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为什么 Python 不是 lexical scoping？

WBOY
WBOYOriginal
2016-06-06 16:23:051175browse

这似乎背离了现代程序设计语言的一般设计思路。

比如

<span class="k">def</span> <span class="nf">foo</span><span class="p">():</span>
    <span class="n">x</span> <span class="o">=</span> <span class="mi">1</span>
    <span class="k">def</span> <span class="nf">inner</span><span class="p">():</span>
        <span class="k">return</span> <span class="n">x</span> <span class="o">+</span> <span class="mi">1</span>
    <span class="n">x</span> <span class="o">=</span> <span class="mi">3</span>
    <span class="k">print</span> <span class="n">inner</span><span class="p">()</span>

回复内容：

这段代码是lexical scoping，静态作用域是指我们可以根据代码文本的位置，确定变量的存在区域。

按照python的LEGB（Local，Enclosing，Global，Built-in）规则，当调用inner()时，x实际上是在foo的scope中找到的。inner之所能够访问foo中的x，是因为inner is inside the text of foo，这正是lexical的含义。
为什么 Python 不是 lexical scoping？
Bash是Dynamic Scoping的
<code class="language-bash"><span class="nv">x</span><span class="o">=</span><span class="m">1</span>                                                                                                 
<span class="k">function</span> g <span class="o">()</span> <span class="o">{</span> <span class="nb">echo</span> <span class="nv">$x</span> <span class="p">;</span> <span class="nv">x</span><span class="o">=</span><span class="m">2</span> <span class="p">;</span> <span class="o">}</span> 
<span class="k">function</span> f <span class="o">()</span> <span class="o">{</span> <span class="nb">local </span><span class="nv">x</span><span class="o">=</span><span class="m">3</span> <span class="p">;</span> g <span class="p">;</span> <span class="o">}</span> 
f  <span class="c">#f中的g执行时打印出的x是3而不是1</span>
<span class="nb">echo</span> <span class="nv">$x</span>  <span class="c">#这时打印出的x是1</span>
</code>
你以为Python是

<code class="language-ocaml"><span class="k">let</span> <span class="n">foo</span> <span class="bp">()</span> <span class="o">=</span>
  <span class="k">let</span> <span class="n">x</span> <span class="o">=</span> <span class="mi">1</span> <span class="k">in</span>
    <span class="k">let</span> <span class="n">inner</span> <span class="bp">()</span> <span class="o">=</span> <span class="n">x</span> <span class="o">+</span> <span class="mi">1</span> <span class="k">in</span>
      <span class="k">let</span> <span class="n">x</span> <span class="o">=</span> <span class="mi">3</span> <span class="k">in</span>
        <span class="n">print</span> <span class="o">(</span><span class="n">inner</span> <span class="bp">()</span><span class="o">)</span>
</code>
python的闭包里的自由变量是按引用传递的，而不是按值传递，所以会有这个结果。
这和scoping没有关系。
C++的lambda就可以选择capture by copy或者capture by reference. 根据之前阅读Python源码的经验(如果记错请指正)，在题主的例子里面，这个inner是一个闭包。闭包在Python里面的实现方式是保存一个通往外部namespace的指针(可以理解成一个dictionary)。

楼主可以参看这个例子
<code class="language-python"><span class="k">def</span> <span class="nf">foo</span><span class="p">():</span>
	<span class="k">def</span> <span class="nf">inner</span><span class="p">():</span>
		<span class="k">return</span> <span class="n">x</span> <span class="o">+</span> <span class="mi">1</span>
	<span class="n">x</span> <span class="o">=</span> <span class="mi">1</span>
	<span class="k">print</span> <span class="n">inner</span><span class="p">()</span> <span class="c"># output 2</span>
	<span class="n">x</span> <span class="o">=</span> <span class="mi">2</span>
	<span class="k">print</span> <span class="n">inner</span><span class="p">()</span> <span class="c"># output 3</span>
</code>

