python中对 函数 闭包 的理解
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最近学到 函数 闭包的时候，似懂非懂、迷迷糊糊的样子，很是头疼，今天就特意查了下关于闭包的知识，现将我自己的理解分享如下！
一、python 闭包定义
首先，关于闭包，百度百科是这样解释的：
闭包是指可以包含自由（未绑定到特定对象）变量的代码块；这些变量不是在这个代码块内或者任何全局上下文中定义的，而是在定义代码块的环境中定义（局部变量）。“闭包” 一词来源于以下两者的结合：要执行的代码块（由于自由变量被包含在代码块中，这些自由变量以及它们引用的对象没有被释放）和为自由变量提供绑定的计算环境（作用域）。
当然，这种学术性的解释对于我们小白来说，只能是更加的晦涩难懂。
从python对闭包的定义来说：
必须是内部定义的函数，该函数包含对外部作用域而不是全局作用域名字的引用！
可能这么说还是有点难理解，那咱们换一种方式理解：
python中的闭包从表现形式上可以解释为：如果在一个内部函数里，对在外部作用域（但不是在全局作用域）的变量进行引用，那么内部函数就被认为是闭包.
举个简单的例子来说明，可能会好理解一些：
def f1():
x = 12
def f2():
print(x)
return f2
f = f1()
print(type(f))
print(f)
f()
代码执行之后的结果如下：
<class ‘function‘>
<function f1.<locals>.f2 at 0x00000000028EC9D8>
12
结合这段简单的代码和定义来说明闭包：
如果在一个内部函数里：f2()就是这个内部函数，
对在外部作用域（但不是在全局作用域）的变量进行引用：x就是被引用的变量，x在外部作用域f1()里面，但不在全局作用域里。
则这个内部函数f2()就是一个闭包。
从函数的定义角度出发：函数可以嵌套使用，可以把函数当成参数或返回值进行传递，函数也可以作为容器类型的元素，作为一个变量可以去赋值。
在这个简单的例子当中，函数f1()中嵌套定义另一个函数f2()，内部的函数f2()引用了外部函数f1()的变量，函数f2()被当成夹带外部变量的 对象 返回给f1()，这样就形成一个闭包函数。
实例化输出的时候 定义的 f=f1()中， 其实 f 就是函数f2()(不准确但是好理解)。这里需要注意的是，f 是一个函数名，加上（）就能当作函数来用了。（因为f2()返回的是一串函数执行过程的包）
二、闭包的用途
1、闭包就是为了不动原函数里面的代码，还要给它增加‘新功能’的一种手段。
最直接的例子就是爬取网站代码的闭包函数，不使用的时候，不用执行闭包函数，当需要爬取网站数据时，直接将定义的函数名后加上（），当作函数来使用。
1 from urllib.request import urlopen 2 def get(url): 3 def index(): 4 return urlopen(url).read() 5 return index 6 7 python=get(‘http://www.python.org‘) 8 print(python) #主函数的内存 9 print(python()) #查看爬取网址之后，获得的网址的具体内容 10 print(python.__closure__[0].cell_contents) #将下载下来的内容翻译，以网址的形式显示下载的网站。
执行结果如下：
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2 3 5 8 13 21 34 55 89 144 233 377 610 987</span></code></pre></div>\n <div class="slide-copy"><h1>Functions Defined</h1>\r\n<p>The core of extensible programming is defining functions. Python allows mandatory and optional arguments, keyword arguments, and even arbitrary argument lists. <a href="//docs.python.org/3/tutorial/controlflow.html#defining-functions">More about defining functions in Python 3</a></p></div>\n </li>\n \n <li>\n <div class="slide-code"><pre><code><span class="comment"># Python 3: List comprehensions</span>\r\n>>> fruits = [\‘Banana\‘, \‘Apple\‘, \‘Lime\‘]\r\n>>> loud_fruits = [fruit.upper() for fruit in fruits]\r\n>>> print(loud_fruits)\r\n<span class="output">[\‘BANANA\‘, \‘APPLE\‘, \‘LIME\‘]</span>\r\n\r\n<span class="comment"># List and the enumerate function</span>\r\n>>> list(enumerate(fruits))\r\n<span class="output">[(0, \‘Banana\‘), (1, \‘Apple\‘), (2, \‘Lime\‘)]</span></code></pre></div>\n <div class="slide-copy"><h1>Compound Data Types</h1>\r\n<p>Lists (known as arrays in other languages) are one of the compound data types that Python understands. Lists can be indexed, sliced and manipulated with other built-in functions. <a href="//docs.python.org/3/tutorial/introduction.html#lists">More about lists in Python 3</a></p></div>\n </li>\n \n <li>\n <div class="slide-code"><pre><code><span class="comment"># Python 3: Simple arithmetic</span>\r\n>>> 1 / 2\r\n<span class="output">0.5</span>\r\n>>> 2 ** 3\r\n<span class="output">8</span>\r\n>>> 17 / 3 <span class="comment"># classic division returns a float</span>\r\n<span class="output">5.666666666666667</span>\r\n>>> 17 // 3 <span class="comment"># floor division</span>\r\n<span class="output">5</span></code></pre></div>\n <div class="slide-copy"><h1>Intuitive Interpretation</h1>\r\n<p>Calculations are simple with Python, and expression syntax is straightforward: the operators <code>+</code>, <code>-</code>, <code>*</code> and <code>/</code> work as expected; parentheses <code>()</code> can be used for grouping. <a href="http://docs.python.org/3/tutorial/introduction.html#using-python-as-a-calculator">More about simple math functions in Python 3</a>.