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<span id="tut-morecontrol"></span><h1>4. 深入了解流程控制<a class="headerlink" href="#more-control-flow-tools" title="本標題的永久連結">¶</a></h1>
<p>除了剛才介紹的 <a class="reference internal" href="../reference/compound_stmts.html#while"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">while</span></code></a>，Python 擁有在其他程式語言中常用的流程控制語法，並有ㄧ些不一樣的改變。</p>
<div class="section" id="if-statements">
<span id="tut-if"></span><h2>4.1. <a class="reference internal" href="../reference/compound_stmts.html#if"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">if</span></code></a> 陳述式<a class="headerlink" href="#if-statements" title="本標題的永久連結">¶</a></h2>
<p>或許最常見的陳述式種類就是 <a class="reference internal" href="../reference/compound_stmts.html#if"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">if</span></code></a> 了。舉例來說：</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">x</span> <span class="o">=</span> <span class="nb">int</span><span class="p">(</span><span class="nb">input</span><span class="p">(</span><span class="s2">"Please enter an integer: "</span><span class="p">))</span>
<span class="go">Please enter an integer: 42</span>
<span class="gp">>>> </span><span class="k">if</span> <span class="n">x</span> <span class="o"><</span> <span class="mi">0</span><span class="p">:</span>
<span class="gp">... </span> <span class="n">x</span> <span class="o">=</span> <span class="mi">0</span>
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s1">'Negative changed to zero'</span><span class="p">)</span>
<span class="gp">... </span><span class="k">elif</span> <span class="n">x</span> <span class="o">==</span> <span class="mi">0</span><span class="p">:</span>
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s1">'Zero'</span><span class="p">)</span>
<span class="gp">... </span><span class="k">elif</span> <span class="n">x</span> <span class="o">==</span> <span class="mi">1</span><span class="p">:</span>
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s1">'Single'</span><span class="p">)</span>
<span class="gp">... </span><span class="k">else</span><span class="p">:</span>
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s1">'More'</span><span class="p">)</span>
<span class="gp">...</span>
<span class="go">More</span>
</pre></div>
</div>
<p>可以有零個或多個 <a class="reference internal" href="../reference/compound_stmts.html#elif"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">elif</span></code></a> 段落，且 <a class="reference internal" href="../reference/compound_stmts.html#else"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">else</span></code></a> 段落可有可無。關鍵字 <a class="reference internal" href="../reference/compound_stmts.html#elif"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">elif</span></code></a> 只是「else if」的縮寫，並且用來避免過多的縮排。一個 <a class="reference internal" href="../reference/compound_stmts.html#if"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">if</span></code></a> … <a class="reference internal" href="../reference/compound_stmts.html#elif"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">elif</span></code></a> … <a class="reference internal" href="../reference/compound_stmts.html#elif"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">elif</span></code></a> … 序列可以用來替代其他語言中出現的 <code class="docutils literal notranslate"><span class="pre">switch</span></code> 或 <code class="docutils literal notranslate"><span class="pre">case</span></code> 陳述式。</p>
</div>
<div class="section" id="for-statements">
<span id="tut-for"></span><h2>4.2. <a class="reference internal" href="../reference/compound_stmts.html#for"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">for</span></code></a> 陳述式<a class="headerlink" href="#for-statements" title="本標題的永久連結">¶</a></h2>
<p id="index-0">在 Python 中的 <a class="reference internal" href="../reference/compound_stmts.html#for"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">for</span></code></a> 陳述式可能不同於在 C 或 Pascal 中所看到的使用方式。與其只能选代 (iterate) 一個等差級數（如 Pascal），或給與使用者定義选代步進方式與結束條件（如 C），Python 的 <a class="reference internal" href="../reference/compound_stmts.html#for"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">for</span></code></a> 陳述选代任何序列（list 或者字串）的元素，以他們出現在序列中的順序。例如（無意雙關）：</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="c1"># Measure some strings:</span>
<span class="gp">... </span><span class="n">words</span> <span class="o">=</span> <span class="p">[</span><span class="s1">'cat'</span><span class="p">,</span> <span class="s1">'window'</span><span class="p">,</span> <span class="s1">'defenestrate'</span><span class="p">]</span>
<span class="gp">>>> </span><span class="k">for</span> <span class="n">w</span> <span class="ow">in</span> <span class="n">words</span><span class="p">:</span>
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="n">w</span><span class="p">,</span> <span class="nb">len</span><span class="p">(</span><span class="n">w</span><span class="p">))</span>
<span class="gp">...</span>
<span class="go">cat 3</span>
<span class="go">window 6</span>
<span class="go">defenestrate 12</span>
</pre></div>
</div>
<p>如果你在迴圈中需要修改一個你正在选代的序列（例如重複一些選擇的元素），那麼會建議你先建立一個序列的拷貝。选代序列並不暗示建立新的拷貝。此時 slice 語法就讓這件事十分容易完成：</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">for</span> <span class="n">w</span> <span class="ow">in</span> <span class="n">words</span><span class="p">[:]:</span> <span class="c1"># Loop over a slice copy of the entire list.</span>
<span class="gp">... </span> <span class="k">if</span> <span class="nb">len</span><span class="p">(</span><span class="n">w</span><span class="p">)</span> <span class="o">></span> <span class="mi">6</span><span class="p">:</span>
<span class="gp">... </span> <span class="n">words</span><span class="o">.</span><span class="n">insert</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="n">w</span><span class="p">)</span>
<span class="gp">...</span>
<span class="gp">>>> </span><span class="n">words</span>
<span class="go">['defenestrate', 'cat', 'window', 'defenestrate']</span>
</pre></div>
</div>
<p>With <code class="docutils literal notranslate"><span class="pre">for</span> <span class="pre">w</span> <span class="pre">in</span> <span class="pre">words:</span></code>, the example would attempt to create an infinite list,
inserting <code class="docutils literal notranslate"><span class="pre">defenestrate</span></code> over and over again.</p>
</div>
<div class="section" id="the-range-function">
<span id="tut-range"></span><h2>4.3. <a class="reference internal" href="../library/stdtypes.html#range" title="range"><code class="xref py py-func docutils literal notranslate"><span class="pre">range()</span></code></a> 函式<a class="headerlink" href="#the-range-function" title="本標題的永久連結">¶</a></h2>
<p>如果你需要选代一個數列的話，使用內建 <a class="reference internal" href="../library/stdtypes.html#range" title="range"><code class="xref py py-func docutils literal notranslate"><span class="pre">range()</span></code></a> 函式就很方便。它可以生成一等差級數：</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </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="mi">5</span><span class="p">):</span>
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="n">i</span><span class="p">)</span>
<span class="gp">...</span>
<span class="go">0</span>
<span class="go">1</span>
<span class="go">2</span>
<span class="go">3</span>
<span class="go">4</span>
</pre></div>
</div>
<p>給定的結束值永遠不會出現在生成的序列中；<code class="docutils literal notranslate"><span class="pre">range(10)</span></code> 生成的 10 個數值，即對應存取一個長度為 10 的序列內每一個元素的索引值。也可以讓 range 從其他數值計數，或者給定不同的級距（甚至為負；有時稱之為 『step』）：</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="nb">range</span><span class="p">(</span><span class="mi">5</span><span class="p">,</span> <span class="mi">10</span><span class="p">)</span>
<span class="mi">5</span><span class="p">,</span> <span class="mi">6</span><span class="p">,</span> <span class="mi">7</span><span class="p">,</span> <span class="mi">8</span><span class="p">,</span> <span class="mi">9</span>
<span class="nb">range</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="mi">10</span><span class="p">,</span> <span class="mi">3</span><span class="p">)</span>
<span class="mi">0</span><span class="p">,</span> <span class="mi">3</span><span class="p">,</span> <span class="mi">6</span><span class="p">,</span> <span class="mi">9</span>