这分明就是lexical scoping嘛，譬如说等价的C#代码


<code class="language-csharp"><span class="k">void</span> <span class="nf">Foo</span><span class="p">()</span>
<span class="p">{</span>
    <span class="kt">int</span> <span class="n">x</span><span class="p">=</span><span class="m">1</span><span class="p">;</span>
    <span class="n">Func</span><span class="p"><span class="kt">int</span><span class="p">></span> <span class="n">inner</span> <span class="p">=</span> <span class="p">()=></span><span class="n">x</span><span class="p">+</span><span class="m">1</span><span class="p">;</span>
    <span class="n">x</span><span class="p">=</span><span class="m">3</span><span class="p">;</span>
    <span class="n">Console</span><span class="p">.</span><span class="n">WriteLine</span><span class="p">(</span><span class="n">inner</span><span class="p">());</span>
<span class="p">}</span>
</span></code>
把楼主的代码改写成 lua 可以看看 Python 和 Lua 在处理上的不同：

第一个例子结果都是一样的，因为变量绑定的是引用而不是值。第二个例子：

<code class="language-lua"><span class="k">function</span> <span class="nf">foo</span><span class="p">()</span>
	<span class="k">function</span> <span class="nf">inner</span><span class="p">()</span>
		<span class="k">return</span> <span class="n">x</span> <span class="o">+</span> <span class="mi">1</span>
	<span class="k">end</span>
	<span class="kd">local</span> <span class="n">x</span> <span class="o">=</span> <span class="mi">3</span>
	<span class="nb">print</span><span class="p">(</span><span class="n">inner</span><span class="p">())</span>
<span class="k">end</span>

<span class="n">foo</span><span class="p">()</span>
</code>
题主所说的 现代化的编程语言指的是什么？ js经过这么多代的更新迭代，现在也是这样~

<code class="language-js"><span class="p">(</span><span class="kd">function</span> <span class="nx">foo</span><span class="p">()</span> <span class="p">{</span>
  <span class="kd">function</span> <span class="nx">inner</span><span class="p">()</span> <span class="p">{</span>
    <span class="k">return</span> <span class="nx">x</span><span class="o">+</span><span class="mi">1</span><span class="p">;</span>
  <span class="p">}</span>
  <span class="nx">x</span> <span class="o">=</span> <span class="mi">3</span><span class="p">;</span>
  <span class="nx">console</span><span class="p">.</span><span class="nx">log</span><span class="p">(</span><span class="nx">inner</span><span class="p">());</span>
<span class="p">})();</span>
</code>
&lt;span class=&quot;k&quot;&gt;def&lt;/span&gt; &lt;span class=&quot;nf&quot;&gt;foo&lt;/span&gt;&lt;span class=&quot;p&quot;&gt;():&lt;/span&gt;
    &lt;span class=&quot;n&quot;&gt;x&lt;/span&gt; &lt;span class=&quot;o&quot;&gt;=&lt;/span&gt; &lt;span class=&quot;mi&quot;&gt;1&lt;/span&gt;
    &lt;span class=&quot;k&quot;&gt;def&lt;/span&gt; &lt;span class=&quot;nf&quot;&gt;inner&lt;/span&gt;&lt;span class=&quot;p&quot;&gt;():&lt;/span&gt;
        &lt;span class=&quot;k&quot;&gt;return&lt;/span&gt; &lt;span class=&quot;n&quot;&gt;x&lt;/span&gt; &lt;span class=&quot;o&quot;&gt;+&lt;/span&gt; &lt;span class=&quot;mi&quot;&gt;1&lt;/span&gt;
    &lt;span class=&quot;n&quot;&gt;x&lt;/span&gt; &lt;span class=&quot;o&quot;&gt;=&lt;/span&gt; &lt;span class=&quot;mi&quot;&gt;3&lt;/span&gt;
    &lt;span class=&quot;k&quot;&gt;print&lt;/span&gt; &lt;span class=&quot;n&quot;&gt;inner&lt;/span&gt;&lt;span class=&quot;p&quot;&gt;()&lt;/span&gt;
<span class="n">foo</span><span class="p">()</span>  
<span class="c"># 输出4</span>
我觉得完全没有背离啊。。
输出感觉当前 x 走的 這個問題很好回答：x是inner函數的環境變量，所以在inner的定義中出現的x其實就是對定義外面的，也就是inner的環境變量的一個引用而已。

函數只有在被調用的時候才會執行，你前面將置為1，後面又改為3，不過是改變了x的引用值而已，相當於給x重新賦值。然後執行inner函數，使用x所對應的值為3，因此答案就是4了。
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