装饰器接受一个函数,添加一些功能并返回它。 在本教程中,您将学习如何创建装饰器以及为什么要使用它。
Python Decorators
Python 中的装饰器
Python 有一个有趣的函数,称为装饰器,可为现有代码添加功能。
这也称为元编程,因为程序的一部分试图在编译时修改程序的另一部分。
学习装饰器的先决条件
为了了解装饰器,我们必须首先了解 Python 的一些基本知识。
我们必须对以下事实感到满意:Python 中的所有内容(是!甚至是类)都是对象。 我们定义的名称只是绑定到这些对象的标识符。 函数也不例外,它们也是对象(带有属性)。 各种不同的名称可以绑定到同一函数对象。
这是一个例子。
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<span class="pl-k">def</span> <span class="pl-en">first</span>(<span class="pl-s1">msg</span>): <span class="pl-en">print</span>(<span class="pl-s1">msg</span>) <span class="pl-en">first</span>(<span class="pl-s">"Hello"</span>) <span class="pl-s1">second</span> <span class="pl-c1">=</span> <span class="pl-s1">first</span> <span class="pl-en">second</span>(<span class="pl-s">"Hello"</span>) |
输出
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<span class="pl-v">Hello</span> <span class="pl-v">Hello</span> |
当您运行代码时,两个函数first
和second
给出相同的输出。 在这里,名称first
和second
指的是同一函数对象。
现在事情开始变得怪异了。
可以将函数作为参数传递给另一个函数。
如果您在 Python 中使用过map
,filter
和reduce
之类的函数,那么您已经知道这一点。
将其他函数作为参数的此类函数也称为高阶函数。 这是这种函数的一个例子。
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<span class="pl-k">def</span> <span class="pl-en">inc</span>(<span class="pl-s1">x</span>): <span class="pl-k">return</span> <span class="pl-s1">x</span> <span class="pl-c1">+</span> <span class="pl-c1">1</span> <span class="pl-k">def</span> <span class="pl-en">dec</span>(<span class="pl-s1">x</span>): <span class="pl-k">return</span> <span class="pl-s1">x</span> <span class="pl-c1">-</span> <span class="pl-c1">1</span> <span class="pl-k">def</span> <span class="pl-en">operate</span>(<span class="pl-s1">func</span>, <span class="pl-s1">x</span>): <span class="pl-s1">result</span> <span class="pl-c1">=</span> <span class="pl-en">func</span>(<span class="pl-s1">x</span>) <span class="pl-k">return</span> <span class="pl-s1">result</span> |
我们调用函数如下。
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<span class="pl-c1">>></span><span class="pl-c1">></span> <span class="pl-en">operate</span>(<span class="pl-s1">inc</span>,<span class="pl-c1">3</span>) <span class="pl-c1">4</span> <span class="pl-c1">>></span><span class="pl-c1">></span> <span class="pl-en">operate</span>(<span class="pl-s1">dec</span>,<span class="pl-c1">3</span>) <span class="pl-c1">2</span> |
此外,一个函数可以返回另一个函数。
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<span class="pl-k">def</span> <span class="pl-en">is_called</span>(): <span class="pl-k">def</span> <span class="pl-en">is_returned</span>(): <span class="pl-en">print</span>(<span class="pl-s">"Hello"</span>) <span class="pl-k">return</span> <span class="pl-s1">is_returned</span> <span class="pl-s1">new</span> <span class="pl-c1">=</span> <span class="pl-en">is_called</span>() <span class="pl-c"># Outputs "Hello"</span> <span class="pl-en">new</span>() |
输出:
1 |
<span class="pl-v">Hello</span> |
在这里,is_returned()
是一个嵌套函数,每次调用is_called()
时都会定义并返回该函数。
最后,我们必须了解 Python 中的 C losures。
回到装饰器
函数和方法被称为可调用,因为它们可以被调用。
实际上,任何实现特殊__call__()
方法的对象都称为可调用对象。 因此,从最基本的意义上讲,装饰器是可调用的,可返回可调用的。
基本上,装饰器接受一个函数,添加一些功能并返回它。
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<span class="pl-k">def</span> <span class="pl-en">make_pretty</span>(<span class="pl-s1">func</span>): <span class="pl-k">def</span> <span class="pl-en">inner</span>(): <span class="pl-en">print</span>(<span class="pl-s">"I got decorated"</span>) <span class="pl-en">func</span>() <span class="pl-k">return</span> <span class="pl-s1">inner</span> <span class="pl-k">def</span> <span class="pl-en">ordinary</span>(): <span class="pl-en">print</span>(<span class="pl-s">"I am ordinary"</span>) |
在 shell 中运行以下代码时,
