When you use a decorator, you're replacing one function with another. In other words, if you have a decorator
def logged(func): def with_logging(*args, **kwargs): print func.__name__ + " was called" return func(*args, **kwargs) return with_logging
then when you say
@logged def f(x): """does some math""" return x + x * x
it's exactly the same as saying
def f(x): """does some math""" return x + x * x f = logged(f)
and your function f is replaced with the function with_logging. Unfortunately, this means that if you then say
print f.__name__
it will print with_logging because that's the name of your new function. In fact, if you look at the docstring for f, it will be blank because with_logging has no docstring, and so the docstring you wrote won't be there anymore. Also, if you look at the pydoc result for that function, it won't be listed as taking one argument x; instead it'll be listed as taking *args and **kwargs because that's what with_logging takes.
If using a decorator always meant losing this information about a function, it would be a serious problem. That's why we have functools.wraps. This takes a function used in a decorator and adds the functionality of copying over the function name, docstring, arguments list, etc. And since wraps is itself a decorator, the following code does the correct thing:
from functools import wraps def logged(func): @wraps(func) def with_logging(*args, **kwargs): print func.__name__ + " was called" return func(*args, **kwargs) return with_logging @logged def f(x): """does some math""" return x + x * x print f.__name__ # prints 'f' print f.__doc__ # prints 'does some math'
python 装饰器和 functools 模块
什么是装饰器？
在 python 语言里第一次看到装饰器不免让人想到设计模式中的装饰模式——动态地给一个对象添加一些额外的职责，就增加功能来说，装饰模式比生成子类更为灵活。
好吧，python 中的装饰器显然和装饰模式毫无关系。那 python 中的装饰器到底是什么呢？
简而言之，装饰器提供了一种方法，在函数和类定义语句的末尾插入自动运行代码。python 中有两种装饰器：函数装饰器和类装饰器。
函数装饰器
简单的装饰器例子：
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def decorator(F):
print "I'm decorator"
return F
@decorator
def foo():
print 'Hello World!'
foo()
"""
I'm decorator
Hello World!
"""
decorator(foo)()
"""
I'm decorator
Hello World!
"""
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从上面运行后结果看出，装饰器就是一个能够返回可调用对象（函数）的可调用对象（函数）。
具有封闭作用域的装饰器
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def decorator(func):
print 'in decorator'
def wrapper(*args):
print 'in decorator wrapper'
wrapper._calls += 1
print "calls = %d" % (wrapper._calls)
func(*args)
wrapper._calls = 0
return wrapper
@decorator
def foo(x, y):
print "x = %d, y = %d" % (x, y)
foo(1, 2)
"""
in decorator
in decorator wrapper
calls = 1
x = 1, y = 2
"""
foo(2, 3)
"""
in decorator wrapper
calls = 2
x = 2, y = 3
"""
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可以看出第一次调用 foo(1, 2) 时，相当于
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foo = decorator(foo)
foo(1, 2)
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第二次调用 foo(2, 3) 时 foo 已经为 decorator(foo) 的返回值了
再看看一个装饰器类来实现的：
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class decorator:
def __init__(self, func):
print 'in decorator __init__'
self.func = func
self.calls = 0
def __call__(self, *args):
print 'in decorator __call__'
self.calls += 1
print "calls = %d" % (self.calls)
self.func(*args)
@decorator
def foo(x, y):
print "x = %d, y = %d" % (x, y)
foo(1, 2)
"""
in decorator __init__
in decorator __call__
calls = 1
x = 1, y = 2
"""
foo(2, 3)
"""
in decorator __call__
calls = 2
x = 2, y = 3
"""
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装饰器参数
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def decorator_wrapper(a, b):
print 'in decorator_wrapper'
print "a = %d, b = %d" % (a, b)
def decorator(func):
print 'in decorator'
def wrapper(*args):
print 'in wrapper'
func(*args)
return wrapper
return decorator
@decorator_wrapper(1, 2) # 这里先回执行 decorator_wrapper(1, 2), 返回 decorator 相当于 @decorator
def foo(word):
print word
foo('Hello World!')
"""
in decorator_wrapper
a = 1, b = 2
in decorator
in wrapper
Hello World!
"""
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functools 模块
functools 模块中有三个主要的函数 partial(), update_wrapper() 和 wraps(), 下面我们分别来看一下吧。
partial(func[,args][, *keywords])
看源码时发现这个函数不是用 python 写的，而是用 C 写的，但是帮助文档中给出了用 python 实现的代码，如下：
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def partial(func, *args, **keywords):
def newfunc(*fargs, **fkeywords):
newkeywords = keywords.copy()
newkeywords.update(fkeywords)
return func(*(args + fargs), **newkeywords)
newfunc.func = func
newfunc.args = args
newfunc.keywords = keywords
return newfunc
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OK，可能一下子没看明白，那么继续往下看，看一下是怎么用的。我们知道 python 中有个 int([x[,base]]) 函数，作用是把字符串转换为一个普通的整型。如果要把所有输入的二进制数转为整型，那么就要这样写 int('11', base=2)。这样写起来貌似不太方便，那么我们就能用 partial 来实现值传递一个参数就能转换二进制数转为整型的方法。
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from functools import partial
int2 = partial(int, base=2)
print int2('11')
print int2('101')
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update_wrapper(wrapper, wrapped[, assigned][, updated])
看这个函数的源代码发现，它就是把被封装的函数的 module, name, doc 和 dict 复制到封装的函数中去，源码如下，很简单的几句：
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WRAPPER_ASSIGNMENTS = ('__module__', '__name__', '__doc__')
WRAPPER_UPDATES = ('__dict__',)
def update_wrapper(wrapper, wrapped, assigned = WRAPPER_ASSIGNMENTS, updated = WRAPPER_UPDATES):
for attr in assigned:
setattr(wrapper, attr, getattr(wrapped, attr))
for attr in updated:
getattr(wrapper, attr).update(getattr(wrapped, attr, {}))
return wrapper
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具体如何用我们可以往下看一下。
wraps(wrapped[, assigned][, updated])
wraps() 函数把用 partial() 把 update_wrapper() 给封装了一下。
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def wraps(wrapped, assigned = WRAPPER_ASSIGNMENTS, updated = WRAPPER_UPDATES):
return partial(update_wrapper, wrapped=wrapped,
assigned=assigned, updated=updated)
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好，接下来看一下是如何使用的，这才恍然大悟，一直在很多开源项目的代码中看到如下使用。
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from functools import wraps
def my_decorator(f):
@wraps(f)
def wrapper(*args, **kwds):
print 'Calling decorated function'
return f(*args, **kwds)
return wrapper
@my_decorator
def example():
"""这里是文档注释"""
print 'Called example function'
example()
"""
Calling decorated function
Called example function
"""
print example.__name__
print example.__doc__
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