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<h2 class="title"><a name="kgp.attributes"></a>9.6. Accessing element attributes
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<div class="abstract">
<p><span class="acronym">XML</span> elements can have one or more attributes, and it is incredibly simple to access them once you have parsed an <span class="acronym">XML</span> document.
</p>
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<p>For this section, you'll be using the <tt class="filename">binary.xml</tt> grammar file that you saw in the <a href="searching.html" title="9.5. Searching for elements">previous section</a>.
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<td colspan="2" align="left" valign="top" width="99%">This section may be a little confusing, because of some overlapping terminology. Elements in an <span class="acronym">XML</span> document have attributes, and <span class="application">Python</span> objects also have attributes. When you parse an <span class="acronym">XML</span> document, you get a bunch of <span class="application">Python</span> objects that represent all the pieces of the <span class="acronym">XML</span> document, and some of these <span class="application">Python</span> objects represent attributes of the <span class="acronym">XML</span> elements. But the (<span class="application">Python</span>) objects that represent the (<span class="acronym">XML</span>) attributes also have (<span class="application">Python</span>) attributes, which are used to access various parts of the (<span class="acronym">XML</span>) attribute that the object represents. I told you it was confusing. I am open to suggestions on how to distinguish these
more clearly.
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<div class="example"><a name="d0e24787"></a><h3 class="title">Example 9.24. Accessing element attributes</h3><pre class="screen">
<tt class="prompt">>>> </tt><span class="userinput">xmldoc = minidom.parse(<span class='pystring'>'binary.xml'</span>)</span>
<tt class="prompt">>>> </tt><span class="userinput">reflist = xmldoc.getElementsByTagName(<span class='pystring'>'ref'</span>)</span>
<tt class="prompt">>>> </tt><span class="userinput">bitref = reflist[0]</span>
<tt class="prompt">>>> </tt><span class="userinput"><span class='pykeyword'>print</span> bitref.toxml()</span>
<span class="computeroutput"><ref id="bit">
<p>0</p>
<p>1</p>
</ref></span>
<tt class="prompt">>>> </tt><span class="userinput">bitref.attributes</span> <a name="kgp.attributes.1.1"></a><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12">
<span class="computeroutput"><xml.dom.minidom.NamedNodeMap instance at 0x81e0c9c></span>
<tt class="prompt">>>> </tt><span class="userinput">bitref.attributes.keys()</span> <a name="kgp.attributes.1.2"></a><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12"> <a name="kgp.attributes.1.3"></a><img src="../images/callouts/3.png" alt="3" border="0" width="12" height="12">
<span class="computeroutput">[u'id']</span>
<tt class="prompt">>>> </tt><span class="userinput">bitref.attributes.values()</span> <a name="kgp.attributes.1.4"></a><img src="../images/callouts/4.png" alt="4" border="0" width="12" height="12">
<span class="computeroutput">[<xml.dom.minidom.Attr instance at 0x81d5044>]</span>
<tt class="prompt">>>> </tt><span class="userinput">bitref.attributes[<span class='pystring'>"id"</span>]</span> <a name="kgp.attributes.1.5"></a><img src="../images/callouts/5.png" alt="5" border="0" width="12" height="12">
<span class="computeroutput"><xml.dom.minidom.Attr instance at 0x81d5044></span></pre><div class="calloutlist">
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<td valign="top" align="left">Each <tt class="classname">Element</tt> object has an attribute called <tt class="literal">attributes</tt>, which is a <tt class="classname">NamedNodeMap</tt> object. This sounds scary, but it's not, because a <tt class="classname">NamedNodeMap</tt> is an object that <a href="../object_oriented_framework/userdict.html" title="5.5. Exploring UserDict: A Wrapper Class">acts like a dictionary</a>, so you already know how to use it.
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<td valign="top" align="left">Treating the <tt class="classname">NamedNodeMap</tt> as a dictionary, you can get a list of the names of the attributes of this element by using <tt class="function">attributes.keys()</tt>. This element has only one attribute, <tt class="literal">'id'</tt>.
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<td valign="top" align="left">Attribute names, like all other text in an <span class="acronym">XML</span> document, are stored in <a href="unicode.html" title="9.4. Unicode">unicode</a>.
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<td valign="top" align="left">Again treating the <tt class="classname">NamedNodeMap</tt> as a dictionary, you can get a list of the values of the attributes by using <tt class="function">attributes.values()</tt>. The values are themselves objects, of type <tt class="classname">Attr</tt>. You'll see how to get useful information out of this object in the next example.
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<td valign="top" align="left">Still treating the <tt class="classname">NamedNodeMap</tt> as a dictionary, you can access an individual attribute by name, using normal dictionary syntax. (Readers who have been
paying extra-close attention will already know how the <tt class="classname">NamedNodeMap</tt> class accomplishes this neat trick: by defining a <a href="../object_oriented_framework/special_class_methods.html" title="5.6. Special Class Methods"><tt class="function">__getitem__</tt> special method</a>. Other readers can take comfort in the fact that they don't need to understand how it works in order to use it effectively.)
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<div class="example"><a name="d0e24922"></a><h3 class="title">Example 9.25. Accessing individual attributes</h3><pre class="screen">
<tt class="prompt">>>> </tt><span class="userinput">a = bitref.attributes[<span class='pystring'>"id"</span>]</span>
<tt class="prompt">>>> </tt><span class="userinput">a</span>
<span class="computeroutput"><xml.dom.minidom.Attr instance at 0x81d5044></span>
<tt class="prompt">>>> </tt><span class="userinput">a.name</span> <a name="kgp.attributes.2.1"></a><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12">
<span class="computeroutput">u'id'</span>
<tt class="prompt">>>> </tt><span class="userinput">a.value</span> <a name="kgp.attributes.2.2"></a><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12">
<span class="computeroutput">u'bit'</span></pre><div class="calloutlist">
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<td width="12" valign="top" align="left"><a href="#kgp.attributes.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">The <tt class="classname">Attr</tt> object completely represents a single <span class="acronym">XML</span> attribute of a single <span class="acronym">XML</span> element. The name of the attribute (the same name as you used to find this object in the <tt class="literal">bitref.attributes</tt> <tt class="classname">NamedNodeMap</tt> pseudo-dictionary) is stored in <tt class="literal">a.name</tt>.
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<td valign="top" align="left">The actual text value of this <span class="acronym">XML</span> attribute is stored in <tt class="literal">a.value</tt>.
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<td colspan="2" align="left" valign="top" width="99%">Like a dictionary, attributes of an <span class="acronym">XML</span> element have no ordering. Attributes may <span class="emphasis"><em>happen to be</em></span> listed in a certain order in the original <span class="acronym">XML</span> document, and the <tt class="classname">Attr</tt> objects may <span class="emphasis"><em>happen to be</em></span> listed in a certain order when the <span class="acronym">XML</span> document is parsed into <span class="application">Python</span> objects, but these orders are arbitrary and should carry no special meaning. You should always access individual attributes
by name, like the keys of a dictionary.
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