(self):
X.var +=1return X.var
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
Decorators are used either to define a function’s properties or as boilerplate that alters it; it’s possible but counter-intuitive for them to return completely different functions. Looking at the other responses here, it seems like one of the most common uses is to limit the scope of some other process – be it logging, profiling, security checks, etc.
CherryPy uses object-dispatching to match URLs to objects and, eventually, methods. Decorators on those methods signal whether or not CherryPy is even allowed to use those methods. For example, adapted from the tutorial:
class HelloWorld:
...
def secret(self):
return "You shouldn't be here."
@cherrypy.expose
def index(self):
return "Hello world!"
cherrypy.quickstart(HelloWorld())
I used them recently, while working on social networking web application. For Community/Groups, I was supposed to give membership authorization to create new discussion and reply to a message you have to be the member of that particular group. So, I wrote a decorator @membership_required and put that where I required in my view.
回答 11
我用这个装饰器来修复参数
def fill_it(arg):if isinstance(arg, int):return"wan"+ str(arg)else:try:# number present as stringif str(int(arg))== arg:return"wan"+ arg
else:# This should never happenedraiseException("I dont know this "+ arg)print"What arg?"exceptValueError, e:return arg
def fill_wanname(func):def wrapper(arg):
filled = fill_it(arg)return func(filled)return wrapper
@fill_wannamedef get_iface_of(wanname):global __iface_config__
return __iface_config__[wanname]['iface']
def fill_it(arg):
if isinstance(arg, int):
return "wan" + str(arg)
else:
try:
# number present as string
if str(int(arg)) == arg:
return "wan" + arg
else:
# This should never happened
raise Exception("I dont know this " + arg)
print "What arg?"
except ValueError, e:
return arg
def fill_wanname(func):
def wrapper(arg):
filled = fill_it(arg)
return func(filled)
return wrapper
@fill_wanname
def get_iface_of(wanname):
global __iface_config__
return __iface_config__[wanname]['iface']
this written when I refactor some functions need to passed argument “wanN” but in my old codes, I passed N or ‘N’ only
回答 12
装饰器可用于轻松创建函数方法变量。
def static_var(varname, value):'''
Decorator to create a static variable for the specified function
@param varname: static variable name
@param value: initial value for the variable
'''def decorate(func):
setattr(func, varname, value)return func
return decorate
@static_var("count",0)def mainCallCount():
mainCallCount.count +=1
Decorator can be used to easily create function method variables.
def static_var(varname, value):
'''
Decorator to create a static variable for the specified function
@param varname: static variable name
@param value: initial value for the variable
'''
def decorate(func):
setattr(func, varname, value)
return func
return decorate
@static_var("count", 0)
def mainCallCount():
mainCallCount.count += 1