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<h2 class="title"><a name="kgp.parse"></a>9.3. Parsing <span class="acronym">XML</span></h2>
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<div></div>
</div>
<div class="abstract">
<p>As I was saying, actually parsing an <span class="acronym">XML</span> document is very simple: one line of code. Where you go from there is up to you.
</p>
</div>
<div class="example"><a name="d0e23245"></a><h3 class="title">Example 9.8. Loading an <span class="acronym">XML</span> document (for real this time)
</h3><pre class="screen">
<tt class="prompt">>>> </tt><span class="userinput"><span class='pykeyword'>from</span> xml.dom <span class='pykeyword'>import</span> minidom</span> <a name="kgp.parse.1.1"></a><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12">
<tt class="prompt">>>> </tt><span class="userinput">xmldoc = minidom.parse(<span class='pystring'>'~/diveintopython/common/py/kgp/binary.xml'</span>)</span> <a name="kgp.parse.1.2"></a><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12">
<tt class="prompt">>>> </tt><span class="userinput">xmldoc</span> <a name="kgp.parse.1.3"></a><img src="../images/callouts/3.png" alt="3" border="0" width="12" height="12">
<span class="computeroutput"><xml.dom.minidom.Document instance at 010BE87C></span>
<tt class="prompt">>>> </tt><span class="userinput"><span class='pykeyword'>print</span> xmldoc.toxml()</span> <a name="kgp.parse.1.4"></a><img src="../images/callouts/4.png" alt="4" border="0" width="12" height="12">
<span class="computeroutput"><?xml version="1.0" ?>
<grammar>
<ref id="bit">
<p>0</p>
<p>1</p>
</ref>
<ref id="byte">
<p><xref id="bit"/><xref id="bit"/><xref id="bit"/><xref id="bit"/>\
<xref id="bit"/><xref id="bit"/><xref id="bit"/><xref id="bit"/></p>
</ref>
</grammar></span></pre><div class="calloutlist">
<table border="0" summary="Callout list">
<tr>
<td width="12" valign="top" align="left"><a href="#kgp.parse.1.1"><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">As you saw in the <a href="packages.html" title="9.2. Packages">previous section</a>, this imports the <tt class="filename">minidom</tt> module from the <tt class="filename">xml.dom</tt> package.
</td>
</tr>
<tr>
<td width="12" valign="top" align="left"><a href="#kgp.parse.1.2"><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">Here is the one line of code that does all the work: <tt class="function">minidom.parse</tt> takes one argument and returns a parsed representation of the <span class="acronym">XML</span> document. The argument can be many things; in this case, it's simply a filename of an <span class="acronym">XML</span> document on my local disk. (To follow along, you'll need to change the path to point to your downloaded examples directory.)
But you can also pass a <a href="../file_handling/file_objects.html" title="6.2. Working with File Objects">file object</a>, or even a <a href="../html_processing/extracting_data.html#dialect.extract.urllib" title="Example 8.5. Introducing urllib">file-like object</a>. You'll take advantage of this flexibility later in this chapter.
</td>
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<tr>
<td width="12" valign="top" align="left"><a href="#kgp.parse.1.3"><img src="../images/callouts/3.png" alt="3" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">The object returned from <tt class="function">minidom.parse</tt> is a <tt class="classname">Document</tt> object, a descendant of the <tt class="classname">Node</tt> class. This <tt class="classname">Document</tt> object is the root level of a complex tree-like structure of interlocking <span class="application">Python</span> objects that completely represent the <span class="acronym">XML</span> document you passed to <tt class="function">minidom.parse</tt>.
</td>
</tr>
<tr>
<td width="12" valign="top" align="left"><a href="#kgp.parse.1.4"><img src="../images/callouts/4.png" alt="4" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left"><tt class="function">toxml</tt> is a method of the <tt class="classname">Node</tt> class (and is therefore available on the <tt class="classname">Document</tt> object you got from <tt class="function">minidom.parse</tt>). <tt class="function">toxml</tt> prints out the <span class="acronym">XML</span> that this <tt class="classname">Node</tt> represents. For the <tt class="classname">Document</tt> node, this prints out the entire <span class="acronym">XML</span> document.