<span class="nb">range</span><span class="p">(</span><span class="o">-</span><span class="mi">10</span><span class="p">,</span> <span class="o">-</span><span class="mi">100</span><span class="p">,</span> <span class="o">-</span><span class="mi">30</span><span class="p">)</span>
<span class="o">-</span><span class="mi">10</span><span class="p">,</span> <span class="o">-</span><span class="mi">40</span><span class="p">,</span> <span class="o">-</span><span class="mi">70</span>
</pre></div>
</div>
<p>欲选代一個序列的索引值們，你可以搭配使用 <a class="reference internal" href="../library/stdtypes.html#range" title="range"><code class="xref py py-func docutils literal notranslate"><span class="pre">range()</span></code></a> 和 <a class="reference internal" href="../library/functions.html#len" title="len"><code class="xref py py-func docutils literal notranslate"><span class="pre">len()</span></code></a> 如下：</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">a</span> <span class="o">=</span> <span class="p">[</span><span class="s1">'Mary'</span><span class="p">,</span> <span class="s1">'had'</span><span class="p">,</span> <span class="s1">'a'</span><span class="p">,</span> <span class="s1">'little'</span><span class="p">,</span> <span class="s1">'lamb'</span><span class="p">]</span>
<span class="gp">>>> </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="nb">len</span><span class="p">(</span><span class="n">a</span><span class="p">)):</span>
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="n">i</span><span class="p">,</span> <span class="n">a</span><span class="p">[</span><span class="n">i</span><span class="p">])</span>
<span class="gp">...</span>
<span class="go">0 Mary</span>
<span class="go">1 had</span>
<span class="go">2 a</span>
<span class="go">3 little</span>
<span class="go">4 lamb</span>
</pre></div>
</div>
<p>然而，在多數的情況，使用 <a class="reference internal" href="../library/functions.html#enumerate" title="enumerate"><code class="xref py py-func docutils literal notranslate"><span class="pre">enumerate()</span></code></a> 函式將更為方便，詳見<a class="reference internal" href="datastructures.html#tut-loopidioms"><span class="std std-ref">迴圈技巧</span></a>。</p>
<p>如果直接印出一個 range 則會出現奇怪的輸出：</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="nb">print</span><span class="p">(</span><span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">))</span>
<span class="go">range(0, 10)</span>
</pre></div>
</div>
<p>在很多情況下，由 <a class="reference internal" href="../library/stdtypes.html#range" title="range"><code class="xref py py-func docutils literal notranslate"><span class="pre">range()</span></code></a> 回傳的物件的行為如同一個 list，但實際上它並不是。它是一個物件在你选代時會回傳想要的序列的連續元素，並不會真正建出這個序列的 list，以節省空間。</p>
<p>我們稱這樣的物件為 <em>iterable</em>（可选代的），意即能作為函式、陳述式中能一直獲取連續元素直到用盡的部件。我們已經看過 <a class="reference internal" href="../reference/compound_stmts.html#for"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">for</span></code></a> 陳述式可做為如此的 <em>iterator</em>（选代器）。<a class="reference internal" href="../library/stdtypes.html#list" title="list"><code class="xref py py-func docutils literal notranslate"><span class="pre">list()</span></code></a> 函式為另一個例子，他可以自 iterable（可选代物件）建立 list：</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="nb">list</span><span class="p">(</span><span class="nb">range</span><span class="p">(</span><span class="mi">5</span><span class="p">))</span>
<span class="go">[0, 1, 2, 3, 4]</span>
</pre></div>
</div>
<p>待會我們可以看到更多函式回傳 iterable 和接受 iterable 為引數。</p>
</div>
<div class="section" id="break-and-continue-statements-and-else-clauses-on-loops">
<span id="tut-break"></span><h2>4.4. <a class="reference internal" href="../reference/simple_stmts.html#break"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">break</span></code></a> 和 <a class="reference internal" href="../reference/simple_stmts.html#continue"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">continue</span></code></a> 陳述、迴圈內 <a class="reference internal" href="../reference/compound_stmts.html#else"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">else</span></code></a> 段落<a class="headerlink" href="#break-and-continue-statements-and-else-clauses-on-loops" title="本標題的永久連結">¶</a></h2>
<p>The <a class="reference internal" href="../reference/simple_stmts.html#break"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">break</span></code></a> statement, like in C, breaks out of the innermost enclosing
<a class="reference internal" href="../reference/compound_stmts.html#for"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">for</span></code></a> or <a class="reference internal" href="../reference/compound_stmts.html#while"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">while</span></code></a> loop.</p>
<p>迴圈可以帶有一個 <code class="docutils literal notranslate"><span class="pre">else</span></code> 段落。當迴圈歷遍选代的 list （在 <a class="reference internal" href="../reference/compound_stmts.html#for"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">for</span></code></a> 中）或條件為偽（在 <a class="reference internal" href="../reference/compound_stmts.html#while"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">while</span></code></a> 中）時，這個段落會被執行；但迴圈被 <a class="reference internal" href="../reference/simple_stmts.html#break"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">break</span></code></a> 陳述終止時則不會。底下尋找質數的迴圈即示範了這個行為：</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">for</span> <span class="n">n</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">2</span><span class="p">,</span> <span class="mi">10</span><span class="p">):</span>
<span class="gp">... </span> <span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">2</span><span class="p">,</span> <span class="n">n</span><span class="p">):</span>
<span class="gp">... </span> <span class="k">if</span> <span class="n">n</span> <span class="o">%</span> <span class="n">x</span> <span class="o">==</span> <span class="mi">0</span><span class="p">:</span>
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="n">n</span><span class="p">,</span> <span class="s1">'equals'</span><span class="p">,</span> <span class="n">x</span><span class="p">,</span> <span class="s1">'*'</span><span class="p">,</span> <span class="n">n</span><span class="o">//</span><span class="n">x</span><span class="p">)</span>
<span class="gp">... </span> <span class="k">break</span>
<span class="gp">... </span> <span class="k">else</span><span class="p">:</span>
<span class="gp">... </span> <span class="c1"># loop fell through without finding a factor</span>
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="n">n</span><span class="p">,</span> <span class="s1">'is a prime number'</span><span class="p">)</span>
<span class="gp">...</span>
<span class="go">2 is a prime number</span>
<span class="go">3 is a prime number</span>
<span class="go">4 equals 2 * 2</span>
<span class="go">5 is a prime number</span>
<span class="go">6 equals 2 * 3</span>
<span class="go">7 is a prime number</span>
<span class="go">8 equals 2 * 4</span>
<span class="go">9 equals 3 * 3</span>
</pre></div>
</div>
<p>（沒錯，這是正確的程式碼。請看仔細：<code class="docutils literal notranslate"><span class="pre">else</span></code> 段落屬於 <a class="reference internal" href="../reference/compound_stmts.html#for"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">for</span></code></a> 迴圈，<strong>並非</strong> <a class="reference internal" href="../reference/compound_stmts.html#if"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">if</span></code></a> 陳述。）</p>
<p>當 <code class="docutils literal notranslate"><span class="pre">else</span></code> 段落與迴圈使用時，相較於搭配 <a class="reference internal" href="../reference/compound_stmts.html#if"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">if</span></code></a> 陳述使用，它的行為與搭配 <a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a> 陳述使用時更為相似：<a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a> 的 <code class="docutils literal notranslate"><span class="pre">else</span></code> 段落在沒有任何例外 (exception) 時執行，而迴圈的 <code class="docutils literal notranslate"><span class="pre">else</span></code> 段落在沒有任何 <code class="docutils literal notranslate"><span class="pre">break</span></code> 時執行。更多有關 <a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a> 陳述和例外的介紹，見<a class="reference internal" href="errors.html#tut-handling"><span class="std std-ref">處理例外</span></a>。</p>
