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<h1 id="things-about-python-named-args">Python 函数的命名参数相关</h1>
<p>
Posted on 三 15 五月 2013 in <a href="https://blog.tonychow.me/category/python.html">python</a>
• 2 min read
</p>
</header>
<div>
<h3>起因</h3>
<p>今天师弟问了一个关于 Python 函数参数的一个问题：</p>
<div class="highlight"><pre><span></span><code><span class="c1">#1</span>
<span class="k">def</span> <span class="nf">func</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="n">l</span><span class="o">=</span><span class="p">[]):</span>
<span class="k">pass</span>
<span class="c1">#2</span>
<span class="k">def</span> <span class="nf">func</span><span class="p">(</span><span class="n">x</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>
</code></pre></div>
<blockquote>
<p>为啥第一个函数会把 l 每次调用完的值保留下来？</p>
</blockquote>
<p>起初我认为问的是这两个函数使用的时候，为何会保持对传入的参数 l 的修改。从这个方面来讲，是因为 Python 对于数据赋值的处理的原因。</p>
<!--more-->
<p>在 Python 中，赋值是传引用的。一个列表，比如 [1, 2, 3]，或者一个字符串，'tonychow'，这些对象在创建的时候会在内存中分配一段空间。如果将这些对象赋值给一个变量名，那就会导致在 Python 的命名空间中该变量名指向内存中这个对象。对该变量名的操作就是对内存中这个对象的操作。所以如果尝试直接将一个变量 a 赋值给另外一个变量 b ，导致的后果是，命名空间中，这两个变量名 a 和 b 指向内存中同样一个对象，也就是所谓传引用赋值。对其中任意一个变量的操作，实质是对该对象进行操作，所以同样的操作后结果也会可以在另外一个变量中看到。如下：</p>
<div class="highlight"><pre><span></span><code>>>> <span class="nv">a</span> <span class="o">=</span> <span class="o">[</span><span class="m">1</span>, <span class="m">2</span>, <span class="m">3</span><span class="o">]</span>
>>> <span class="nv">b</span> <span class="o">=</span> a
>>> a.pop<span class="o">()</span>
<span class="m">3</span>
>>> b
<span class="o">[</span><span class="m">1</span>, <span class="m">2</span><span class="o">]</span>
>>> a
<span class="o">[</span><span class="m">1</span>, <span class="m">2</span><span class="o">]</span>
>>>
</code></pre></div>
<p>从上面的代码可以看到，在将 a 赋值给 b 之后，对 a 列表调用 pop 方法，导致的是 b 列表也发生了变化。我们还可以通过 Python 内置的 globals 函数和 id 函数来加深这个理解。globals 函数将会返回一个字典，这个字典是当前的全局符号表。而 id 函数则会返回一个对象的标识，可以将其返回值看作是这个对象在内存中的地址。</p>
<div class="highlight"><pre><span></span><code><span class="o">>>></span> <span class="nb">globals</span><span class="p">()</span>
<span class="p">{</span><span class="s1">'a'</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="s1">'b'</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="s1">'__builtins__'</span><span class="p">:</span> <span class="o"><</span><span class="n">module</span> <span class="s1">'__builtin__'</span> <span class="p">(</span><span class="n">built</span><span class="o">-</span><span class="ow">in</span><span class="p">)</span><span class="o">></span><span class="p">,</span>
<span class="s1">'value'</span><span class="p">:</span> <span class="kc">None</span><span class="p">,</span> <span class="s1">'__package__'</span><span class="p">:</span> <span class="kc">None</span><span class="p">,</span>
<span class="s1">'key'</span><span class="p">:</span> <span class="s1">'__doc__'</span><span class="p">,</span> <span class="s1">'__name__'</span><span class="p">:</span> <span class="s1">'__main__'</span><span class="p">,</span> <span class="s1">'__doc__'</span><span class="p">:</span> <span class="kc">None</span><span class="p">}</span>
<span class="o">>>></span> <span class="nb">id</span><span class="p">(</span><span class="n">a</span><span class="p">)</span>
<span class="mi">3077280588</span><span class="n">L</span>
<span class="o">>>></span> <span class="nb">id</span><span class="p">(</span><span class="n">b</span><span class="p">)</span>
<span class="mi">3077280588</span><span class="n">L</span>
<span class="o">>>></span>
</code></pre></div>
<p>可以看到，a 和 b 都在当前的全局字符表中，他们的值也都是一致的。此外，id 函数的结果明确地说明了 a 和 b 这两个变量名都是指向了内存中的同一个对象。而在 Python 中，调用函数的时候，传入参数，也是进行传引用的赋值。所以我师弟说的这两个函数都会保留对于传入参数的修改，也就是：</p>
<div class="highlight"><pre><span></span><code><span class="o">>>></span> <span class="k">def</span> <span class="nf">func</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="o">...</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="o">...</span> <span class="n">l</span> <span class="o">=</span> <span class="p">[]</span>
<span class="o">...</span> <span class="n">l</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span>
<span class="o">...</span>
