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<h2 class="title"><a name="fileinfo.classattributes"></a>5.8. Introducing Class Attributes
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<div class="abstract">
<p>You already know about <a href="userdict.html#fileinfo.userdict.init.example" title="Example 5.9. Defining the UserDict Class">data attributes</a>, which are variables owned by a specific instance of a class. <span class="application">Python</span> also supports class attributes, which are variables owned by the class itself.
</p>
</div>
<div class="example"><a name="fileinfo.classattributes.intro"></a><h3 class="title">Example 5.17. Introducing Class Attributes</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">
<tr>
<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> is the class itself, not any particular instance of the class.
</td>
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<tr>
<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> is a class attribute: literally, an attribute of the class. It is available before creating any instances of the class.
</td>
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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>
</td>
<td valign="top" align="left">Class attributes are available both through direct reference to the class and through any instance of the class.</td>
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</div><a name="compare.classattr.java"></a><table class="note" border="0" summary="">
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<td colspan="2" align="left" valign="top" width="99%">In <span class="application">Java</span>, both static variables (called class attributes in <span class="application">Python</span>) and instance variables (called data attributes in <span class="application">Python</span>) are defined immediately after the class definition (one with the <tt class="literal">static</tt> keyword, one without). In <span class="application">Python</span>, only class attributes can be defined here; data attributes are defined in the <tt class="function">__init__</tt> method.
</td>
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</table>
<p>Class attributes can be used as class-level constants (which is how you use them in <tt class="classname">MP3FileInfo</tt>), but they are not really constants. You can also change them.
</p><a name="d0e13711"></a><table class="note" border="0" summary="">
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<td colspan="2" align="left" valign="top" width="99%">There are no constants in <span class="application">Python</span>. Everything can be changed if you try hard enough. This fits with one of the core principles of <span class="application">Python</span>: bad behavior should be discouraged but not banned. If you really want to change the value of <tt class="literal">None</tt>, you can do it, but don't come running to me when your code is impossible to debug.
</td>
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</table>
<div class="example"><a name="fileinfo.classattributes.writeable.example"></a><h3 class="title">Example 5.18. Modifying Class Attributes</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">
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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>
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<td valign="top" align="left"><tt class="varname">count</tt> is a class attribute of the <tt class="classname">counter</tt> class.
</td>
</tr>
<tr>
<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> is a built-in attribute of every class instance (of every class). It is a reference to the class that <tt class="varname">self</tt> is an instance of (in this case, the <tt class="classname">counter</tt> class).
</td>
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<tr>
<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>
</td>
<td valign="top" align="left">Because <tt class="varname">count</tt> is a class attribute, it is available through direct reference to the class, before you have created any instances of the
class.
</td>
</tr>
<tr>
<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">Creating an instance of the class calls the <tt class="function">__init__</tt> method, which increments the class attribute <tt class="varname">count</tt> by <tt class="constant">1</tt>. This affects the class itself, not just the newly created instance.
</td>
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<tr>
<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">Creating a second instance will increment the class attribute <tt class="varname">count</tt> again. Notice how the class attribute is shared by the class and all instances of the class.
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