按照python官方文檔，函數(shù)定義可以被一個(gè)或者多個(gè)decorator包起來(lái)，解釋如下：

@f1(arg)
@f2
def func(): pass

等同于

def func(): pass
func = f1(arg)(f2(func))

而且：Decorator expressions are evaluated when the function is defined, in the scope that contains the function definition

Example 1. 無(wú)參數(shù)裝飾器

def hello(fn):
    def wrapper():
        print "hello, %s" % fn.__name__
        fn()
        print "goodby, %s" % fn.__name__
    return wrapper
 
@hello
def foo():
    print "i am foo"

解釋

• 編譯器做如下轉(zhuǎn)換：

@hello
def foo     # ===> hello(foo)

• foo = hello(foo) = wrapper
• foo此時(shí)僅僅是一個(gè)函數(shù)，沒(méi)有被調(diào)用，foo()才會(huì)調(diào)用

這種裝飾器，僅僅將在已有函數(shù)的基礎(chǔ)之上，附加額外的裝飾功能。
到底什么時(shí)候調(diào)用，由使用者決定

Example 2. 帶參數(shù)裝飾器

def hello(dummy=""):
    def wrapper(fn):
        print "hello, %s" % fn.__name__
        fn()
        print "goodby, %s" % fn.__name__
    return wrapper
 
@hello("")
def foo():
    print "i am foo"

解釋

• 編譯器做如下轉(zhuǎn)換：

@hello("")
def foo     # ===> hello("")(foo)

• foo = hello("")(foo)
• hello("") = wrapper
• foo = wrapper(foo)
• 此時(shí)編譯器已經(jīng)執(zhí)行wrapper函數(shù)
• 由于wrapper沒(méi)有返回值，foo=None

這種裝飾器就不一樣了，函數(shù)定義期間就被賦予執(zhí)行的行為
設(shè)想：這可以被用來(lái)做某些初始化動(dòng)作

從上面的例子分析可以看到，被decorator的函數(shù)其實(shí)已經(jīng)是另外一個(gè)函數(shù)了，對(duì)于最前面那個(gè)hello.py的例子來(lái)說(shuō)，如果你查詢(xún)一下foo.__name__的話，你會(huì)發(fā)現(xiàn)其輸出的是“wrapper”，而不是我們期望的“foo”，這會(huì)給我們的程序埋一些坑。所以，Python的functool包中提供了一個(gè)叫wrap的decorator來(lái)消除這樣的副作用

from functools import wraps
def hello(fn):
    @wraps(fn)
    def wrapper():
        print "hello, %s" % fn.__name__
        fn()
        print "goodby, %s" % fn.__name__
    return wrapper
 
@hello
def foo():
    '''foo help doc'''
    print "i am foo"
    pass
 
foo()
print foo.__name__ #輸出 foo
print foo.__doc__  #輸出 foo help doc

應(yīng)用1. 給函數(shù)調(diào)用做緩存

from functools import wraps
def memo(fn):
    cache = {}
    miss = object()
 
    @wraps(fn)
    def wrapper(*args):
        result = cache.get(args, miss)
        if result is miss:
            result = fn(*args)
            cache[args] = result
        return result
 
    return wrapper
 
@memo
def fib(n):
    if n < 2:
        return n
    return fib(n - 1) + fib(n - 2)

應(yīng)用2. 注冊(cè)回調(diào)函數(shù)

class MyApp():
    def __init__(self):
        self.func_map = {}
 
    def register(self, name):
        def func_wrapper(func):
            self.func_map[name] = func
            return func
        return func_wrapper
 
    def call_method(self, name=None):
        func = self.func_map.get(name, None)
        if func is None:
            raise Exception("No function registered against - " + str(name))
        return func()
 
app = MyApp()
 
@app.register('/')
def main_page_func():
    return "This is the main page."
 
@app.register('/next_page')
def next_page_func():
    return "This is the next page."
 
print app.call_method('/')
print app.call_method('/next_page')

參考自：coolshell