<span class="o">>>></span> <span class="n">bar</span> <span class="o">=</span> <span class="p">[</span><span class="mi">2</span><span class="p">]</span>
<span class="o">>>></span> <span class="n">bar</span>
<span class="p">[</span><span class="mi">2</span><span class="p">]</span>
<span class="o">>>></span> <span class="n">func</span><span class="p">(</span><span class="n">bar</span><span class="p">)</span>
<span class="o">>>></span> <span class="n">func</span><span class="p">(</span><span class="n">bar</span><span class="p">)</span>
<span class="o">>>></span> <span class="n">bar</span>
<span class="p">[</span><span class="mi">2</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="mi">1</span><span class="p">]</span>
<span class="o">>>></span>
</code></pre></div>
<p>题外话，在 Python 内置的数据类型中，有两种不同的数据类型。一种是可变类型(mutable)，比如 list ， dict 等;另外一种就是不可变类型(immutable)，比如字符串或者 tuple。</p>
<p>可是后来师弟贴出了另外一段代码：</p>
<div class="highlight"><pre><span></span><code><span class="o">>>></span> <span class="k">def</span> <span class="nf">func2</span><span class="p">(</span><span class="n">items</span><span class="o">=</span><span class="kc">None</span><span class="p">):</span>
<span class="o">...</span> <span class="k">if</span> <span class="n">items</span> <span class="o">==</span> <span class="kc">None</span><span class="p">:</span>
<span class="o">...</span> <span class="n">items</span> <span class="o">=</span> <span class="p">[]</span>
<span class="o">...</span> <span class="n">items</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span>
<span class="o">...</span> <span class="k">return</span> <span class="n">items</span>
<span class="o">...</span>
<span class="o">>>></span> <span class="n">func2</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">func2</span><span class="p">()</span>
<span class="p">[</span><span class="mi">1</span><span class="p">]</span>
<span class="o">>>></span>
</code></pre></div>
<p>这下我明白了，师弟说的不是我想到的那个问题，而是命名参数的问题。</p>
<h3>解决</h3>
<p>说实话这个问题一开始我也没有想到答案。大家在学习 Python 的时候，无论看的是哪本入门书，应该在开始的时候都会看到一句话“ Python 中一切都是对象”。看代码：</p>
<div class="highlight"><pre><span></span><code><span class="o">>>></span> <span class="nb">isinstance</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="nb">int</span><span class="p">)</span>
<span class="kc">True</span>
<span class="o">>>></span> <span class="nb">isinstance</span><span class="p">(</span><span class="s1">'test'</span><span class="p">,</span> <span class="nb">str</span><span class="p">)</span>
<span class="kc">True</span>
<span class="o">>>></span> <span class="k">def</span> <span class="nf">func</span><span class="p">():</span>
<span class="o">...</span> <span class="k">pass</span>
<span class="o">...</span>
<span class="o">>>></span> <span class="nb">type</span><span class="p">(</span><span class="n">func</span><span class="p">)</span>
<span class="o"><</span><span class="nb">type</span> <span class="s1">'function'</span><span class="o">></span>
<span class="o">>>></span> <span class="nb">dir</span><span class="p">(</span><span class="n">func</span><span class="p">)</span>
<span class="p">[</span><span class="s1">'__call__'</span><span class="p">,</span> <span class="s1">'__class__'</span><span class="p">,</span> <span class="s1">'__closure__'</span><span class="p">,</span> <span class="s1">'__code__'</span><span class="p">,</span> <span class="s1">'__defaults__'</span><span class="p">,</span>
<span class="s1">'__delattr__'</span><span class="p">,</span> <span class="s1">'__dict__'</span><span class="p">,</span> <span class="s1">'__doc__'</span><span class="p">,</span> <span class="s1">'__format__'</span><span class="p">,</span> <span class="s1">'__get__'</span><span class="p">,</span>
<span class="s1">'__getattribute__'</span><span class="p">,</span> <span class="s1">'__globals__'</span><span class="p">,</span> <span class="s1">'__hash__'</span><span class="p">,</span> <span class="s1">'__init__'</span><span class="p">,</span>
<span class="s1">'__module__'</span><span class="p">,</span> <span class="s1">'__name__'</span><span class="p">,</span> <span class="s1">'__new__'</span><span class="p">,</span> <span class="s1">'__reduce__'</span><span class="p">,</span> <span class="s1">'__reduce_ex__'</span><span class="p">,</span>
