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<meta property="og:description" content="python 默认值的陷阱 def test(a, l = []): for i in range(a): l.append(i) print(l) >>> test(2) >>> test(2, [1]) >>> test(3) 输出结果 [0, 1] [1, 0, 1] [0, 1, 0, 1, 2] 最后一行为什么是[0, 1, 0, 1, 2], 而不是期望中的[0, 1, 2] ？ 官方文档是这样解释的 Default values are computed once …" />
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<meta name="twitter:description" content="python 默认值的陷阱 def test(a, l = []): for i in range(a): l.append(i) print(l) >>> test(2) >>> test(2, [1]) >>> test(3) 输出结果 [0, 1] [1, 0, 1] [0, 1, 0, 1, 2] 最后一行为什么是[0, 1, 0, 1, 2], 而不是期望中的[0, 1, 2] ？ 官方文档是这样解释的 Default values are computed once …">
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Python 默认参数
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<p>python 默认值的陷阱</p>
<div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">test</span><span class="p">(</span><span class="n">a</span><span class="p">,</span> <span class="n">l</span> <span class="o">=</span> <span class="p">[]):</span>
<span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="n">a</span><span class="p">):</span>
<span class="n">l</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">i</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="n">l</span><span class="p">)</span>
</pre></div>
<div class="highlight"><pre><span></span><span class="o">>>></span> <span class="n">test</span><span class="p">(</span><span class="mi">2</span><span class="p">)</span>
<span class="o">>>></span> <span class="n">test</span><span class="p">(</span><span class="mi">2</span><span class="p">,</span> <span class="p">[</span><span class="mi">1</span><span class="p">])</span>
<span class="o">>>></span> <span class="n">test</span><span class="p">(</span><span class="mi">3</span><span class="p">)</span>
</pre></div>
<p>输出结果</p>
<div class="highlight"><pre><span></span><span class="p">[</span><span class="mi">0</span><span class="p">,</span> <span class="mi">1</span><span class="p">]</span>
<span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="mi">1</span><span class="p">]</span>
<span class="p">[</span><span class="mi">0</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">]</span>
</pre></div>
<p>最后一行为什么是<code>[0, 1, 0, 1, 2]</code>, 而不是期望中的<code>[0, 1, 2]</code> ？<br>
官方文档是这样解释的</p>
<blockquote>
<p>Default values are computed once, then re-used.</p>
</blockquote>
<p>因为默认值在函数定义的时候被初始化，而且只初始化一次, 后面重复使用。又因为列表可变的，顾不指定默认值的时候会不断的追加到默认值列表中。</p>
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