</td>
</tr>
</table>
</div>
</div>
<p>Now that you have an <span class="acronym">XML</span> document in memory, you can start traversing through it.
</p>
<div class="example"><a name="kgp.parse.gettingchildnodes.example"></a><h3 class="title">Example 9.9. Getting child nodes</h3><pre class="screen">
<tt class="prompt">>>> </tt><span class="userinput">xmldoc.childNodes</span> <a name="kgp.parse.2.1"></a><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12">
<span class="computeroutput">[<DOM Element: grammar at 17538908>]</span>
<tt class="prompt">>>> </tt><span class="userinput">xmldoc.childNodes[0]</span> <a name="kgp.parse.2.2"></a><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12">
<span class="computeroutput"><DOM Element: grammar at 17538908></span>
<tt class="prompt">>>> </tt><span class="userinput">xmldoc.firstChild</span> <a name="kgp.parse.2.3"></a><img src="../images/callouts/3.png" alt="3" border="0" width="12" height="12">
<span class="computeroutput"><DOM Element: grammar at 17538908></span></pre><div class="calloutlist">
<table border="0" summary="Callout list">
<tr>
<td width="12" valign="top" align="left"><a href="#kgp.parse.2.1"><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">Every <tt class="classname">Node</tt> has a <tt class="function">childNodes</tt> attribute, which is a list of the <tt class="classname">Node</tt> objects. A <tt class="classname">Document</tt> always has only one child node, the root element of the <span class="acronym">XML</span> document (in this case, the <tt class="sgmltag-element">grammar</tt> element).
</td>
</tr>
<tr>
<td width="12" valign="top" align="left"><a href="#kgp.parse.2.2"><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">To get the first (and in this case, the only) child node, just use regular list syntax. Remember, there is nothing special
going on here; this is just a regular <span class="application">Python</span> list of regular <span class="application">Python</span> objects.
</td>
</tr>
<tr>
<td width="12" valign="top" align="left"><a href="#kgp.parse.2.3"><img src="../images/callouts/3.png" alt="3" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">Since getting the first child node of a node is a useful and common activity, the <tt class="classname">Node</tt> class has a <tt class="function">firstChild</tt> attribute, which is synonymous with <tt class="literal">childNodes[0]</tt>. (There is also a <tt class="function">lastChild</tt> attribute, which is synonymous with <tt class="literal">childNodes[-1]</tt>.)
</td>
</tr>
</table>
</div>
</div>
<div class="example"><a name="d0e23458"></a><h3 class="title">Example 9.10. <tt class="function">toxml</tt> works on any node
</h3><pre class="screen">
<tt class="prompt">>>> </tt><span class="userinput">grammarNode = xmldoc.firstChild</span>
<tt class="prompt">>>> </tt><span class="userinput"><span class='pykeyword'>print</span> grammarNode.toxml()</span> <a name="kgp.parse.3.1"></a><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12">
<span class="computeroutput"><grammar>
<ref id="bit">
<p>0</p>
<p>1</p>
</ref>
<ref id="byte">
<p><xref id="bit"/><xref id="bit"/><xref id="bit"/><xref id="bit"/>\
<xref id="bit"/><xref id="bit"/><xref id="bit"/><xref id="bit"/></p>
</ref>
</grammar></span></pre><div class="calloutlist">
<table border="0" summary="Callout list">
<tr>
<td width="12" valign="top" align="left"><a href="#kgp.parse.3.1"><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">Since the <tt class="function">toxml</tt> method is defined in the <tt class="classname">Node</tt> class, it is available on any <span class="acronym">XML</span> node, not just the <tt class="classname">Document</tt> element.
