python中的 @ 修饰符

今天学习廖老师的python教程，碰到了修饰符'@'，不太了解，查看了下官方文档。

简单的整理下：

@dec2
@dec1
def func(arg1, arg2, ...):
pass

　等价于

def func(arg1, arg2, ...):
pass
func = dec2(dec1(func))

使用示例：

在comp.lang.python 和 python-dev的大部分讨论集中在更简捷地使用内置修饰符staticmethod() 和 classmethod() 上。但修饰符的功能远比这强大。下面会对它的使用进行一些讲解：

1.定义一个执行即退出的函数。注意，这个函数并不像通常情况那样，被真正包裹。

def onexit(f):
import atexit
atexit.register(f)
return f

@onexit
def func():
...

（Note that this example is probably not suitable for real usage, but is for example purposes only.）

注意，这个示例可能并不能准确表达在实际中的使用，它只是做一个示例。

2.定义一个只能产生一个实例的类（有实例后，这个类不能再产生新的实例）。注意，一旦这个类失效了(估计意思是保存在下文的singleton中字典中的相应键失效)，就会促使程序员让这个类产生更多的实例。（来自于python-dev的Shane Hathaway）

def singleton(cls):
instances = {}
def getinstance():
if cls not in instances:
instances[cls] = cls()
return instances[cls]
return getinstance

@singleton
class MyClass:
...

3.Add attributes to a function. (Based on an example posted by Anders Munch on python-dev.)

def attrs(**kwds):
def decorate(f):
for k in kwds:
setattr(f, k, kwds[k])
return f
return decorate

@attrs(versionadded="2.2",
author="Guido van Rossum")
def mymethod(f):
...

4.Enforce function argument and return types. Note that this copies the func_name attribute from the old to the new function. func_name was made writable in Python 2.4a3:

def accepts(*types):
def check_accepts(f):
assert len(types) == f.func_code.co_argcount
def new_f(*args, **kwds):
for (a, t) in zip(args, types):
assert isinstance(a, t), \
"arg %r does not match %s" % (a,t)
return f(*args, **kwds)
new_f.func_name = f.func_name
return new_f
return check_accepts

def returns(rtype):
def check_returns(f):
def new_f(*args, **kwds):
result = f(*args, **kwds)
assert isinstance(result, rtype), \
"return value %r does not match %s" % (result,rtype)
return result
new_f.func_name = f.func_name
return new_f
return check_returns

@accepts(int, (int,float))
@returns((int,float))
def func(arg1, arg2):
return arg1 * arg2

5.Declare that a class implements a particular (set of) interface(s). This is from a posting by Bob Ippolito on python-dev based on experience with PyProtocols [27].

def provides(*interfaces):
"""
An actual, working, implementation of provides for
the current implementation of PyProtocols.  Not
particularly important for the PEP text.
"""
def provides(typ):
declareImplementation(typ, instancesProvide=interfaces)
return typ
return provides

class IBar(Interface):
"""Declare something about IBar here"""

@provides(IBar)
class Foo(object):
"""Implement something here..."""