<p><a class="reference internal" href="../reference/simple_stmts.html#continue"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">continue</span></code></a> 陳述，亦承襲於 C 語言，讓迴圈於下個选代起繼續執行：</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">for</span> <span class="n">num</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">2</span><span class="p">,</span> <span class="mi">10</span><span class="p">):</span>
<span class="gp">... </span> <span class="k">if</span> <span class="n">num</span> <span class="o">%</span> <span class="mi">2</span> <span class="o">==</span> <span class="mi">0</span><span class="p">:</span>
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s2">"Found an even number"</span><span class="p">,</span> <span class="n">num</span><span class="p">)</span>
<span class="gp">... </span> <span class="k">continue</span>
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s2">"Found a number"</span><span class="p">,</span> <span class="n">num</span><span class="p">)</span>
<span class="go">Found an even number 2</span>
<span class="go">Found a number 3</span>
<span class="go">Found an even number 4</span>
<span class="go">Found a number 5</span>
<span class="go">Found an even number 6</span>
<span class="go">Found a number 7</span>
<span class="go">Found an even number 8</span>
<span class="go">Found a number 9</span>
</pre></div>
</div>
</div>
<div class="section" id="pass-statements">
<span id="tut-pass"></span><h2>4.5. <a class="reference internal" href="../reference/simple_stmts.html#pass"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">pass</span></code></a> 陳述式<a class="headerlink" href="#pass-statements" title="本標題的永久連結">¶</a></h2>
<p><a class="reference internal" href="../reference/simple_stmts.html#pass"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">pass</span></code></a> 陳述不執行任何動作。它用在語法上需要一個陳述但不需要執行任何動作的時候。例如：</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">while</span> <span class="kc">True</span><span class="p">:</span>
<span class="gp">... </span> <span class="k">pass</span> <span class="c1"># Busy-wait for keyboard interrupt (Ctrl+C)</span>
<span class="gp">...</span>
</pre></div>
</div>
<p>這經常用於定義一個最簡單的類別：</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">class</span> <span class="nc">MyEmptyClass</span><span class="p">:</span>
<span class="gp">... </span> <span class="k">pass</span>
<span class="gp">...</span>
</pre></div>
</div>
<p><a class="reference internal" href="../reference/simple_stmts.html#pass"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">pass</span></code></a> 亦可作為一個函式或條件判斷主體的預留位置，它可幫助你以更宏觀的角度思考並撰寫新的程式碼。<a class="reference internal" href="../reference/simple_stmts.html#pass"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">pass</span></code></a> 可自動忽略：</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">def</span> <span class="nf">initlog</span><span class="p">(</span><span class="o">*</span><span class="n">args</span><span class="p">):</span>
<span class="gp">... </span> <span class="k">pass</span> <span class="c1"># Remember to implement this!</span>
<span class="gp">...</span>
</pre></div>
</div>
</div>
<div class="section" id="defining-functions">
<span id="tut-functions"></span><h2>4.6. 定義函式 (function)<a class="headerlink" href="#defining-functions" title="本標題的永久連結">¶</a></h2>
<p>We can create a function that writes the Fibonacci series to an arbitrary
boundary:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">def</span> <span class="nf">fib</span><span class="p">(</span><span class="n">n</span><span class="p">):</span> <span class="c1"># write Fibonacci series up to n</span>
<span class="gp">... </span> <span class="sd">"""Print a Fibonacci series up to n."""</span>
<span class="gp">... </span> <span class="n">a</span><span class="p">,</span> <span class="n">b</span> <span class="o">=</span> <span class="mi">0</span><span class="p">,</span> <span class="mi">1</span>
<span class="gp">... </span> <span class="k">while</span> <span class="n">a</span> <span class="o"><</span> <span class="n">n</span><span class="p">:</span>
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="n">a</span><span class="p">,</span> <span class="n">end</span><span class="o">=</span><span class="s1">' '</span><span class="p">)</span>
<span class="gp">... </span> <span class="n">a</span><span class="p">,</span> <span class="n">b</span> <span class="o">=</span> <span class="n">b</span><span class="p">,</span> <span class="n">a</span><span class="o">+</span><span class="n">b</span>
<span class="gp">... </span> <span class="nb">print</span><span class="p">()</span>
<span class="gp">...</span>
<span class="gp">>>> </span><span class="c1"># Now call the function we just defined:</span>
<span class="gp">... </span><span class="n">fib</span><span class="p">(</span><span class="mi">2000</span><span class="p">)</span>
<span class="go">0 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 1597</span>
</pre></div>
</div>
<p id="index-1">The keyword <a class="reference internal" href="../reference/compound_stmts.html#def"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">def</span></code></a> introduces a function <em>definition</em>. It must be
followed by the function name and the parenthesized list of formal parameters.
The statements that form the body of the function start at the next line, and
must be indented.</p>
<p>The first statement of the function body can optionally be a string literal;
this string literal is the function’s documentation string, or <em class="dfn">docstring</em>.
(More about docstrings can be found in the section <a class="reference internal" href="#tut-docstrings"><span class="std std-ref">說明文件字串</span></a>.)
There are tools which use docstrings to automatically produce online or printed
documentation, or to let the user interactively browse through code; it’s good
practice to include docstrings in code that you write, so make a habit of it.</p>
<p>The <em>execution</em> of a function introduces a new symbol table used for the local
variables of the function. More precisely, all variable assignments in a
function store the value in the local symbol table; whereas variable references
first look in the local symbol table, then in the local symbol tables of
enclosing functions, then in the global symbol table, and finally in the table
of built-in names. Thus, global variables cannot be directly assigned a value
within a function (unless named in a <a class="reference internal" href="../reference/simple_stmts.html#global"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">global</span></code></a> statement), although they
may be referenced.</p>
<p>The actual parameters (arguments) to a function call are introduced in the local
symbol table of the called function when it is called; thus, arguments are
passed using <em>call by value</em> (where the <em>value</em> is always an object <em>reference</em>,
not the value of the object). <a class="footnote-reference" href="#id2" id="id1">[1]</a> When a function calls another function, a new
local symbol table is created for that call.</p>
<p>A function definition introduces the function name in the current symbol table.
The value of the function name has a type that is recognized by the interpreter
as a user-defined function. This value can be assigned to another name which
can then also be used as a function. This serves as a general renaming
mechanism:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">fib</span>
<span class="go"><function fib at 10042ed0></span>
<span class="gp">>>> </span><span class="n">f</span> <span class="o">=</span> <span class="n">fib</span>
<span class="gp">>>> </span><span class="n">f</span><span class="p">(</span><span class="mi">100</span><span class="p">)</span>
<span class="go">0 1 1 2 3 5 8 13 21 34 55 89</span>
</pre></div>
</div>
<p>Coming from other languages, you might object that <code class="docutils literal notranslate"><span class="pre">fib</span></code> is not a function but
a procedure since it doesn’t return a value. In fact, even functions without a
<a class="reference internal" href="../reference/simple_stmts.html#return"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">return</span></code></a> statement do return a value, albeit a rather boring one. This
value is called <code class="docutils literal notranslate"><span class="pre">None</span></code> (it’s a built-in name). Writing the value <code class="docutils literal notranslate"><span class="pre">None</span></code> is
normally suppressed by the interpreter if it would be the only value written.
