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<h2 class="wrapper title">Python 3.8新特性——仅限位置形参</h2>
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<span>Author：</span><span>flypython</span> | <span>Date: </span><span>2019-04-02</span> | <span>Category：</span><span><a href="/fly/Python新特性/" title="Python新特性">Python新特性</a></span>
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<h2 id="仅限位置形参"><a href="#仅限位置形参" class="headerlink" title="仅限位置形参"></a>仅限位置形参</h2><p>Positional-only parameters官方翻译为仅限位置形参，也可以理解为只接受位置参数。意思就是，它只是一个位置参数，不接受关键字传参。</p>
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<p>语法： </p>
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<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">def funx(a,b,/): # / 指明，前面的a,b参数是仅限位置形参</span><br><span class="line"> pass</span><br></pre></td></tr></table></figure>
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<p>函数形参语法<code>/</code> 用来指明某些函数形参必须使用仅限位置而非关键字参数</p>
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<p>其实，Python内置的很多C函数接口都是这种形式，比如</p>
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<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line">>>> import builtins</span><br><span class="line">>>> help(__builtins__.divmod)</span><br><span class="line">Help on built-in function divmod in module builtins:</span><br><span class="line"></span><br><span class="line">divmod(x, y, /)</span><br><span class="line"> Return the tuple (x//y, x%y). Invariant: div*y + mod == x.</span><br></pre></td></tr></table></figure>
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<p>很多函数后面都有 <code>/</code>来表明，左边的这些参数只接受位置参数。</p>
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<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line">>>> divmod(1,2)</span><br><span class="line">(0, 1)</span><br><span class="line">>>> divmod(x=1,y=2) </span><br><span class="line">Traceback (most recent call last):</span><br><span class="line"> File "<stdin>", line 1, in <module></span><br><span class="line">TypeError: divmod() takes no keyword arguments</span><br><span class="line">>>></span><br></pre></td></tr></table></figure>
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<p>指定关键字的参数会报语法错误，它的用途就是强制使用者用位置参数来传参。</p>
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<h4 id="官方例子"><a href="#官方例子" class="headerlink" title="官方例子"></a>官方例子</h4><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line">>>> def f(a,b,/,**kwargs):</span><br><span class="line">... print(a,b,kwargs)</span><br><span class="line">...</span><br><span class="line">>>> f(10,20,a=1,b=2,c=3)</span><br><span class="line">10 20 {'a': 1, 'b': 2, 'c': 3}</span><br></pre></td></tr></table></figure>
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<p>由于在 <code>/</code> 左侧的形参不会被公开为可用关键字</p>
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<p>这里的a,b 为仅限位置参数，最后a,b会被赋值了两次。<br>位置参数赋值一次，关键字参数赋值一次，关键字参数以kwargs字典的形式存在，需要通过 <code>kwargs['a'],kwargs['b']</code>访问。</p>
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<p>现在我们来看一下，添加了仅限位置形参之后的函数参数形式</p>
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<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">def name(positional_only_parameters, /, positional_or_keyword_parameters,</span><br><span class="line"> *, keyword_only_parameters):</span><br></pre></td></tr></table></figure>
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<p>包括了仅限位置形参， <code>/</code>, 位置形参或者关键字参数 ,<code>*</code>，仅限关键字参数。</p>
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<p><img src="https://tva1.sinaimg.cn/large/006tNbRwly1gai73g9r3ej30so09mq3k.jpg" alt></p>
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<p>最后，我们可以定义以下形式的函数</p>
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<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line">def name(p1, p2, /, p_or_kw, *, kw):</span><br><span class="line">def name(p1, p2=None, /, p_or_kw=None, *, kw):</span><br><span class="line">def name(p1, p2=None, /, *, kw):</span><br><span class="line">def name(p1, p2=None, /):</span><br><span class="line">def name(p1, p2, /, p_or_kw):</span><br><span class="line">def name(p1, p2, /):</span><br></pre></td></tr></table></figure>
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<h4 id="参考"><a href="#参考" class="headerlink" title="参考"></a>参考</h4><ul>
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<li><a href="https://docs.python.org/zh-cn/3.8/whatsnew/3.8.html" target="_blank" rel="noopener">https://docs.python.org/zh-cn/3.8/whatsnew/3.8.html</a></li>
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<li><a href="https://docs.python.org/zh-cn/3/howto/clinic.html" target="_blank" rel="noopener">https://docs.python.org/zh-cn/3/howto/clinic.html</a></li>
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<li><a href="https://www.python.org/dev/peps/pep-0570" target="_blank" rel="noopener">https://www.python.org/dev/peps/pep-0570</a></li>
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