python 装饰器和 functools 模块

男娘i 2022-07-26 11:17 155阅读 0赞

## 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： ###

@decorator
def foo():
 print 'Hello World!'
# 上面等价于 foo = decorator(foo)

foo()
"""
I'm decorator
Hello World!
"""

decorator(foo)() # 所以这里的输出与 foo() 相同
"""
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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 <td style="margin:0px; padding:0px 0px 0px 15px; border-left-width:1px; border-left-style:solid; border-left-color:rgb(255,255,255); color:rgb(102,102,102)"> <pre style="margin-top:0px; margin-bottom:0px; padding:0px; font-family:Monaco,Menlo,Consolas,'Courier New',monospace; border:none; overflow:auto; background:rgb(238,238,238)">def decorator(func): # 装饰器函数
 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) # 第一次调用
"""
in decorator
in decorator wrapper
calls = 1
x = 1, y = 2
"""

foo(2, 3) # 第二次调用
"""
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

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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</pre> </td> 
 <td style="margin:0px; padding:0px 0px 0px 15px; border-left-width:1px; border-left-style:solid; border-left-color:rgb(255,255,255); color:rgb(102,102,102)"> <pre style="margin-top:0px; margin-bottom:0px; padding:0px; font-family:Monaco,Menlo,Consolas,'Courier New',monospace; border:none; overflow:auto; background:rgb(238,238,238)">class decorator: # 一个装饰器类
 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) # 第一次调用
"""
in decorator __init__
in decorator __call__
calls = 1
x = 1, y = 2
"""

foo(2, 3) # 第二次调用
"""
in decorator __call__
calls = 2
x = 2, y = 3
"""
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### 装饰器参数 ###

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 <td style="margin:0px; padding:0px 15px 0px 0px; color:rgb(153,153,153); border-right-width:1px; border-right-style:solid; border-right-color:rgb(221,221,221); text-align:right"> <pre style="margin-top:0px; margin-bottom:0px; padding:0px; font-family:Monaco,Menlo,Consolas,'Courier New',monospace; border:none; overflow:auto; background:rgb(238,238,238)">1
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</pre> </td> 
 <td style="margin:0px; padding:0px 0px 0px 15px; border-left-width:1px; border-left-style:solid; border-left-color:rgb(255,255,255); color:rgb(102,102,102)"> <pre style="margin-top:0px; margin-bottom:0px; padding:0px; font-family:Monaco,Menlo,Consolas,'Courier New',monospace; border:none; overflow:auto; background:rgb(238,238,238)">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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 <td style="margin:0px; padding:0px 0px 0px 15px; border-left-width:1px; border-left-style:solid; border-left-color:rgb(255,255,255); color:rgb(102,102,102)"> <pre style="margin-top:0px; margin-bottom:0px; padding:0px; font-family:Monaco,Menlo,Consolas,'Courier New',monospace; border:none; overflow:auto; background:rgb(238,238,238)">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。

print int2('11') # 3
print int2('101') # 5
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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：

<table style="margin:0px; padding:0px; border-spacing:0px"> 
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</pre> </td> 
 <td style="margin:0px; padding:0px 0px 0px 15px; border-left-width:1px; border-left-style:solid; border-left-color:rgb(255,255,255); color:rgb(102,102,102)"> <pre style="margin-top:0px; margin-bottom:0px; padding:0px; font-family:Monaco,Menlo,Consolas,'Courier New',monospace; border:none; overflow:auto; background:rgb(238,238,238)">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。

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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</pre> </td> 
 <td style="margin:0px; padding:0px 0px 0px 15px; border-left-width:1px; border-left-style:solid; border-left-color:rgb(255,255,255); color:rgb(102,102,102)"> <pre style="margin-top:0px; margin-bottom:0px; padding:0px; font-family:Monaco,Menlo,Consolas,'Courier New',monospace; border:none; overflow:auto; background:rgb(238,238,238)">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()

# 下面是输出
"""
Calling decorated function
Called example function
"""
print example.__name__ # 'example'
print example.__doc__ # '这里是文档注释'
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