<span class="s1">'__repr__'</span><span class="p">,</span> <span class="s1">'__setattr__'</span><span class="p">,</span> <span class="s1">'__sizeof__'</span><span class="p">,</span> <span class="s1">'__str__'</span><span class="p">,</span> <span class="s1">'__subclasshook__'</span><span class="p">,</span>
<span class="s1">'func_closure'</span><span class="p">,</span> <span class="s1">'func_code'</span><span class="p">,</span> <span class="s1">'func_defaults'</span><span class="p">,</span> <span class="s1">'func_dict'</span><span class="p">,</span> <span class="s1">'func_doc'</span><span class="p">,</span>
<span class="s1">'func_globals'</span><span class="p">,</span> <span class="s1">'func_name'</span><span class="p">]</span>
<span class="o">>>></span> <span class="n">func</span><span class="o">.</span><span class="vm">__code__</span>
<span class="o"><</span><span class="n">code</span> <span class="nb">object</span> <span class="n">func</span> <span class="n">at</span> <span class="mh">0xb76c8410</span><span class="p">,</span> <span class="n">file</span> <span class="s2">"<stdin>"</span><span class="p">,</span> <span class="n">line</span> <span class="mi">1</span><span class="o">></span>
<span class="o">>>></span> <span class="n">func</span><span class="o">.</span><span class="vm">__name__</span>
<span class="s1">'func'</span>
<span class="o">>>></span>
</code></pre></div>
<p>对的，数字1是一个对象，字符串 'test' 也是一个对象，甚至一个函数也是一个类型为 function 的对象，也有一堆的属性和方法。对于 function 对象而言，有一个特殊属性 <strong>defaults</strong> ，这个属性用一个元组保存了是这个 function 对象的命名参数的缺省值，如下：</p>
<div class="highlight"><pre><span></span><code><span class="o">>>></span> <span class="k">def</span> <span class="nf">func</span><span class="p">(</span><span class="n">a</span><span class="o">=</span><span class="mi">1</span><span class="p">,</span> <span class="n">b</span><span class="o">=</span><span class="mi">2</span><span class="p">):</span>
<span class="o">...</span> <span class="k">pass</span>
<span class="o">...</span>
<span class="o">>>></span> <span class="n">func</span><span class="o">.</span><span class="vm">__defaults__</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="o">>>></span> <span class="k">def</span> <span class="nf">foo</span><span class="p">(</span><span class="n">a</span><span class="p">,</span> <span class="n">b</span><span class="p">):</span>
<span class="o">...</span> <span class="k">pass</span>
<span class="o">...</span>
<span class="o">>>></span> <span class="n">foo</span><span class="o">.</span><span class="vm">__defaults__</span> <span class="ow">is</span> <span class="kc">None</span>
<span class="kc">True</span>
<span class="o">>>></span> <span class="k">def</span> <span class="nf">func_no</span><span class="p">():</span>
<span class="o">...</span> <span class="k">pass</span>
<span class="o">...</span>
<span class="o">>>></span> <span class="n">func_no</span><span class="o">.</span><span class="vm">__defaults__</span> <span class="ow">is</span> <span class="kc">None</span>
<span class="kc">True</span>
<span class="o">>>></span>
</code></pre></div>
<p>如果一个函数有命名参数，则按顺序保存了命名参数的缺省值。如果这个函数命名参数没有缺省值或者没有命名参数，则为 None 。回到问题，为什么第一个函数中指定缺省值为 [] 会导致随着执行过程中，缺省参数的值会被保留下来呢？代码如下：</p>
<div class="highlight"><pre><span></span><code><span class="o">>>></span> <span class="k">def</span> <span class="nf">foo</span><span class="p">(</span><span class="n">l</span><span class="o">=</span><span class="p">[]):</span>
<span class="o">...</span> <span class="n">l</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span>
<span class="o">...</span> <span class="k">return</span> <span class="n">l</span>
<span class="o">...</span>
<span class="o">>>></span> <span class="n">foo</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">foo</span><span class="p">()</span>
<span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">1</span><span class="p">]</span>
<span class="o">>>></span> <span class="n">foo</span><span class="p">()</span>
<span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="mi">1</span><span class="p">]</span>
<span class="o">>>></span>
</code></pre></div>
