python为什么对象之间会共享默认值？

这种类型的缺陷通常会惹恼新手程序员。考虑这个函数

<span class="k">def</span> <span class="nf">foo</span><span class="p">(</span><span class="n">mydict</span><span class="o">=</span><span class="p">{}):</span>  <span class="c1"># Danger: shared reference to one dict for all calls</span>
    <span class="o">...</span> <span class="n">compute</span> <span class="n">something</span> <span class="o">...</span>
    <span class="n">mydict</span><span class="p">[</span><span class="n">key</span><span class="p">]</span> <span class="o">=</span> <span class="n">value</span>
    <span class="k">return</span> <span class="n">mydict</span>

www#gaodaima.com来源gaodai$ma#com搞$代*码网搞代码

第一次调用此函数时，mydict 包含一项。第二次，mydict 包含两项，因为当 foo() 开始执行时， mydict 中已经有一项了。

函数调用经常被期望为默认值创建新的对象。但实际情况并非如此。默认值会在函数定义时一次性地创建。如果对象发生改变，就如本示例中的字典那样，则对函数的后续调用将会引用这个被改变的对象。

按照定义，不可变对象例如数字、字符串、元组和 None 因为不可变所以是安全的。对可变对象例如字典、列表和类实例的改变则可能造成迷惑。

由于这一特性，在编程中应遵循的一项好习惯是不使用可变对象作为默认值。而应使用 None 作为默认值和函数中的值，检查值为 None 的形参并创建相应的列表、字典或其他可变对象。例如，不要这样写:

<span class="k">def</span> <span class="nf">foo</span><span class="p">(</span><span class="n">mydict</span><span class="o">=</span><span class="p">{}):</span>
    <span class="o">...</span>

而要这样写:

<span class="k">def</span> <span class="nf">foo</span><span class="p">(</span><span class="n">mydict</span><span class="o">=</span><span class="kc">None</span><span class="p">):</span>
    <span class="k">if</span> <span class="n">mydict</span> <span class="ow">is</span> <span class="kc">None</span><span class="p">:</span>
        <span class="n">mydict</span> <span class="o">=</span> <span class="p">{}</span>  <span class="c1"># create a new dict for local namespace</span>

这一特性有时会很有用处。当你有一个需要进行大量耗时计算的函数时，一个常见技巧是将每次调用函数的参数和结果值缓存起来，并在同样的值被再次请求时返回缓存的值。这称为“记忆”，具体实现方式可以是这样的:

<span class="c1"># Callers can only provide two parameters and optionally pass _cache by keyword</span>
<span class="k">def</span> <span class="nf">expensive</span><span class="p">(</span><span class="n">arg1</span><span class="p">,</span> <span class="n">arg2</span><span class="p">,</span> <span class="o">*</span><span class="p">,</span> <span class="n">_cache</span><span class="o">=</span><span class="p">{}):</span>
    <span class="k">if</span> <span class="p">(</span><span class="n">arg1</span><span class="p">,</span> <span class="n">arg2</span><span class="p">)</span> <span class="ow">in</span> <span class="n">_cache</span><span class="p">:</span>
        <span class="k">return</span> <span class="n">_cache</span><span class="p">[(</span><span class="n">arg1</span><span class="p">,</span> <span class="n">arg2</span><span class="p">)]</span>

    <span class="c1"># Calculate the value</span>
    <span class="n">result</span> <span class="o">=</span> <span class="o">...</span> <span class="n">expensive</span> <span class="n">computation</span> <span class="o">...</span>
    <span class="n">_cache</span><span class="p">[(</span><span class="n">arg1</span><span class="p">,</span> <span class="n">arg2</span><span class="p">)]</span> <span class="o">=</span> <span class="n">result</span>           <span class="c1"># Store result in the cache</span>
    <span class="k">return</span> <span class="n">result</span>

你也可以使用包含一个字典的全局变量而不使用参数默认值；这完全取决于个人偏好。

来源：搞代码网：原文地址：https://www.gaodaima.com

搞代码网（gaodaima.com）提供的所有资源部分来自互联网，如果有侵犯您的版权或其他权益，请说明详细缘由并提供版权或权益证明然后发送到邮箱[email protected]‍，我们会在看到邮件的第一时间内为您处理，或直接联系QQ：872152909。本网站采用BY-NC-SA协议进行授权
转载请注明原文链接：python为什么对象之间会共享默认值？

Hi，您需要填写昵称和邮箱！