You can see it if you really want to using <a class="reference internal" href="../library/functions.html#print" title="print"><code class="xref py py-func docutils literal notranslate"><span class="pre">print()</span></code></a>:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">fib</span><span class="p">(</span><span class="mi">0</span><span class="p">)</span>
<span class="gp">>>> </span><span class="nb">print</span><span class="p">(</span><span class="n">fib</span><span class="p">(</span><span class="mi">0</span><span class="p">))</span>
<span class="go">None</span>
</pre></div>
</div>
<p>It is simple to write a function that returns a list of the numbers of the
Fibonacci series, instead of printing it:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">def</span> <span class="nf">fib2</span><span class="p">(</span><span class="n">n</span><span class="p">):</span> <span class="c1"># return Fibonacci series up to n</span>
<span class="gp">... </span> <span class="sd">"""Return a list containing the Fibonacci series up to n."""</span>
<span class="gp">... </span> <span class="n">result</span> <span class="o">=</span> <span class="p">[]</span>
<span class="gp">... </span> <span class="n">a</span><span class="p">,</span> <span class="n">b</span> <span class="o">=</span> <span class="mi">0</span><span class="p">,</span> <span class="mi">1</span>
<span class="gp">... </span> <span class="k">while</span> <span class="n">a</span> <span class="o"><</span> <span class="n">n</span><span class="p">:</span>
<span class="gp">... </span> <span class="n">result</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">a</span><span class="p">)</span> <span class="c1"># see below</span>
<span class="gp">... </span> <span class="n">a</span><span class="p">,</span> <span class="n">b</span> <span class="o">=</span> <span class="n">b</span><span class="p">,</span> <span class="n">a</span><span class="o">+</span><span class="n">b</span>
<span class="gp">... </span> <span class="k">return</span> <span class="n">result</span>
<span class="gp">...</span>
<span class="gp">>>> </span><span class="n">f100</span> <span class="o">=</span> <span class="n">fib2</span><span class="p">(</span><span class="mi">100</span><span class="p">)</span> <span class="c1"># call it</span>
<span class="gp">>>> </span><span class="n">f100</span> <span class="c1"># write the result</span>
<span class="go">[0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89]</span>
</pre></div>
</div>
<p>This example, as usual, demonstrates some new Python features:</p>
<ul class="simple">
<li>The <a class="reference internal" href="../reference/simple_stmts.html#return"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">return</span></code></a> statement returns with a value from a function.
<a class="reference internal" href="../reference/simple_stmts.html#return"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">return</span></code></a> without an expression argument returns <code class="docutils literal notranslate"><span class="pre">None</span></code>. Falling off
the end of a function also returns <code class="docutils literal notranslate"><span class="pre">None</span></code>.</li>
<li>The statement <code class="docutils literal notranslate"><span class="pre">result.append(a)</span></code> calls a <em>method</em> of the list object
<code class="docutils literal notranslate"><span class="pre">result</span></code>. A method is a function that 『belongs』 to an object and is named
<code class="docutils literal notranslate"><span class="pre">obj.methodname</span></code>, where <code class="docutils literal notranslate"><span class="pre">obj</span></code> is some object (this may be an expression),
and <code class="docutils literal notranslate"><span class="pre">methodname</span></code> is the name of a method that is defined by the object’s type.
Different types define different methods. Methods of different types may have
the same name without causing ambiguity. (It is possible to define your own
object types and methods, using <em>classes</em>, see <a class="reference internal" href="classes.html#tut-classes"><span class="std std-ref">Classes</span></a>)
The method <code class="xref py py-meth docutils literal notranslate"><span class="pre">append()</span></code> shown in the example is defined for list objects; it
adds a new element at the end of the list. In this example it is equivalent to
<code class="docutils literal notranslate"><span class="pre">result</span> <span class="pre">=</span> <span class="pre">result</span> <span class="pre">+</span> <span class="pre">[a]</span></code>, but more efficient.</li>
</ul>
</div>
<div class="section" id="more-on-defining-functions">
<span id="tut-defining"></span><h2>4.7. More on Defining Functions<a class="headerlink" href="#more-on-defining-functions" title="本標題的永久連結">¶</a></h2>
<p>It is also possible to define functions with a variable number of arguments.
There are three forms, which can be combined.</p>
<div class="section" id="default-argument-values">
<span id="tut-defaultargs"></span><h3>4.7.1. Default Argument Values<a class="headerlink" href="#default-argument-values" title="本標題的永久連結">¶</a></h3>
<p>The most useful form is to specify a default value for one or more arguments.
This creates a function that can be called with fewer arguments than it is
defined to allow. For example:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">ask_ok</span><span class="p">(</span><span class="n">prompt</span><span class="p">,</span> <span class="n">retries</span><span class="o">=</span><span class="mi">4</span><span class="p">,</span> <span class="n">reminder</span><span class="o">=</span><span class="s1">'Please try again!'</span><span class="p">):</span>
<span class="k">while</span> <span class="kc">True</span><span class="p">:</span>
<span class="n">ok</span> <span class="o">=</span> <span class="nb">input</span><span class="p">(</span><span class="n">prompt</span><span class="p">)</span>
<span class="k">if</span> <span class="n">ok</span> <span class="ow">in</span> <span class="p">(</span><span class="s1">'y'</span><span class="p">,</span> <span class="s1">'ye'</span><span class="p">,</span> <span class="s1">'yes'</span><span class="p">):</span>
<span class="k">return</span> <span class="kc">True</span>
<span class="k">if</span> <span class="n">ok</span> <span class="ow">in</span> <span class="p">(</span><span class="s1">'n'</span><span class="p">,</span> <span class="s1">'no'</span><span class="p">,</span> <span class="s1">'nop'</span><span class="p">,</span> <span class="s1">'nope'</span><span class="p">):</span>
<span class="k">return</span> <span class="kc">False</span>
<span class="n">retries</span> <span class="o">=</span> <span class="n">retries</span> <span class="o">-</span> <span class="mi">1</span>
<span class="k">if</span> <span class="n">retries</span> <span class="o"><</span> <span class="mi">0</span><span class="p">:</span>
<span class="k">raise</span> <span class="ne">ValueError</span><span class="p">(</span><span class="s1">'invalid user response'</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="n">reminder</span><span class="p">)</span>
</pre></div>
</div>
<p>This function can be called in several ways:</p>
<ul class="simple">
<li>giving only the mandatory argument:
<code class="docutils literal notranslate"><span class="pre">ask_ok('Do</span> <span class="pre">you</span> <span class="pre">really</span> <span class="pre">want</span> <span class="pre">to</span> <span class="pre">quit?')</span></code></li>
<li>giving one of the optional arguments:
<code class="docutils literal notranslate"><span class="pre">ask_ok('OK</span> <span class="pre">to</span> <span class="pre">overwrite</span> <span class="pre">the</span> <span class="pre">file?',</span> <span class="pre">2)</span></code></li>
<li>or even giving all arguments:
<code class="docutils literal notranslate"><span class="pre">ask_ok('OK</span> <span class="pre">to</span> <span class="pre">overwrite</span> <span class="pre">the</span> <span class="pre">file?',</span> <span class="pre">2,</span> <span class="pre">'Come</span> <span class="pre">on,</span> <span class="pre">only</span> <span class="pre">yes</span> <span class="pre">or</span> <span class="pre">no!')</span></code></li>