<p>其实通过上面的罗嗦一大堆，答案很容易就可以得到了：foo 是一个 function 类型的对象，这个对象中有个 <strong>defaults</strong> 属性，保存了命名参数 l 的值，而在一次次的调用过程中，因为没有传入参数，所以实际上 foo 函数改变的是命名参数的缺省值。也就是师弟所说的这个函数在一次次调用中保留了对命名参数l的结果的修改。而师弟贴出的第二个函数的命名参数缺省值是 None ，实质上就是没有缺省值，所以l的值修改没有在调用中保留下来。是不是真的这样？我们来看下：</p>
<div class="highlight"><pre><span></span><code><span class="o">>>></span> <span class="k">def</span> <span class="nf">foo</span><span class="p">(</span><span class="n">l</span><span class="o">=</span><span class="p">[]):</span>
<span class="o">...</span> <span class="nb">print</span> <span class="s1">'default_arg_addr:'</span> <span class="o">+</span> <span class="nb">str</span><span class="p">(</span><span class="nb">id</span><span class="p">(</span><span class="n">l</span><span class="p">))</span>
<span class="o">...</span> <span class="n">l</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span>
<span class="o">...</span> <span class="nb">print</span> <span class="s1">'changed_var_addr:'</span> <span class="o">+</span> <span class="nb">str</span><span class="p">(</span><span class="nb">id</span><span class="p">(</span><span class="n">l</span><span class="p">))</span>
<span class="o">...</span> <span class="nb">print</span> <span class="n">l</span>
<span class="o">...</span>
<span class="o">>>></span> <span class="nb">id</span><span class="p">(</span><span class="n">foo</span><span class="o">.</span><span class="vm">__defaults__</span><span class="p">[</span><span class="mi">0</span><span class="p">])</span>
<span class="mi">3077402860</span><span class="n">L</span>
<span class="o">>>></span> <span class="n">foo</span><span class="p">()</span>
<span class="n">default_arg_addr</span><span class="p">:</span><span class="mi">3077402860</span>
<span class="n">changed_var_addr</span><span class="p">:</span><span class="mi">3077402860</span>
<span class="p">[</span><span class="mi">1</span><span class="p">]</span>
<span class="o">>>></span> <span class="n">foo</span><span class="p">()</span>
<span class="n">default_arg_addr</span><span class="p">:</span><span class="mi">3077402860</span>
<span class="n">changed_var_addr</span><span class="p">:</span><span class="mi">3077402860</span>
<span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">1</span><span class="p">]</span>
<span class="o">>>></span> <span class="n">foo</span><span class="o">.</span><span class="vm">__defaults__</span>
<span class="p">([</span><span class="mi">1</span><span class="p">,</span> <span class="mi">1</span><span class="p">],)</span>
<span class="o">>>></span>
</code></pre></div>
<p>上面这个函数 foo 有一个命名参数 l ，它的命名参数缺省值是一个空的列表，虽然是空列表，可是它确确实实是一个对象，已经在内存给它分配了空间。我们可以通过 id 函数的结果看出来。然后是两次的调用 foo 函数可以看到，因为没有传入参数，所以这两次修改的都是这个缺省的命名参数的值，所以可以得到所谓的对 l 的值的修改保留下来了的感觉。</p>
<h3>深入</h3>
<p>首先我们应该明白，在 Python 中，一个对象的实例化和初始化是不同的。一个对象实例化调用的是对象的 <strong>new</strong> 函数，而初始化调用的是 <strong>init</strong> 函数。所以，要深入地去看在 Python 中，函数在实例化的时候到底发生了什么，我们应该要去看 Python 源码。如下，源码版本为 Python2.7.4。</p>
<p>Python2.7.4/Objects/funcobject.c, func_new, L436-L439</p>
<div class="highlight"><pre><span></span><code><span class="k">if</span> <span class="p">(</span><span class="n">defaults</span> <span class="o">!=</span> <span class="n">Py_None</span><span class="p">)</span> <span class="p">{</span>
<span class="n">Py_INCREF</span><span class="p">(</span><span class="n">defaults</span><span class="p">);</span>
<span class="n">newfunc</span><span class="o">-></span><span class="n">func_defaults</span> <span class="o">=</span> <span class="n">defaults</span><span class="p">;</span>
<span class="p">}</span>
</code></pre></div>
<p>Python2.7.4/Include/object.h, L765-L767</p>
<div class="highlight"><pre><span></span><code><span class="cp">#define Py_INCREF(op) ( \</span>
<span class="cp">_Py_INC_REFTOTAL _Py_REF_DEBUG_COMMA \</span>
<span class="cp">((PyObject*)(op))->ob_refcnt++)</span>
</code></pre></div>
<p>上面第一段代码是 funcobject 的 func_new 中的代码，也就应该是 functions 对象的 <strong>new</strong> 函数代码。可以看到，如果 defaults 不是 None，也就是说有值，而我们上面也提到 Python 中一切都是对象，所以就会对这个对象进行 Py_INCREF 操作，并且将这个 defaults 值设定为 func_defaults。Py_INCREF 操作是什么？从第二段代码可以看到，这是一个宏定义，将参数 op 的 ob_refcnt 值加一。ob_refcnt 是什么？refcnt----reference count，这样明白了，就是将该对象的引用计数值加一。在执行了函数函数之后，该命名函数的缺省值对象并没有被销毁，而是随着该函数对象的存在而存在。对这个缺省之对象的修改当然也会被保留下来。</p>
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