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<title>5.8. 类属性介绍</title>
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<h2 class="title"><a name="fileinfo.classattributes"></a>5.8. 类属性介绍
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<div class="abstract">
<p>你已经知道了<a href="userdict.html#fileinfo.userdict.init.example" title="例 5.9. 定义 UserDict 类">数据属性</a>，它们是被一个特定的类实例所拥有的变量。<span class="application">Python</span> 也支持类属性，它们是由类本身所拥有的。
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<div class="example"><a name="fileinfo.classattributes.intro"></a><h3 class="title">例 5.17. 类属性介绍</h3><pre class="programlisting"><span class='pykeyword'>
class</span> MP3FileInfo(FileInfo):
<span class='pystring'>"store ID3v1.0 MP3 tags"</span>
tagDataMap = {<span class='pystring'>"title"</span> : ( 3, 33, stripnulls),
<span class='pystring'>"artist"</span> : ( 33, 63, stripnulls),
<span class='pystring'>"album"</span> : ( 63, 93, stripnulls),
<span class='pystring'>"year"</span> : ( 93, 97, stripnulls),
<span class='pystring'>"comment"</span> : ( 97, 126, stripnulls),
<span class='pystring'>"genre"</span> : (127, 128, ord)}</pre><pre class="screen"><tt class="prompt">>>> </tt><span class="userinput"><span class='pykeyword'>import</span> fileinfo</span>
<tt class="prompt">>>> </tt><span class="userinput">fileinfo.MP3FileInfo</span> <a name="fileinfo.classattributes.1.1"></a><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12">
<span class="computeroutput"><class fileinfo.MP3FileInfo at 01257FDC></span>
<tt class="prompt">>>> </tt><span class="userinput">fileinfo.MP3FileInfo.tagDataMap</span> <a name="fileinfo.classattributes.1.2"></a><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12">
<span class="computeroutput">{'title': (3, 33, <function stripnulls at 0260C8D4>),
'genre': (127, 128, <built-in function ord>),
'artist': (33, 63, <function stripnulls at 0260C8D4>),
'year': (93, 97, <function stripnulls at 0260C8D4>),
'comment': (97, 126, <function stripnulls at 0260C8D4>),
'album': (63, 93, <function stripnulls at 0260C8D4>)}</span>
<tt class="prompt">>>> </tt><span class="userinput">m = fileinfo.MP3FileInfo()</span> <a name="fileinfo.classattributes.1.3"></a><img src="../images/callouts/3.png" alt="3" border="0" width="12" height="12">
<tt class="prompt">>>> </tt><span class="userinput">m.tagDataMap</span>
<span class="computeroutput">{'title': (3, 33, <function stripnulls at 0260C8D4>),
'genre': (127, 128, <built-in function ord>),
'artist': (33, 63, <function stripnulls at 0260C8D4>),
'year': (93, 97, <function stripnulls at 0260C8D4>),
'comment': (97, 126, <function stripnulls at 0260C8D4>),
'album': (63, 93, <function stripnulls at 0260C8D4>)}</span></pre><div class="calloutlist">
<table border="0" summary="Callout list">
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<td width="12" valign="top" align="left"><a href="#fileinfo.classattributes.1.1"><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left"><tt class="classname">MP3FileInfo</tt> 是类本身，不是任何类的特别实例。
</td>
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<td width="12" valign="top" align="left"><a href="#fileinfo.classattributes.1.2"><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left"><tt class="varname">tagDataMap</tt> 是一个类属性：字面的意思，一个类的属性。它在创建任何类实例之前就有效了。
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<td width="12" valign="top" align="left"><a href="#fileinfo.classattributes.1.3"><img src="../images/callouts/3.png" alt="3" border="0" width="12" height="12"></a>
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<td valign="top" align="left">类属性既可以通过直接对类的引用，也可以通过对类的任意实例的引用来使用。</td>
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</div><a name="compare.classattr.java"></a><table class="note" border="0" summary="">
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<td rowspan="2" align="center" valign="top" width="1%"><img src="../images/note.png" alt="注意" title="" width="24" height="24"></td>
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<td colspan="2" align="left" valign="top" width="99%">在 <span class="application">Java</span> 中，静态变量 (在 <span class="application">Python</span> 中叫类属性) 和实例变量 (在 <span class="application">Python</span> 中叫数据属性) 两者都是紧跟在类定义之后定义的 (一个有 <tt class="literal">static</tt> 关键字，一个没有)。在 <span class="application">Python</span> 中，只有类属性可以定义在这里，数据属性定义在 <tt class="function">__init__</tt> 方法中。
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<p>类属性可以作为类级别的常量来使用 (这就是为什么我们在 <tt class="classname">MP3FileInfo</tt> 中使用它们)，但是它们不是真正的常量。你也可以修改它们。