</p></div>\n </li>\n \n <li>\n <div class="slide-code"><pre><code><span class="comment"># Python 3: Simple output (with Unicode)</span>\r\n>>> print("Hello, I\‘m Python!")\r\n<span class="output">Hello, I\‘m Python!</span>\r\n\r\n<span class="comment"># Input, assignment</span>\r\n>>> name = input(\‘What is your name?\\n\‘)\r\n>>> print(\‘Hi, %s.\‘ % name)\r\n<span class="output">What is your name?\r\nPython\r\nHi, Python.</span></code></pre></div>\n <div class="slide-copy"><h1>Quick & Easy to Learn</h1>\r\n<p>Experienced programmers in any other language can pick up Python very quickly, and beginners find the clean syntax and indentation structure easy to learn. <a href="//docs.python.org/3/tutorial/">Whet your appetite</a> with our Python 3 overview.</p>\r\n </div>\n </li>\n \n <li>\n <div class="slide-code"><pre><code><span class="comment"># For loop on a list</span>\r\n>>> numbers = [2, 4, 6, 8]\r\n>>> product = 1\r\n>>> for number in numbers:\r\n... product = product * number\r\n... \r\n>>> print(\‘The product is:\‘, product)\r\n<span class="output">The product is: 384</span></code></pre></div>\n <div class="slide-copy"><h1>All the Flow You’d Expect</h1>\r\n<p>Python knows the usual control flow statements that other languages speak — <code>if</code>, <code>for</code>, <code>while</code> and <code>range</code> — with some of its own twists, of course. <a href="//docs.python.org/3/tutorial/controlflow.html">More control flow tools in Python 3</a></p></div>\n </li>\n \n </ul>\n </div>\n\n\n </div>\n\n \n <div class="introduction">\n <p>Python is a programming language that lets you work quickly <span class="breaker"></span>and integrate systems more effectively. <a class="readmore" href="/doc/">Learn More</a></p>\n </div>\n\n\n </div><!-- end .container -->\n </header>\n\n <div id="content" class="content-wrapper">\n <!-- Main Content Column -->\n <div class="container">\n\n <section class="main-content " role="main">\n\n \n \n\n \n\n \n\n <div class="row">\n\n <div class="small-widget get-started-widget">\n <h2 class="widget-title"><span aria-hidden="true" class="icon-get-started"></span>Get Started</h2>\r\n<p>Whether you\‘re new to programming or an experienced developer, it\‘s easy to learn and use Python.</p>\r\n<p><a href="/about/gettingstarted/">Start with our Beginner’s Guide</a></p>\n </div>\n\n <div class="small-widget download-widget">\n <h2 class="widget-title"><span aria-hidden="true" class="icon-download"></span>Download</h2>\n<p>Python source code and installers are available for download for all versions! 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2、通过外面的一层层的函数传递的参数，让最内层的函数可以直接调用外层函数所有参数，从而实现不动原函数的代码，增加新功能的办法。
在python中，最恰当的例子就是装饰器！不动源代码，通过装饰器给源代码加上新的功能！
1 import time 2 3 def timer(func): 4 def wrapper(*args,**kwargs): 5 start_time = time.time() 6 func(*args,**kwargs) 7 stop_time = time.time() 8 print("run time is %s"%(stop_time-start_time)) 9 return wrapper 10 11 @timer 12 def index(): #index = timer(index) 13 time.sleep(1.5) 14 print("welcome to beijing!") 15 16 @timer 17 def home(name): 18 time.sleep(1) 19 print("welcome to %s home page"%name) 20 21 @timer 22 def auth(name,passwd): 23 time.sleep(0.5) 24 print(name,passwd) 25 26 @timer 27 def tell(): 28 print("---***---") 29 30 index() 31 home("alex") 32 auth("alex","alex3714") 33 tell()
执行结果如下：
welcome to beijing! run time is 1.5000858306884766 welcome to alex home page run time is 1.0000572204589844 alex alex3714 run time is 0.5000283718109131 ---***--- run time is 0.0
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