</td>
</tr>
</table>
</div>
</div>
<div class="example"><a name="kgp.parse.childnodescanbetext.example"></a><h3 class="title">Example 9.11. Child nodes can be text</h3><pre class="screen">
<tt class="prompt">>>> </tt><span class="userinput">grammarNode.childNodes</span> <a name="kgp.parse.4.1"></a><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12">
<span class="computeroutput">[<DOM Text node "\n">, <DOM Element: ref at 17533332>, \
<DOM Text node "\n">, <DOM Element: ref at 17549660>, <DOM Text node "\n">]</span>
<tt class="prompt">>>> </tt><span class="userinput"><span class='pykeyword'>print</span> grammarNode.firstChild.toxml()</span> <a name="kgp.parse.4.2"></a><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12">
<span class="computeroutput">
</span>
<tt class="prompt">>>> </tt><span class="userinput"><span class='pykeyword'>print</span> grammarNode.childNodes[1].toxml()</span> <a name="kgp.parse.4.3"></a><img src="../images/callouts/3.png" alt="3" border="0" width="12" height="12">
<span class="computeroutput"><ref id="bit">
<p>0</p>
<p>1</p>
</ref></span>
<tt class="prompt">>>> </tt><span class="userinput"><span class='pykeyword'>print</span> grammarNode.childNodes[3].toxml()</span> <a name="kgp.parse.4.4"></a><img src="../images/callouts/4.png" alt="4" border="0" width="12" height="12">
<span class="computeroutput"><ref id="byte">
<p><xref id="bit"/><xref id="bit"/><xref id="bit"/><xref id="bit"/>\
<xref id="bit"/><xref id="bit"/><xref id="bit"/><xref id="bit"/></p>
</ref></span>
<tt class="prompt">>>> </tt><span class="userinput"><span class='pykeyword'>print</span> grammarNode.lastChild.toxml()</span> <a name="kgp.parse.4.5"></a><img src="../images/callouts/5.png" alt="5" border="0" width="12" height="12">
<span class="computeroutput">
</span></pre><div class="calloutlist">
<table border="0" summary="Callout list">
<tr>
<td width="12" valign="top" align="left"><a href="#kgp.parse.4.1"><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">Looking at the <span class="acronym">XML</span> in <tt class="filename">binary.xml</tt>, you might think that the <tt class="sgmltag-element">grammar</tt> has only two child nodes, the two <tt class="sgmltag-element">ref</tt> elements. But you're missing something: the carriage returns! After the <tt class="literal">'<grammar>'</tt> and before the first <tt class="literal">'<ref>'</tt> is a carriage return, and this text counts as a child node of the <tt class="sgmltag-element">grammar</tt> element. Similarly, there is a carriage return after each <tt class="literal">'</ref>'</tt>; these also count as child nodes. So <tt class="literal">grammar.childNodes</tt> is actually a list of 5 objects: 3 <tt class="classname">Text</tt> objects and 2 <tt class="classname">Element</tt> objects.
</td>
</tr>
<tr>
<td width="12" valign="top" align="left"><a href="#kgp.parse.4.2"><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">The first child is a <tt class="classname">Text</tt> object representing the carriage return after the <tt class="literal">'<grammar>'</tt> tag and before the first <tt class="literal">'<ref>'</tt> tag.
</td>
</tr>
<tr>
<td width="12" valign="top" align="left"><a href="#kgp.parse.4.3"><img src="../images/callouts/3.png" alt="3" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">The second child is an <tt class="classname">Element</tt> object representing the first <tt class="sgmltag-element">ref</tt> element.
</td>
</tr>
<tr>
<td width="12" valign="top" align="left"><a href="#kgp.parse.4.4"><img src="../images/callouts/4.png" alt="4" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">The fourth child is an <tt class="classname">Element</tt> object representing the second <tt class="sgmltag-element">ref</tt> element.
</td>
</tr>
<tr>
<td width="12" valign="top" align="left"><a href="#kgp.parse.4.5"><img src="../images/callouts/5.png" alt="5" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">The last child is a <tt class="classname">Text</tt> object representing the carriage return after the <tt class="literal">'</ref>'</tt> end tag and before the <tt class="literal">'</grammar>'</tt> end tag.
