248 lines
10 KiB
Markdown
248 lines
10 KiB
Markdown
# Python:用 lxml 解析 XML
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> 原文：<https://www.blog.pythonlibrary.org/2010/11/20/python-parsing-xml-with-lxml/>
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上次，我们看了 Python 的一个内置 XML 解析器。在本文中，我们将看看有趣的第三方包，来自 [codespeak](http://codespeak.net/lxml/) 的 lxml。它使用了 ElementTree API 等。lxml 包支持 XPath 和 XSLT，包括一个用于 SAX 的 API 和一个与 C/Pyrex 模块兼容的 C 级 API。我们只是用它做一些简单的事情。
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无论如何，对于本文，我们将使用 minidom 解析示例中的例子，看看如何用 lxml 解析这些例子。下面是一个 XML 示例，它来自一个为跟踪约会而编写的程序:
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```py
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<appointment><begin>1181251680</begin>
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<uid>040000008200E000</uid>
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<alarmtime>1181572063</alarmtime>
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<state><location><duration>1800</duration>
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<subject>Bring pizza home</subject></location></state></appointment>
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<appointment><begin>1234360800</begin>
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<duration>1800</duration>
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<subject>Check MS Office website for updates</subject>
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<location><uid>604f4792-eb89-478b-a14f-dd34d3cc6c21-1234360800</uid>
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<state>dismissed</state></location></appointment>
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```
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上面的 XML 显示了两个约会。从纪元开始的开始时间以秒为单位；uid 是基于开始时间和一个密钥(我认为)的哈希生成的；报警时间是自该时期以来的秒数，但应该小于开始时间；而状态就是任命有没有被打盹儿，有没有被辞退。其余的就不言自明了。现在让我们看看如何解析它。
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```py
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from lxml import etree
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from StringIO import StringIO
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#----------------------------------------------------------------------
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def parseXML(xmlFile):
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"""
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Parse the xml
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"""
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f = open(xmlFile)
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xml = f.read()
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f.close()
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tree = etree.parse(StringIO(xml))
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context = etree.iterparse(StringIO(xml))
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for action, elem in context:
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if not elem.text:
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text = "None"
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else:
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text = elem.text
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print elem.tag + " => " + text
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if __name__ == "__main__":
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parseXML("example.xml")
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```
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首先，我们导入所需的模块，即来自 **lxml** 包的 **etree** 模块和来自内置 StringIO 模块的 **StringIO** 函数。我们的 **parseXML** 函数接受一个参数:所讨论的 XML 文件的路径。我们打开文件，阅读并关闭它。现在有趣的部分来了！我们使用 etree 的 parse 函数来解析从 StringIO 模块返回的 XML 代码。出于我不完全理解的原因，parse 函数需要一个类似文件的对象。
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无论如何，接下来我们迭代上下文(即 lxml.etree.iterparse 对象)并提取标记元素。我们添加条件语句 **if** 来用单词“None”替换空字段，以使输出更加清晰。仅此而已。
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## 解析图书示例
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这个例子的结果有点蹩脚。大多数情况下，您希望保存提取的数据并对其进行处理，而不仅仅是将其输出到 stdout。因此，对于我们的下一个例子，我们将创建一个数据结构来包含结果。这个例子的数据结构将是一个字典列表。我们将在这里使用 MSDN 图书的例子:
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```py
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<book id="bk101"><author>Gambardella, Matthew</author>
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<title>XML Developer's Guide</title>
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<genre>Computer</genre>
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<price>44.95</price>
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<publish_date>2000-10-01</publish_date>
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<description>An in-depth look at creating applications
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with XML.</description></book>
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<book id="bk102"><author>Ralls, Kim</author>
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<title>Midnight Rain</title>
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<genre>Fantasy</genre>
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<price>5.95</price>
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<publish_date>2000-12-16</publish_date>
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<description>A former architect battles corporate zombies,
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an evil sorceress, and her own childhood to become queen
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of the world.</description></book>
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<book id="bk103"><author>Corets, Eva</author>
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<title>Maeve Ascendant</title>
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<genre>Fantasy</genre>
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<price>5.95</price>
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<publish_date>2000-11-17</publish_date>
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<description>After the collapse of a nanotechnology
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society in England, the young survivors lay the
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foundation for a new society.</description></book>
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<book id="bk104"><author>Corets, Eva</author>
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<title>Oberon's Legacy</title>
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<genre>Fantasy</genre>
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<price>5.95</price>
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<publish_date>2001-03-10</publish_date>
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<description>In post-apocalypse England, the mysterious
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agent known only as Oberon helps to create a new life
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for the inhabitants of London. Sequel to Maeve
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Ascendant.</description></book>
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<book id="bk105"><author>Corets, Eva</author>
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<title>The Sundered Grail</title>
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<genre>Fantasy</genre>
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<price>5.95</price>
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<publish_date>2001-09-10</publish_date>
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<description>The two daughters of Maeve, half-sisters,
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battle one another for control of England. Sequel to
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Oberon's Legacy.</description></book>
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<book id="bk106"><author>Randall, Cynthia</author>
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<title>Lover Birds</title>
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<genre>Romance</genre>
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<price>4.95</price>
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<publish_date>2000-09-02</publish_date>
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<description>When Carla meets Paul at an ornithology
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conference, tempers fly as feathers get ruffled.</description></book>
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<book id="bk107"><author>Thurman, Paula</author>
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<title>Splish Splash</title>
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<genre>Romance</genre>
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<price>4.95</price>
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<publish_date>2000-11-02</publish_date>