</ul>
<p>This example also introduces the <a class="reference internal" href="../reference/expressions.html#in"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">in</span></code></a> keyword. This tests whether or
not a sequence contains a certain value.</p>
<p>The default values are evaluated at the point of function definition in the
<em>defining</em> scope, so that</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">i</span> <span class="o">=</span> <span class="mi">5</span>
<span class="k">def</span> <span class="nf">f</span><span class="p">(</span><span class="n">arg</span><span class="o">=</span><span class="n">i</span><span class="p">):</span>
<span class="nb">print</span><span class="p">(</span><span class="n">arg</span><span class="p">)</span>
<span class="n">i</span> <span class="o">=</span> <span class="mi">6</span>
<span class="n">f</span><span class="p">()</span>
</pre></div>
</div>
<p>will print <code class="docutils literal notranslate"><span class="pre">5</span></code>.</p>
<p><strong>Important warning:</strong> The default value is evaluated only once. This makes a
difference when the default is a mutable object such as a list, dictionary, or
instances of most classes. For example, the following function accumulates the
arguments passed to it on subsequent calls:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">f</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="n">L</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">a</span><span class="p">)</span>
<span class="k">return</span> <span class="n">L</span>
<span class="nb">print</span><span class="p">(</span><span class="n">f</span><span class="p">(</span><span class="mi">1</span><span class="p">))</span>
<span class="nb">print</span><span class="p">(</span><span class="n">f</span><span class="p">(</span><span class="mi">2</span><span class="p">))</span>
<span class="nb">print</span><span class="p">(</span><span class="n">f</span><span class="p">(</span><span class="mi">3</span><span class="p">))</span>
</pre></div>
</div>
<p>This will print</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></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">2</span><span class="p">]</span>
<span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">]</span>
</pre></div>
</div>
<p>If you don’t want the default to be shared between subsequent calls, you can
write the function like this instead:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">f</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="kc">None</span><span class="p">):</span>
<span class="k">if</span> <span class="n">L</span> <span class="ow">is</span> <span class="kc">None</span><span class="p">:</span>
<span class="n">L</span> <span class="o">=</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">a</span><span class="p">)</span>
<span class="k">return</span> <span class="n">L</span>
</pre></div>
</div>
</div>
<div class="section" id="keyword-arguments">
<span id="tut-keywordargs"></span><h3>4.7.2. Keyword Arguments<a class="headerlink" href="#keyword-arguments" title="本標題的永久連結">¶</a></h3>
<p>Functions can also be called using <a class="reference internal" href="../glossary.html#term-keyword-argument"><span class="xref std std-term">keyword arguments</span></a>
of the form <code class="docutils literal notranslate"><span class="pre">kwarg=value</span></code>. For instance, the following function:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">parrot</span><span class="p">(</span><span class="n">voltage</span><span class="p">,</span> <span class="n">state</span><span class="o">=</span><span class="s1">'a stiff'</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s1">'voom'</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="s1">'Norwegian Blue'</span><span class="p">):</span>
<span class="nb">print</span><span class="p">(</span><span class="s2">"-- This parrot wouldn't"</span><span class="p">,</span> <span class="n">action</span><span class="p">,</span> <span class="n">end</span><span class="o">=</span><span class="s1">' '</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="s2">"if you put"</span><span class="p">,</span> <span class="n">voltage</span><span class="p">,</span> <span class="s2">"volts through it."</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="s2">"-- Lovely plumage, the"</span><span class="p">,</span> <span class="nb">type</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="s2">"-- It's"</span><span class="p">,</span> <span class="n">state</span><span class="p">,</span> <span class="s2">"!"</span><span class="p">)</span>
</pre></div>
</div>
<p>accepts one required argument (<code class="docutils literal notranslate"><span class="pre">voltage</span></code>) and three optional arguments
(<code class="docutils literal notranslate"><span class="pre">state</span></code>, <code class="docutils literal notranslate"><span class="pre">action</span></code>, and <code class="docutils literal notranslate"><span class="pre">type</span></code>). This function can be called in any
of the following ways:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">parrot</span><span class="p">(</span><span class="mi">1000</span><span class="p">)</span> <span class="c1"># 1 positional argument</span>
<span class="n">parrot</span><span class="p">(</span><span class="n">voltage</span><span class="o">=</span><span class="mi">1000</span><span class="p">)</span> <span class="c1"># 1 keyword argument</span>
<span class="n">parrot</span><span class="p">(</span><span class="n">voltage</span><span class="o">=</span><span class="mi">1000000</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s1">'VOOOOOM'</span><span class="p">)</span> <span class="c1"># 2 keyword arguments</span>
<span class="n">parrot</span><span class="p">(</span><span class="n">action</span><span class="o">=</span><span class="s1">'VOOOOOM'</span><span class="p">,</span> <span class="n">voltage</span><span class="o">=</span><span class="mi">1000000</span><span class="p">)</span> <span class="c1"># 2 keyword arguments</span>
<span class="n">parrot</span><span class="p">(</span><span class="s1">'a million'</span><span class="p">,</span> <span class="s1">'bereft of life'</span><span class="p">,</span> <span class="s1">'jump'</span><span class="p">)</span> <span class="c1"># 3 positional arguments</span>
<span class="n">parrot</span><span class="p">(</span><span class="s1">'a thousand'</span><span class="p">,</span> <span class="n">state</span><span class="o">=</span><span class="s1">'pushing up the daisies'</span><span class="p">)</span> <span class="c1"># 1 positional, 1 keyword</span>
</pre></div>
</div>
<p>but all the following calls would be invalid:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">parrot</span><span class="p">()</span> <span class="c1"># required argument missing</span>
<span class="n">parrot</span><span class="p">(</span><span class="n">voltage</span><span class="o">=</span><span class="mf">5.0</span><span class="p">,</span> <span class="s1">'dead'</span><span class="p">)</span> <span class="c1"># non-keyword argument after a keyword argument</span>
<span class="n">parrot</span><span class="p">(</span><span class="mi">110</span><span class="p">,</span> <span class="n">voltage</span><span class="o">=</span><span class="mi">220</span><span class="p">)</span> <span class="c1"># duplicate value for the same argument</span>
<span class="n">parrot</span><span class="p">(</span><span class="n">actor</span><span class="o">=</span><span class="s1">'John Cleese'</span><span class="p">)</span> <span class="c1"># unknown keyword argument</span>
</pre></div>
</div>
<p>In a function call, keyword arguments must follow positional arguments.
All the keyword arguments passed must match one of the arguments
accepted by the function (e.g. <code class="docutils literal notranslate"><span class="pre">actor</span></code> is not a valid argument for the
<code class="docutils literal notranslate"><span class="pre">parrot</span></code> function), and their order is not important. This also includes
non-optional arguments (e.g. <code class="docutils literal notranslate"><span class="pre">parrot(voltage=1000)</span></code> is valid too).
No argument may receive a value more than once.