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<span class="pl-c1">>></span><span class="pl-c1">></span> <span class="pl-en">ordinary</span>() <span class="pl-v">I</span> <span class="pl-s1">am</span> <span class="pl-s1">ordinary</span> <span class="pl-c1">>></span><span class="pl-c1">></span> <span class="pl-c"># let's decorate this ordinary function</span> <span class="pl-c1">>></span><span class="pl-c1">></span> <span class="pl-s1">pretty</span> <span class="pl-c1">=</span> <span class="pl-en">make_pretty</span>(<span class="pl-s1">ordinary</span>) <span class="pl-c1">>></span><span class="pl-c1">></span> <span class="pl-en">pretty</span>() <span class="pl-v">I</span> <span class="pl-s1">got</span> <span class="pl-s1">decorated</span> <span class="pl-v">I</span> <span class="pl-s1">am</span> <span class="pl-s1">ordinary</span> |
在上面显示的示例中,make_pretty()
是装饰器。 在分配步骤中:
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<span class="pl-s1">pretty</span> <span class="pl-c1">=</span> <span class="pl-en">make_pretty</span>(<span class="pl-s1">ordinary</span>) |
装饰了函数ordinary()
,并将返回的函数命名为pretty
。
我们可以看到装饰器函数在原始函数中添加了一些新功能。 这类似于包装礼物。 装饰器充当包装器。 装饰的对象(内部实际礼物)的性质不会改变。 但是现在,它看起来很漂亮(因为它已经被装饰了)。
通常,我们装饰一个函数并将其重新分配为
1 |
<span class="pl-s1">ordinary</span> <span class="pl-c1">=</span> <span class="pl-en">make_pretty</span>(<span class="pl-s1">ordinary</span>). |
这是一个常见的构造,因此,Python 具有简化此语法的语法。
我们可以将@
符号与装饰器函数的名称一起使用,并将其放置在要装饰的函数的定义上方。 例如,
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<span class="pl-en">@<span class="pl-s1">make_pretty</span></span> <span class="pl-k">def</span> <span class="pl-en">ordinary</span>(): <span class="pl-en">print</span>(<span class="pl-s">"I am ordinary"</span>) |
相当于
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<span class="pl-k">def</span> <span class="pl-en">ordinary</span>(): <span class="pl-en">print</span>(<span class="pl-s">"I am ordinary"</span>) <span class="pl-s1">ordinary</span> <span class="pl-c1">=</span> <span class="pl-en">make_pretty</span>(<span class="pl-s1">ordinary</span>) |
这只是实现装饰器的语法糖。
带参装饰器
上面的装饰器很简单,并且只能用于没有任何参数的函数。 如果我们的函数具有以下参数:
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<span class="pl-k">def</span> <span class="pl-en">divide</span>(<span class="pl-s1">a</span>, <span class="pl-s1">b</span>): <span class="pl-k">return</span> <span class="pl-s1">a</span><span class="pl-c1">/</span><span class="pl-s1">b</span> |
该函数具有两个参数,a
和b
。 我们知道,如果我们将b
传递为 0,将会产生错误。
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<span class="pl-c1">>></span><span class="pl-c1">></span> <span class="pl-en">divide</span>(<span class="pl-c1">2</span>,<span class="pl-c1">5</span>) <span class="pl-c1">0.4</span> <span class="pl-c1">>></span><span class="pl-c1">></span> <span class="pl-en">divide</span>(<span class="pl-c1">2</span>,<span class="pl-c1">0</span>) <span class="pl-v">Traceback</span> (<span class="pl-s1">most</span> <span class="pl-s1">recent</span> <span class="pl-s1">call</span> <span class="pl-s1">last</span>): ... <span class="pl-v">ZeroDivisionError</span>: <span class="pl-s1">division</span> <span class="pl-s1">by</span> <span class="pl-s1">zero</span> |
现在,让我们做一个装饰器来检查这种情况是否会导致错误。
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<span class="pl-k">def</span> <span class="pl-en">smart_divide</span>(<span class="pl-s1">func</span>): <span class="pl-k">def</span> <span class="pl-en">inner</span>(<span class="pl-s1">a</span>, <span class="pl-s1">b</span>): <span class="pl-en">print</span>(<span class="pl-s">"I am going to divide"</span>, <span class="pl-s1">a</span>, <span class="pl-s">"and"</span>, <span class="pl-s1">b</span>) <span class="pl-k">if</span> <span class="pl-s1">b</span> <span class="pl-c1">==</span> <span class="pl-c1">0</span>: <span class="pl-en">print</span>(<span class="pl-s">"Whoops! cannot divide"</span>) <span class="pl-k">return</span> <span class="pl-k">return</span> <span class="pl-en">func</span>(<span class="pl-s1">a</span>, <span class="pl-s1">b</span>) <span class="pl-k">return</span> <span class="pl-s1">inner</span> <span class="pl-en">@<span class="pl-s1">smart_divide</span></span> <span class="pl-k">def</span> <span class="pl-en">divide</span>(<span class="pl-s1">a</span>, <span class="pl-s1">b</span>): <span class="pl-en">print</span>(<span class="pl-s1">a</span><span class="pl-c1">/</span><span class="pl-s1">b</span>) |