</td>
</tr>
</table>
</div>
</div>
<div class="example"><a name="d0e23628"></a><h3 class="title">Example 9.12. Drilling down all the way to text</h3><pre class="screen">
<tt class="prompt">>>> </tt><span class="userinput">grammarNode</span>
<span class="computeroutput"><DOM Element: grammar at 19167148></span>
<tt class="prompt">>>> </tt><span class="userinput">refNode = grammarNode.childNodes[1]</span> <a name="kgp.parse.5.1"></a><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12">
<tt class="prompt">>>> </tt><span class="userinput">refNode</span>
<span class="computeroutput"><DOM Element: ref at 17987740></span>
<tt class="prompt">>>> </tt><span class="userinput">refNode.childNodes</span> <a name="kgp.parse.5.2"></a><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12">
<span class="computeroutput">[<DOM Text node "\n">, <DOM Text node " ">, <DOM Element: p at 19315844>, \
<DOM Text node "\n">, <DOM Text node " ">, \
<DOM Element: p at 19462036>, <DOM Text node "\n">]</span>
<tt class="prompt">>>> </tt><span class="userinput">pNode = refNode.childNodes[2]</span>
<tt class="prompt">>>> </tt><span class="userinput">pNode</span>
<span class="computeroutput"><DOM Element: p at 19315844></span>
<tt class="prompt">>>> </tt><span class="userinput"><span class='pykeyword'>print</span> pNode.toxml()</span> <a name="kgp.parse.5.3"></a><img src="../images/callouts/3.png" alt="3" border="0" width="12" height="12">
<span class="computeroutput"><p>0</p></span>
<tt class="prompt">>>> </tt><span class="userinput">pNode.firstChild</span> <a name="kgp.parse.5.4"></a><img src="../images/callouts/4.png" alt="4" border="0" width="12" height="12">
<span class="computeroutput"><DOM Text node "0"></span>
<tt class="prompt">>>> </tt><span class="userinput">pNode.firstChild.data</span> <a name="kgp.parse.5.5"></a><img src="../images/callouts/5.png" alt="5" border="0" width="12" height="12">
<span class="computeroutput">u'0'</span></pre><div class="calloutlist">
<table border="0" summary="Callout list">
<tr>
<td width="12" valign="top" align="left"><a href="#kgp.parse.5.1"><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">As you saw in the previous example, the first <tt class="sgmltag-element">ref</tt> element is <tt class="literal">grammarNode.childNodes[1]</tt>, since childNodes[0] is a <tt class="classname">Text</tt> node for the carriage return.
</td>
</tr>
<tr>
<td width="12" valign="top" align="left"><a href="#kgp.parse.5.2"><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">The <tt class="sgmltag-element">ref</tt> element has its own set of child nodes, one for the carriage return, a separate one for the spaces, one for the <tt class="sgmltag-element">p</tt> element, and so forth.
</td>
</tr>
<tr>
<td width="12" valign="top" align="left"><a href="#kgp.parse.5.3"><img src="../images/callouts/3.png" alt="3" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">You can even use the <tt class="function">toxml</tt> method here, deeply nested within the document.
</td>
</tr>
<tr>
<td width="12" valign="top" align="left"><a href="#kgp.parse.5.4"><img src="../images/callouts/4.png" alt="4" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">The <tt class="sgmltag-element">p</tt> element has only one child node (you can't tell that from this example, but look at <tt class="literal">pNode.childNodes</tt> if you don't believe me), and it is a <tt class="classname">Text</tt> node for the single character <tt class="literal">'0'</tt>.
</td>
</tr>
<tr>
<td width="12" valign="top" align="left"><a href="#kgp.parse.5.5"><img src="../images/callouts/5.png" alt="5" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">The <tt class="literal">.data</tt> attribute of a <tt class="classname">Text</tt> node gives you the actual string that the text node represents. But what is that <tt class="literal">'u'</tt> in front of the string? The answer to that deserves its own section.
</td>
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<p class="copyright">Copyright © 2000, 2001, 2002, 2003, 2004 <a href="mailto:mark@diveintopython.org">Mark Pilgrim</a></p>
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