Here’s an example that fails due to this restriction:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">def</span> <span class="nf">function</span><span class="p">(</span><span class="n">a</span><span class="p">):</span>
<span class="gp">... </span> <span class="k">pass</span>
<span class="gp">...</span>
<span class="gp">>>> </span><span class="n">function</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="n">a</span><span class="o">=</span><span class="mi">0</span><span class="p">)</span>
<span class="gt">Traceback (most recent call last):</span>
File <span class="nb">"<stdin>"</span>, line <span class="m">1</span>, in <span class="n"><module></span>
<span class="gr">TypeError</span>: <span class="n">function() got multiple values for keyword argument 'a'</span>
</pre></div>
</div>
<p>When a final formal parameter of the form <code class="docutils literal notranslate"><span class="pre">**name</span></code> is present, it receives a
dictionary (see <a class="reference internal" href="../library/stdtypes.html#typesmapping"><span class="std std-ref">Mapping Types — dict</span></a>) containing all keyword arguments except for
those corresponding to a formal parameter. This may be combined with a formal
parameter of the form <code class="docutils literal notranslate"><span class="pre">*name</span></code> (described in the next subsection) which
receives a tuple containing the positional arguments beyond the formal parameter
list. (<code class="docutils literal notranslate"><span class="pre">*name</span></code> must occur before <code class="docutils literal notranslate"><span class="pre">**name</span></code>.) For example, if we define a
function like this:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">cheeseshop</span><span class="p">(</span><span class="n">kind</span><span class="p">,</span> <span class="o">*</span><span class="n">arguments</span><span class="p">,</span> <span class="o">**</span><span class="n">keywords</span><span class="p">):</span>
<span class="nb">print</span><span class="p">(</span><span class="s2">"-- Do you have any"</span><span class="p">,</span> <span class="n">kind</span><span class="p">,</span> <span class="s2">"?"</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="s2">"-- I'm sorry, we're all out of"</span><span class="p">,</span> <span class="n">kind</span><span class="p">)</span>
<span class="k">for</span> <span class="n">arg</span> <span class="ow">in</span> <span class="n">arguments</span><span class="p">:</span>
<span class="nb">print</span><span class="p">(</span><span class="n">arg</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="s2">"-"</span> <span class="o">*</span> <span class="mi">40</span><span class="p">)</span>
<span class="k">for</span> <span class="n">kw</span> <span class="ow">in</span> <span class="n">keywords</span><span class="p">:</span>
<span class="nb">print</span><span class="p">(</span><span class="n">kw</span><span class="p">,</span> <span class="s2">":"</span><span class="p">,</span> <span class="n">keywords</span><span class="p">[</span><span class="n">kw</span><span class="p">])</span>
</pre></div>
</div>
<p>It could be called like this:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">cheeseshop</span><span class="p">(</span><span class="s2">"Limburger"</span><span class="p">,</span> <span class="s2">"It's very runny, sir."</span><span class="p">,</span>
<span class="s2">"It's really very, VERY runny, sir."</span><span class="p">,</span>
<span class="n">shopkeeper</span><span class="o">=</span><span class="s2">"Michael Palin"</span><span class="p">,</span>
<span class="n">client</span><span class="o">=</span><span class="s2">"John Cleese"</span><span class="p">,</span>
<span class="n">sketch</span><span class="o">=</span><span class="s2">"Cheese Shop Sketch"</span><span class="p">)</span>
</pre></div>
</div>
<p>and of course it would print:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>-- Do you have any Limburger ?
-- I'm sorry, we're all out of Limburger
It's very runny, sir.
It's really very, VERY runny, sir.
----------------------------------------
shopkeeper : Michael Palin
client : John Cleese
sketch : Cheese Shop Sketch
</pre></div>
</div>
<p>Note that the order in which the keyword arguments are printed is guaranteed
to match the order in which they were provided in the function call.</p>
</div>
<div class="section" id="arbitrary-argument-lists">
<span id="tut-arbitraryargs"></span><h3>4.7.3. Arbitrary Argument Lists<a class="headerlink" href="#arbitrary-argument-lists" title="本標題的永久連結">¶</a></h3>
<p id="index-2">Finally, the least frequently used option is to specify that a function can be
called with an arbitrary number of arguments. These arguments will be wrapped
up in a tuple (see <a class="reference internal" href="datastructures.html#tut-tuples"><span class="std std-ref">Tuples 和序列 (Sequences)</span></a>). Before the variable number of arguments,
zero or more normal arguments may occur.</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">write_multiple_items</span><span class="p">(</span><span class="n">file</span><span class="p">,</span> <span class="n">separator</span><span class="p">,</span> <span class="o">*</span><span class="n">args</span><span class="p">):</span>
<span class="n">file</span><span class="o">.</span><span class="n">write</span><span class="p">(</span><span class="n">separator</span><span class="o">.</span><span class="n">join</span><span class="p">(</span><span class="n">args</span><span class="p">))</span>
</pre></div>
</div>
<p>Normally, these <code class="docutils literal notranslate"><span class="pre">variadic</span></code> arguments will be last in the list of formal
parameters, because they scoop up all remaining input arguments that are
passed to the function. Any formal parameters which occur after the <code class="docutils literal notranslate"><span class="pre">*args</span></code>
parameter are 『keyword-only』 arguments, meaning that they can only be used as
keywords rather than positional arguments.</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">def</span> <span class="nf">concat</span><span class="p">(</span><span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="n">sep</span><span class="o">=</span><span class="s2">"/"</span><span class="p">):</span>
<span class="gp">... </span> <span class="k">return</span> <span class="n">sep</span><span class="o">.</span><span class="n">join</span><span class="p">(</span><span class="n">args</span><span class="p">)</span>
<span class="gp">...</span>
<span class="gp">>>> </span><span class="n">concat</span><span class="p">(</span><span class="s2">"earth"</span><span class="p">,</span> <span class="s2">"mars"</span><span class="p">,</span> <span class="s2">"venus"</span><span class="p">)</span>
<span class="go">'earth/mars/venus'</span>
<span class="gp">>>> </span><span class="n">concat</span><span class="p">(</span><span class="s2">"earth"</span><span class="p">,</span> <span class="s2">"mars"</span><span class="p">,</span> <span class="s2">"venus"</span><span class="p">,</span> <span class="n">sep</span><span class="o">=</span><span class="s2">"."</span><span class="p">)</span>
<span class="go">'earth.mars.venus'</span>
</pre></div>
</div>
</div>
<div class="section" id="unpacking-argument-lists">
<span id="tut-unpacking-arguments"></span><h3>4.7.4. Unpacking Argument Lists<a class="headerlink" href="#unpacking-argument-lists" title="本標題的永久連結">¶</a></h3>
<p>The reverse situation occurs when the arguments are already in a list or tuple
but need to be unpacked for a function call requiring separate positional
arguments. For instance, the built-in <a class="reference internal" href="../library/stdtypes.html#range" title="range"><code class="xref py py-func docutils literal notranslate"><span class="pre">range()</span></code></a> function expects separate
<em>start</em> and <em>stop</em> arguments. If they are not available separately, write the
function call with the <code class="docutils literal notranslate"><span class="pre">*</span></code>-operator to unpack the arguments out of a list
or tuple:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="nb">list</span><span class="p">(</span><span class="nb">range</span><span class="p">(</span><span class="mi">3</span><span class="p">,</span> <span class="mi">6</span><span class="p">))</span> <span class="c1"># normal call with separate arguments</span>
<span class="go">[3, 4, 5]</span>