如果出现错误情况,此新实现将返回None
。
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<span class="pl-c1">>></span><span class="pl-c1">></span> <span class="pl-en">divide</span>(<span class="pl-c1">2</span>,<span class="pl-c1">5</span>) <span class="pl-v">I</span> <span class="pl-s1">am</span> <span class="pl-s1">going</span> <span class="pl-s1">to</span> <span class="pl-s1">divide</span> <span class="pl-c1">2</span> <span class="pl-c1">and</span> <span class="pl-c1">5</span> <span class="pl-c1">0.4</span> <span class="pl-c1">>></span><span class="pl-c1">></span> <span class="pl-en">divide</span>(<span class="pl-c1">2</span>,<span class="pl-c1">0</span>) <span class="pl-v">I</span> <span class="pl-s1">am</span> <span class="pl-s1">going</span> <span class="pl-s1">to</span> <span class="pl-s1">divide</span> <span class="pl-c1">2</span> <span class="pl-c1">and</span> <span class="pl-c1">0</span> <span class="pl-v">Whoops</span>! <span class="pl-s1">cannot</span> <span class="pl-s1">divide</span> |
通过这种方式,我们可以修饰带有参数的函数。
敏锐的观察者会注意到,装饰器内部嵌套的inner()
函数的参数与其装饰的函数的参数相同。 考虑到这一点,现在我们可以使通用装饰器可以使用任意数量的参数。
在 Python 中,此魔术是通过function(*args, **kwargs)
完成的。 这样,args
将是位置参数的元组,kwargs
将是关键字参数的字典。 这样的装饰器的一个例子是:
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<span class="pl-k">def</span> <span class="pl-en">works_for_all</span>(<span class="pl-s1">func</span>): <span class="pl-k">def</span> <span class="pl-en">inner</span>(<span class="pl-c1">*</span><span class="pl-s1">args</span>, <span class="pl-c1">**</span><span class="pl-s1">kwargs</span>): <span class="pl-en">print</span>(<span class="pl-s">"I can decorate any function"</span>) <span class="pl-k">return</span> <span class="pl-en">func</span>(<span class="pl-c1">*</span><span class="pl-s1">args</span>, <span class="pl-c1">**</span><span class="pl-s1">kwargs</span>) <span class="pl-k">return</span> <span class="pl-s1">inner</span> |
Python 中的链接装饰器
可以在 Python 中链接多个装饰器。
这就是说,一个函数可以用不同(或相同)的装饰器多次装饰。 我们只需将装饰器放置在所需函数之上。
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<span class="pl-k">def</span> <span class="pl-en">star</span>(<span class="pl-s1">func</span>): <span class="pl-k">def</span> <span class="pl-en">inner</span>(<span class="pl-c1">*</span><span class="pl-s1">args</span>, <span class="pl-c1">**</span><span class="pl-s1">kwargs</span>): <span class="pl-en">print</span>(<span class="pl-s">"*"</span> <span class="pl-c1">*</span> <span class="pl-c1">30</span>) <span class="pl-en">func</span>(<span class="pl-c1">*</span><span class="pl-s1">args</span>, <span class="pl-c1">**</span><span class="pl-s1">kwargs</span>) <span class="pl-en">print</span>(<span class="pl-s">"*"</span> <span class="pl-c1">*</span> <span class="pl-c1">30</span>) <span class="pl-k">return</span> <span class="pl-s1">inner</span> <span class="pl-k">def</span> <span class="pl-en">percent</span>(<span class="pl-s1">func</span>): <span class="pl-k">def</span> <span class="pl-en">inner</span>(<span class="pl-c1">*</span><span class="pl-s1">args</span>, <span class="pl-c1">**</span><span class="pl-s1">kwargs</span>): <span class="pl-en">print</span>(<span class="pl-s">"%"</span> <span class="pl-c1">*</span> <span class="pl-c1">30</span>) <span class="pl-en">func</span>(<span class="pl-c1">*</span><span class="pl-s1">args</span>, <span class="pl-c1">**</span><span class="pl-s1">kwargs</span>) <span class="pl-en">print</span>(<span class="pl-s">"%"</span> <span class="pl-c1">*</span> <span class="pl-c1">30</span>) <span class="pl-k">return</span> <span class="pl-s1">inner</span> <span class="pl-en">@<span class="pl-s1">star</span></span> <span class="pl-en">@<span class="pl-s1">percent</span></span> <span class="pl-k">def</span> <span class="pl-en">printer</span>(<span class="pl-s1">msg</span>): <span class="pl-en">print</span>(<span class="pl-s1">msg</span>) <span class="pl-en">printer</span>(<span class="pl-s">"Hello"</span>) |
