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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:

<function get.<locals>.index at 0x0000000002E45B70>
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\n  \n</ul>\n\n                    \n                </nav>\n\n                <div class="header-banner "> <!-- for optional "do-not-print" class -->\n                    \n        <div id="dive-into-python" class="flex-slideshow slideshow">\n\n            <ul class="launch-shell menu" id="launch-shell">\n                <li>\n                    <a class="button prompt" id="start-shell" data-shell-container="#dive-into-python" href="/shell/">&gt;_\n                        <span class="message">Launch Interactive Shell</span>\n                    </a>\n                </li>\n            </ul>\n\n            <ul class="slides menu">\n                \n                <li>\n                    <div class="slide-code"><pre><code><span class="comment"># Python 3: Fibonacci series up to n</span>\r\n>>> def fib(n):\r\n>>>     a, b = 0, 1\r\n>>>     while a &lt; n:\r\n>>>         print(a, end=\‘ \‘)\r\n>>>         a, b = b, a+b\r\n>>>     print()\r\n>>> fib(1000)\r\n<span class="output">0 1 1 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&nbsp;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&nbsp;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&nbsp;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 &amp; 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&nbsp;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&rsquo;d Expect</h1>\r\n<p>Python knows the usual control flow statements that other languages speak &mdash; <code>if</code>, <code>for</code>, <code>while</code> and <code>range</code> &mdash; with some of its own twists, of course. <a href="//docs.python.org/3/tutorial/controlflow.html">More control flow tools in Python&nbsp;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&rsquo;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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Thousands of machines running Linux, IRIX, Compaq Tru64, OS X, Solaris, and Windows join together to provide a production pipeline used by ~800 users daily. Speed of development is key, and Python was a faster way to code (and re-code) the programs that control this production pipeline.</a>\n                            </blockquote>\n\n                            <table cellpadding="0" cellspacing="0" border="0" width="100%" class="quote-from">\n                                <tbody>\n                                    <tr>\n                                        \n                                        <td><p><a href="/success-stories/industrial-light-magic-runs-python/">Industrial Light &amp; Magic Runs on Python</a> <em>by Tim Fortenberry</em></p></td>\n                                    </tr>\n                                </tbody>\n                            </table>\n                            </div>\n                            \n\n                        </div><!-- end .shrubbery -->\n\n                    </div>\n\n                    <div class="medium-widget applications-widget last">\n                        <div 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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

 

python中对 函数 闭包 的理解

标签:hold   asp   显示   data   cin   version   tornado   local   homepage   

原文地址:http://www.cnblogs.com/zh605929205/p/6689528.html

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