〖Python〗-- 函数闭包的理解
2024-04-08 02:50:58
【函数闭包的理解】
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 f2f = 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。
from urllib.request import urlopen
def get(url):
def index():
return urlopen(url).read()
return index
python=get('http://www.python.org')
print(python) #主函数的内存
print(python()) #查看爬取网址之后，获得的网址的具体内容
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>
b'<!doctype html>\n<!--[if lt IE 7]> <html class="no-js ie6 lt-ie7 lt-ie8 lt-ie9"> <![endif]-->\n<!--[if IE 7]> <html class="no-js ie7 lt-ie8 lt-ie9"> <![endif]-->\n<!--[if IE 8]> <html class="no-js ie8 lt-ie9"> <![endif]-->\n<!--[if gt IE 8]><!--><html class="no-js" lang="en" dir="ltr"> <!--<![endif]-->\n\n<head>\n <meta charset="utf-8">\n <meta http-equiv="X-UA-Compatible" content="IE=edge">\n\n <link rel="prefetch" href="//ajax.googleapis.com/ajax/libs/jquery/1.8.2/jquery.min.js">\n\n <meta name="application-name" content="Python.org">\n <meta name="msapplication-tooltip" content="The official home of the Python Programming Language">\n <meta name="apple-mobile-web-app-title" content="Python.org">\n <meta name="apple-mobile-web-app-capable" content="yes">\n <meta name="apple-mobile-web-app-status-bar-style" content="black">\n\n <meta name="viewport" content="width=device-width, initial-scale=1.0">\n <meta name="HandheldFriendly" content="True">\n <meta name="format-detection" content="telephone=no">\n <meta http-equiv="cleartype" content="on">\n <meta http-equiv="imagetoolbar" content="false">\n\n <script src="/static/js/libs/modernizr.js"></script>\n\n <link href="/static/stylesheets/style.css" rel="stylesheet" type="text/css" title="default" />\n <link href="/static/stylesheets/mq.css" rel="stylesheet" type="text/css" media="not print, braille, embossed, speech, tty" />\n \n\n <!--[if (lte IE 8)&(!IEMobile)]>\n <link href="/static/stylesheets/no-mq.css" rel="stylesheet" type="text/css" media="screen" />\n \n \n <![endif]-->\n\n \n <link rel="icon" type="image/x-icon" href="/static/favicon.ico">\n <link rel="apple-touch-icon-precomposed" sizes="144x144" href="/static/apple-touch-icon-144x144-precomposed.png">\n <link rel="apple-touch-icon-precomposed" sizes="114x114" href="/static/apple-touch-icon-114x114-precomposed.png">\n <link rel="apple-touch-icon-precomposed" sizes="72x72" href="/static/apple-touch-icon-72x72-precomposed.png">\n <link rel="apple-touch-icon-precomposed" href="/static/apple-touch-icon-precomposed.png">\n <link rel="apple-touch-icon" href="/static/apple-touch-icon-precomposed.png">\n\n \n <meta name="msapplication-TileImage" content="/static/metro-icon-144x144-precomposed.png"><!-- white shape -->\n <meta name="msapplication-TileColor" content="#3673a5"><!-- python blue -->\n <meta name="msapplication-navbutton-color" content="#3673a5">\n\n <title>Welcome to Python.org</title>\n\n <meta name="description" content="The official home of the Python Programming Language">\n <meta name="keywords" content="Python programming language object oriented web free open source software license documentation download community">\n\n \n <meta property="og:type" content="website">\n <meta property="og:site_name" content="Python.org">\n <meta property="og:title" content="Welcome to Python.org">\n <meta property="og:description" content="The official home of the Python Programming Language">\n \n <meta property="og:image" content="https://www.python.org/static/opengraph-icon-200x200.png">\n <meta property="og:image:secure_url" content="https://www.python.org/static/opengraph-icon-200x200.png">\n \n <meta property="og:url" content="https://www.python.org/">\n\n <link rel="author" href="/static/humans.txt">\n\n \n\n \n <script type="application/ld+json">\n {\n "@context": "http://schema.org",\n "@type": "WebSite",\n "url": "https://www.python.org/",\n "potentialAction": {\n "@type": "SearchAction",\n "target": "https://www.python.org/search/?q={search_term_string}",\n "query-input": "required name=search_term_string"\n }\n }\n </script>\n\n \n <script type="text/javascript">\n var _gaq = _gaq || [];\n _gaq.push([\'_setAccount\', \'UA-39055973-1\']);\n _gaq.push([\'_trackPageview\']);\n\n (function() {\n var ga = document.createElement(\'script\'); ga.type = \'text/javascript\'; ga.async = true;\n ga.src = (\'https:\' == document.location.protocol ? \'https://ssl\' : \'http://www\') + \'.google-analytics.com/ga.js\';\n var s = document.getElementsByTagName(\'script\')[0]; s.parentNode.insertBefore(ga, s);\n })();\n </script>\n \n</head>\n\n<body class="python home" id="homepage">\n\n <div id="touchnav-wrapper">\n\n <div id="nojs" class="do-not-print">\n <p><strong>Notice:</strong> While Javascript is not essential for this website, your interaction with the content will be limited. 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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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http://www.python.orgView Code
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！
import time
def timer(func):
def wrapper(*args,**kwargs):
start_time = time.time()
func(*args,**kwargs)
stop_time = time.time()
print("run time is %s"%(stop_time-start_time))
return wrapper
@timer
def index(): #index = timer(index)
time.sleep(1.5)
print("welcome to beijing!")
@timer
def home(name):
time.sleep(1)
print("welcome to %s home page"%name)
@timer
def auth(name,passwd):
time.sleep(0.5)
print(name,passwd)
@timer
def tell():
print("---***---")
index()
home("alex")
auth("alex","alex3714")
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
转载于:https://www.cnblogs.com/SHENGXIN/p/7421558.html
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