输出:
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<span class="pl-c1">**</span><span class="pl-c1">**</span><span class="pl-c1">**</span><span class="pl-c1">**</span><span class="pl-c1">**</span><span class="pl-c1">**</span><span class="pl-c1">**</span><span class="pl-c1">**</span><span class="pl-c1">**</span><span class="pl-c1">**</span><span class="pl-c1">**</span><span class="pl-c1">**</span><span class="pl-c1">**</span><span class="pl-c1">**</span><span class="pl-c1">**</span> <span class="pl-c1">%</span><span class="pl-c1">%</span><span class="pl-c1">%</span><span class="pl-c1">%</span><span class="pl-c1">%</span><span class="pl-c1">%</span><span class="pl-c1">%</span><span class="pl-c1">%</span><span class="pl-c1">%</span><span class="pl-c1">%</span><span class="pl-c1">%</span><span class="pl-c1">%</span><span class="pl-c1">%</span><span class="pl-c1">%</span><span class="pl-c1">%</span><span class="pl-c1">%</span><span class="pl-c1">%</span><span class="pl-c1">%</span><span class="pl-c1">%</span><span class="pl-c1">%</span><span class="pl-c1">%</span><span class="pl-c1">%</span><span class="pl-c1">%</span><span class="pl-c1">%</span><span class="pl-c1">%</span><span class="pl-c1">%</span><span class="pl-c1">%</span><span class="pl-c1">%</span><span class="pl-c1">%</span><span class="pl-c1">%</span> <span class="pl-v">Hello</span> <span class="pl-c1">%</span><span class="pl-c1">%</span><span class="pl-c1">%</span><span class="pl-c1">%</span><span class="pl-c1">%</span><span class="pl-c1">%</span><span class="pl-c1">%</span><span class="pl-c1">%</span><span class="pl-c1">%</span><span class="pl-c1">%</span><span class="pl-c1">%</span><span class="pl-c1">%</span><span class="pl-c1">%</span><span class="pl-c1">%</span><span class="pl-c1">%</span><span class="pl-c1">%</span><span class="pl-c1">%</span><span class="pl-c1">%</span><span class="pl-c1">%</span><span class="pl-c1">%</span><span class="pl-c1">%</span><span class="pl-c1">%</span><span class="pl-c1">%</span><span class="pl-c1">%</span><span class="pl-c1">%</span><span class="pl-c1">%</span><span class="pl-c1">%</span><span class="pl-c1">%</span><span class="pl-c1">%</span><span class="pl-c1">%</span> <span class="pl-c1">**</span><span class="pl-c1">**</span><span class="pl-c1">**</span><span class="pl-c1">**</span><span class="pl-c1">**</span><span class="pl-c1">**</span><span class="pl-c1">**</span><span class="pl-c1">**</span><span class="pl-c1">**</span><span class="pl-c1">**</span><span class="pl-c1">**</span><span class="pl-c1">**</span><span class="pl-c1">**</span><span class="pl-c1">**</span><span class="pl-c1">**</span> |
上面的语法,
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<span class="pl-en">@<span class="pl-s1">star</span></span> <span class="pl-en">@<span class="pl-s1">percent</span></span> <span class="pl-k">def</span> <span class="pl-en">printer</span>(<span class="pl-s1">msg</span>): <span class="pl-en">print</span>(<span class="pl-s1">msg</span>) |
is equivalent to
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<span class="pl-k">def</span> <span class="pl-en">printer</span>(<span class="pl-s1">msg</span>): <span class="pl-en">print</span>(<span class="pl-s1">msg</span>) <span class="pl-s1">printer</span> <span class="pl-c1">=</span> <span class="pl-en">star</span>(<span class="pl-en">percent</span>(<span class="pl-s1">printer</span>)) |
链接装饰器的顺序很重要。 如果我们将顺序颠倒为
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<span class="pl-en">@<span class="pl-s1">percent</span></span> <span class="pl-en">@<span class="pl-s1">star</span></span> <span class="pl-k">def</span> <span class="pl-en">printer</span>(<span class="pl-s1">msg</span>): <span class="pl-en">print</span>(<span class="pl-s1">msg</span>) |
输出为:
1 2 3 4 5 |
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