<span class="gp">>>> </span><span class="n">args</span> <span class="o">=</span> <span class="p">[</span><span class="mi">3</span><span class="p">,</span> <span class="mi">6</span><span class="p">]</span>
<span class="gp">>>> </span><span class="nb">list</span><span class="p">(</span><span class="nb">range</span><span class="p">(</span><span class="o">*</span><span class="n">args</span><span class="p">))</span> <span class="c1"># call with arguments unpacked from a list</span>
<span class="go">[3, 4, 5]</span>
</pre></div>
</div>
<p id="index-3">In the same fashion, dictionaries can deliver keyword arguments with the <code class="docutils literal notranslate"><span class="pre">**</span></code>-operator:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">def</span> <span class="nf">parrot</span><span class="p">(</span><span class="n">voltage</span><span class="p">,</span> <span class="n">state</span><span class="o">=</span><span class="s1">'a stiff'</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s1">'voom'</span><span class="p">):</span>
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s2">"-- This parrot wouldn't"</span><span class="p">,</span> <span class="n">action</span><span class="p">,</span> <span class="n">end</span><span class="o">=</span><span class="s1">' '</span><span class="p">)</span>
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s2">"if you put"</span><span class="p">,</span> <span class="n">voltage</span><span class="p">,</span> <span class="s2">"volts through it."</span><span class="p">,</span> <span class="n">end</span><span class="o">=</span><span class="s1">' '</span><span class="p">)</span>
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s2">"E's"</span><span class="p">,</span> <span class="n">state</span><span class="p">,</span> <span class="s2">"!"</span><span class="p">)</span>
<span class="gp">...</span>
<span class="gp">>>> </span><span class="n">d</span> <span class="o">=</span> <span class="p">{</span><span class="s2">"voltage"</span><span class="p">:</span> <span class="s2">"four million"</span><span class="p">,</span> <span class="s2">"state"</span><span class="p">:</span> <span class="s2">"bleedin' demised"</span><span class="p">,</span> <span class="s2">"action"</span><span class="p">:</span> <span class="s2">"VOOM"</span><span class="p">}</span>
<span class="gp">>>> </span><span class="n">parrot</span><span class="p">(</span><span class="o">**</span><span class="n">d</span><span class="p">)</span>
<span class="go">-- This parrot wouldn't VOOM if you put four million volts through it. E's bleedin' demised !</span>
</pre></div>
</div>
</div>
<div class="section" id="lambda-expressions">
<span id="tut-lambda"></span><h3>4.7.5. Lambda Expressions<a class="headerlink" href="#lambda-expressions" title="本標題的永久連結">¶</a></h3>
<p>Small anonymous functions can be created with the <a class="reference internal" href="../reference/expressions.html#lambda"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">lambda</span></code></a> keyword.
This function returns the sum of its two arguments: <code class="docutils literal notranslate"><span class="pre">lambda</span> <span class="pre">a,</span> <span class="pre">b:</span> <span class="pre">a+b</span></code>.
Lambda functions can be used wherever function objects are required. They are
syntactically restricted to a single expression. Semantically, they are just
syntactic sugar for a normal function definition. Like nested function
definitions, lambda functions can reference variables from the containing
scope:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">def</span> <span class="nf">make_incrementor</span><span class="p">(</span><span class="n">n</span><span class="p">):</span>
<span class="gp">... </span> <span class="k">return</span> <span class="k">lambda</span> <span class="n">x</span><span class="p">:</span> <span class="n">x</span> <span class="o">+</span> <span class="n">n</span>
<span class="gp">...</span>
<span class="gp">>>> </span><span class="n">f</span> <span class="o">=</span> <span class="n">make_incrementor</span><span class="p">(</span><span class="mi">42</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">f</span><span class="p">(</span><span class="mi">0</span><span class="p">)</span>
<span class="go">42</span>
<span class="gp">>>> </span><span class="n">f</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span>
<span class="go">43</span>
</pre></div>
</div>
<p>The above example uses a lambda expression to return a function. Another use
is to pass a small function as an argument:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">pairs</span> <span class="o">=</span> <span class="p">[(</span><span class="mi">1</span><span class="p">,</span> <span class="s1">'one'</span><span class="p">),</span> <span class="p">(</span><span class="mi">2</span><span class="p">,</span> <span class="s1">'two'</span><span class="p">),</span> <span class="p">(</span><span class="mi">3</span><span class="p">,</span> <span class="s1">'three'</span><span class="p">),</span> <span class="p">(</span><span class="mi">4</span><span class="p">,</span> <span class="s1">'four'</span><span class="p">)]</span>
<span class="gp">>>> </span><span class="n">pairs</span><span class="o">.</span><span class="n">sort</span><span class="p">(</span><span class="n">key</span><span class="o">=</span><span class="k">lambda</span> <span class="n">pair</span><span class="p">:</span> <span class="n">pair</span><span class="p">[</span><span class="mi">1</span><span class="p">])</span>
<span class="gp">>>> </span><span class="n">pairs</span>
<span class="go">[(4, 'four'), (1, 'one'), (3, 'three'), (2, 'two')]</span>
</pre></div>
</div>
</div>
<div class="section" id="documentation-strings">
<span id="tut-docstrings"></span><h3>4.7.6. 說明文件字串<a class="headerlink" href="#documentation-strings" title="本標題的永久連結">¶</a></h3>
<p id="index-4">Here are some conventions about the content and formatting of documentation
strings.</p>
<p>The first line should always be a short, concise summary of the object’s
purpose. For brevity, it should not explicitly state the object’s name or type,
since these are available by other means (except if the name happens to be a
verb describing a function’s operation). This line should begin with a capital
letter and end with a period.</p>
<p>If there are more lines in the documentation string, the second line should be
blank, visually separating the summary from the rest of the description. The
following lines should be one or more paragraphs describing the object’s calling
conventions, its side effects, etc.</p>
<p>The Python parser does not strip indentation from multi-line string literals in
Python, so tools that process documentation have to strip indentation if
desired. This is done using the following convention. The first non-blank line
<em>after</em> the first line of the string determines the amount of indentation for
the entire documentation string. (We can’t use the first line since it is
generally adjacent to the string’s opening quotes so its indentation is not
apparent in the string literal.) Whitespace 「equivalent」 to this indentation is
then stripped from the start of all lines of the string. Lines that are
indented less should not occur, but if they occur all their leading whitespace
should be stripped. Equivalence of whitespace should be tested after expansion
of tabs (to 8 spaces, normally).</p>
<p>Here is an example of a multi-line docstring:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">def</span> <span class="nf">my_function</span><span class="p">():</span>
<span class="gp">... </span> <span class="sd">"""Do nothing, but document it.</span>
<span class="gp">...</span><span class="sd"></span>
<span class="gp">... </span><span class="sd"> No, really, it doesn't do anything.</span>
<span class="gp">... </span><span class="sd"> """</span>
<span class="gp">... </span> <span class="k">pass</span>
<span class="gp">...</span>
<span class="gp">>>> </span><span class="nb">print</span><span class="p">(</span><span class="n">my_function</span><span class="o">.</span><span class="vm">__doc__</span><span class="p">)</span>
<span class="go">Do nothing, but document it.</span>
<span class="go"> No, really, it doesn't do anything.</span>
</pre></div>
</div>
</div>
<div class="section" id="function-annotations">
<span id="tut-annotations"></span><h3>4.7.7. Function Annotations<a class="headerlink" href="#function-annotations" title="本標題的永久連結">¶</a></h3>