</p><a name="d0e13894"></a><table class="note" border="0" summary="">
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<td rowspan="2" align="center" valign="top" width="1%"><img src="../images/note.png" alt="注意" title="" width="24" height="24"></td>
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<td colspan="2" align="left" valign="top" width="99%">在 <span class="application">Python</span> 中没有常量。如果你试图努力的话什么都可以改变。这一点满足 <span class="application">Python</span> 的核心原则之一：坏的行为应该被克服而不是被取缔。如果你真正想改变 <tt class="literal">None</tt> 的值，也可以做到，但当无法调试的时候别来找我。
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<div class="example"><a name="fileinfo.classattributes.writeable.example"></a><h3 class="title">例 5.18. 修改类属性</h3><pre class="screen"><tt class="prompt">>>> </tt><span class="userinput"><span class='pykeyword'>class</span><span class='pyclass'> counter</span>:</span>
<tt class="prompt">... </tt><span class="userinput">count = 0</span> <a name="fileinfo.classattributes.2.1"></a><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12">
<tt class="prompt">... </tt><span class="userinput"><span class='pykeyword'>def</span><span class='pyclass'> __init__</span>(self):</span>
<tt class="prompt">... </tt><span class="userinput"> self.__class__.count += 1</span> <a name="fileinfo.classattributes.2.2"></a><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12">
<tt class="prompt">... </tt>
<tt class="prompt">>>> </tt><span class="userinput">counter</span>
<span class="computeroutput"><class __main__.counter at 010EAECC></span>
<tt class="prompt">>>> </tt><span class="userinput">counter.count</span> <a name="fileinfo.classattributes.2.3"></a><img src="../images/callouts/3.png" alt="3" border="0" width="12" height="12">
<span class="computeroutput">0</span>
<tt class="prompt">>>> </tt><span class="userinput">c = counter()</span>
<tt class="prompt">>>> </tt><span class="userinput">c.count</span> <a name="fileinfo.classattributes.2.4"></a><img src="../images/callouts/4.png" alt="4" border="0" width="12" height="12">
<span class="computeroutput">1</span>
<tt class="prompt">>>> </tt><span class="userinput">counter.count</span>
<span class="computeroutput">1</span>
<tt class="prompt">>>> </tt><span class="userinput">d = counter()</span> <a name="fileinfo.classattributes.2.5"></a><img src="../images/callouts/5.png" alt="5" border="0" width="12" height="12">
<tt class="prompt">>>> </tt><span class="userinput">d.count</span>
<span class="computeroutput">2</span>
<tt class="prompt">>>> </tt><span class="userinput">c.count</span>
<span class="computeroutput">2</span>
<tt class="prompt">>>> </tt><span class="userinput">counter.count</span>
<span class="computeroutput">2</span></pre><div class="calloutlist">
<table border="0" summary="Callout list">
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<td width="12" valign="top" align="left"><a href="#fileinfo.classattributes.2.1"><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left"><tt class="varname">count</tt> 是 <tt class="classname">counter</tt> 类的一个类属性。
</td>
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<td width="12" valign="top" align="left"><a href="#fileinfo.classattributes.2.2"><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left"><tt class="literal">__class__</tt> 是每个类实例的一个内置属性 (也是每个类的)。它是一个类的引用，而 <tt class="varname">self</tt> 是一个类 (在本例中，是 <tt class="classname">counter</tt> 类) 的实例。
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<td width="12" valign="top" align="left"><a href="#fileinfo.classattributes.2.3"><img src="../images/callouts/3.png" alt="3" border="0" width="12" height="12"></a>
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<td valign="top" align="left">因为 <tt class="varname">count</tt> 是一个类属性，它可以在我们创建任何类实例之前，通过直接对类引用而得到。
</td>
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<td width="12" valign="top" align="left"><a href="#fileinfo.classattributes.2.4"><img src="../images/callouts/4.png" alt="4" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">创建一个类实例会调用 <tt class="function">__init__</tt> 方法，它会给类属性 <tt class="varname">count</tt> 加 <tt class="constant">1</tt>。这样会影响到类自身，不只是新创建的实例。
</td>
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<td width="12" valign="top" align="left"><a href="#fileinfo.classattributes.2.5"><img src="../images/callouts/5.png" alt="5" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">创建第二个实例将再次增加类属性 <tt class="varname">count</tt>。注意类属性是如何被类和所有类实例所共享的。
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