<p id="index-5"><a class="reference internal" href="../reference/compound_stmts.html#function"><span class="std std-ref">Function annotations</span></a> are completely optional metadata
information about the types used by user-defined functions (see <span class="target" id="index-6"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-3107"><strong>PEP 3107</strong></a> and
<span class="target" id="index-7"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0484"><strong>PEP 484</strong></a> for more information).</p>
<p>Annotations are stored in the <code class="xref py py-attr docutils literal notranslate"><span class="pre">__annotations__</span></code> attribute of the function
as a dictionary and have no effect on any other part of the function. Parameter
annotations are defined by a colon after the parameter name, followed by an
expression evaluating to the value of the annotation. Return annotations are
defined by a literal <code class="docutils literal notranslate"><span class="pre">-></span></code>, followed by an expression, between the parameter
list and the colon denoting the end of the <a class="reference internal" href="../reference/compound_stmts.html#def"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">def</span></code></a> statement. The
following example has a positional argument, a keyword argument, and the return
value annotated:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">def</span> <span class="nf">f</span><span class="p">(</span><span class="n">ham</span><span class="p">:</span> <span class="nb">str</span><span class="p">,</span> <span class="n">eggs</span><span class="p">:</span> <span class="nb">str</span> <span class="o">=</span> <span class="s1">'eggs'</span><span class="p">)</span> <span class="o">-></span> <span class="nb">str</span><span class="p">:</span>
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s2">"Annotations:"</span><span class="p">,</span> <span class="n">f</span><span class="o">.</span><span class="vm">__annotations__</span><span class="p">)</span>
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s2">"Arguments:"</span><span class="p">,</span> <span class="n">ham</span><span class="p">,</span> <span class="n">eggs</span><span class="p">)</span>
<span class="gp">... </span> <span class="k">return</span> <span class="n">ham</span> <span class="o">+</span> <span class="s1">' and '</span> <span class="o">+</span> <span class="n">eggs</span>
<span class="gp">...</span>
<span class="gp">>>> </span><span class="n">f</span><span class="p">(</span><span class="s1">'spam'</span><span class="p">)</span>
<span class="go">Annotations: {'ham': <class 'str'>, 'return': <class 'str'>, 'eggs': <class 'str'>}</span>
<span class="go">Arguments: spam eggs</span>
<span class="go">'spam and eggs'</span>
</pre></div>
</div>
</div>
</div>
<div class="section" id="intermezzo-coding-style">
<span id="tut-codingstyle"></span><h2>4.8. Intermezzo: Coding Style<a class="headerlink" href="#intermezzo-coding-style" title="本標題的永久連結">¶</a></h2>
<p id="index-8">Now that you are about to write longer, more complex pieces of Python, it is a
good time to talk about <em>coding style</em>. Most languages can be written (or more
concise, <em>formatted</em>) in different styles; some are more readable than others.
Making it easy for others to read your code is always a good idea, and adopting
a nice coding style helps tremendously for that.</p>
<p>For Python, <span class="target" id="index-9"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0008"><strong>PEP 8</strong></a> has emerged as the style guide that most projects adhere to;
it promotes a very readable and eye-pleasing coding style. Every Python
developer should read it at some point; here are the most important points
extracted for you:</p>
<ul>
<li><p class="first">Use 4-space indentation, and no tabs.</p>
<p>4 spaces are a good compromise between small indentation (allows greater
nesting depth) and large indentation (easier to read). Tabs introduce
confusion, and are best left out.</p>
</li>
<li><p class="first">Wrap lines so that they don’t exceed 79 characters.</p>
<p>This helps users with small displays and makes it possible to have several
code files side-by-side on larger displays.</p>
</li>
<li><p class="first">Use blank lines to separate functions and classes, and larger blocks of
code inside functions.</p>
</li>
<li><p class="first">When possible, put comments on a line of their own.</p>
</li>
<li><p class="first">Use docstrings.</p>
</li>
<li><p class="first">Use spaces around operators and after commas, but not directly inside
bracketing constructs: <code class="docutils literal notranslate"><span class="pre">a</span> <span class="pre">=</span> <span class="pre">f(1,</span> <span class="pre">2)</span> <span class="pre">+</span> <span class="pre">g(3,</span> <span class="pre">4)</span></code>.</p>
</li>
<li><p class="first">Name your classes and functions consistently; the convention is to use
<code class="docutils literal notranslate"><span class="pre">CamelCase</span></code> for classes and <code class="docutils literal notranslate"><span class="pre">lower_case_with_underscores</span></code> for functions
and methods. Always use <code class="docutils literal notranslate"><span class="pre">self</span></code> as the name for the first method argument
(see <a class="reference internal" href="classes.html#tut-firstclasses"><span class="std std-ref">A First Look at Classes</span></a> for more on classes and methods).</p>
</li>
<li><p class="first">Don’t use fancy encodings if your code is meant to be used in international
environments. Python’s default, UTF-8, or even plain ASCII work best in any
case.</p>
</li>
<li><p class="first">Likewise, don’t use non-ASCII characters in identifiers if there is only the
slightest chance people speaking a different language will read or maintain
the code.</p>
</li>
</ul>
<p class="rubric">註解</p>
<table class="docutils footnote" frame="void" id="id2" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id1">[1]</a></td><td>Actually, <em>call by object reference</em> would be a better description,
since if a mutable object is passed, the caller will see any changes the
callee makes to it (items inserted into a list).</td></tr>
</tbody>
</table>
</div>
</div>
</div>
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<h3><a href="../contents.html">目錄</a></h3>
<ul>
<li><a class="reference internal" href="#">4. 深入了解流程控制</a><ul>
<li><a class="reference internal" href="#if-statements">4.1. <code class="docutils literal notranslate"><span class="pre">if</span></code> 陳述式</a></li>
<li><a class="reference internal" href="#for-statements">4.2. <code class="docutils literal notranslate"><span class="pre">for</span></code> 陳述式</a></li>
<li><a class="reference internal" href="#the-range-function">4.3. <code class="docutils literal notranslate"><span class="pre">range()</span></code> 函式</a></li>
<li><a class="reference internal" href="#break-and-continue-statements-and-else-clauses-on-loops">4.4. <code class="docutils literal notranslate"><span class="pre">break</span></code> 和 <code class="docutils literal notranslate"><span class="pre">continue</span></code> 陳述、迴圈內 <code class="docutils literal notranslate"><span class="pre">else</span></code> 段落</a></li>
<li><a class="reference internal" href="#pass-statements">4.5. <code class="docutils literal notranslate"><span class="pre">pass</span></code> 陳述式</a></li>
<li><a class="reference internal" href="#defining-functions">4.6. 定義函式 (function)</a></li>
<li><a class="reference internal" href="#more-on-defining-functions">4.7. More on Defining Functions</a><ul>
<li><a class="reference internal" href="#default-argument-values">4.7.1. Default Argument Values</a></li>
<li><a class="reference internal" href="#keyword-arguments">4.7.2. Keyword Arguments</a></li>
<li><a class="reference internal" href="#arbitrary-argument-lists">4.7.3. Arbitrary Argument Lists</a></li>
<li><a class="reference internal" href="#unpacking-argument-lists">4.7.4. Unpacking Argument Lists</a></li>
<li><a class="reference internal" href="#lambda-expressions">4.7.5. Lambda Expressions</a></li>
<li><a class="reference internal" href="#documentation-strings">4.7.6. 說明文件字串</a></li>
<li><a class="reference internal" href="#function-annotations">4.7.7. Function Annotations</a></li>
</ul>
</li>
<li><a class="reference internal" href="#intermezzo-coding-style">4.8. Intermezzo: Coding Style</a></li>
</ul>
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