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<description>A deep sea diver finds true love twenty
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thousand leagues beneath the sea.</description></book>
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<book id="bk108"><author>Knorr, Stefan</author>
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<title>Creepy Crawlies</title>
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<genre>Horror</genre>
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<price>4.95</price>
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<publish_date>2000-12-06</publish_date>
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<description>An anthology of horror stories about roaches,
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centipedes, scorpions and other insects.</description></book>
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<book id="bk109"><author>Kress, Peter</author>
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<title>Paradox Lost</title>
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<genre>Science Fiction</genre>
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<price>6.95</price>
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<publish_date>2000-11-02</publish_date>
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<description>After an inadvertant trip through a Heisenberg
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Uncertainty Device, James Salway discovers the problems
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of being quantum.</description></book>
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<book id="bk110"><author>O'Brien, Tim</author>
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<title>Microsoft .NET: The Programming Bible</title>
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<genre>Computer</genre>
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<price>36.95</price>
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<publish_date>2000-12-09</publish_date>
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<description>Microsoft's .NET initiative is explored in
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detail in this deep programmer's reference.</description></book>
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<book id="bk111"><author>O'Brien, Tim</author>
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<title>MSXML3: A Comprehensive Guide</title>
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<genre>Computer</genre>
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<price>36.95</price>
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<publish_date>2000-12-01</publish_date>
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<description>The Microsoft MSXML3 parser is covered in
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detail, with attention to XML DOM interfaces, XSLT processing,
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SAX and more.</description></book>
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<book id="bk112"><author>Galos, Mike</author>
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<title>Visual Studio 7: A Comprehensive Guide</title>
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<genre>Computer</genre>
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<price>49.95</price>
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<publish_date>2001-04-16</publish_date>
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<description>Microsoft Visual Studio 7 is explored in depth,
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looking at how Visual Basic, Visual C++, C#, and ASP+ are
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integrated into a comprehensive development
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environment.</description></book>
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```
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现在让我们解析它，并把它放到我们的数据结构中！
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```py
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from lxml import etree
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from StringIO import StringIO
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#----------------------------------------------------------------------
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def parseBookXML(xmlFile):
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f = open(xmlFile)
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xml = f.read()
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f.close()
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tree = etree.parse(StringIO(xml))
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print tree.docinfo.doctype
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context = etree.iterparse(StringIO(xml))
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book_dict = {}
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books = []
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for action, elem in context:
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if not elem.text:
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text = "None"
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else:
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text = elem.text
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print elem.tag + " => " + text
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book_dict[elem.tag] = text
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if elem.tag == "book":
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books.append(book_dict)
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book_dict = {}
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return books
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if __name__ == "__main__":
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parseBookXML("example2.xml")
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```
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这个例子与上一个非常相似，所以我们只关注这里的不同之处。在开始迭代上下文之前，我们创建了一个空的 dictionary 对象和一个空的 list。然后在循环内部，我们像这样创建字典:
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```py
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book_dict[elem.tag] = text
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```
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文本为 elem.text 或“无”。最后，如果标签碰巧是“book ”,那么我们在一本书的末尾，需要将字典添加到我们的列表中，并为下一本书重置字典。如你所见，这正是我们所做的。更现实的例子是将提取的数据放入 Book 类。我以前用 json 提要做过后者。
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## 重构代码
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正如我警惕的读者所指出的，我写了一些相当糟糕的代码。所以我对代码进行了一些清理，希望这样会好一点:
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```py
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from lxml import etree
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#----------------------------------------------------------------------
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def parseBookXML(xmlFile):
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""""""
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context = etree.iterparse(xmlFile)
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book_dict = {}
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books = []
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for action, elem in context:
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if not elem.text:
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text = "None"
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else:
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text = elem.text
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print elem.tag + " => " + text
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book_dict[elem.tag] = text
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if elem.tag == "book":
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books.append(book_dict)
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book_dict = {}
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return books
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if __name__ == "__main__":
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parseBookXML("example.xml")
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```
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如您所见，我们完全放弃了 StringIO 模块，将所有文件 I/O 内容放在 lxml 方法调用中。其余都一样。很酷吧？像往常一样，巨蟒摇滚！
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## 包扎
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你从这篇文章中学到什么了吗？我当然希望如此。Python 在其标准库内外都有很多很酷的解析库。一定要检查它们，看看哪一个最适合你的编程方式。
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## 进一步阅读
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* lxml 官方[网站](http://codespeak.net/lxml/index.html)
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* 一篇关于 lxml 的 IBM 文章
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* StringIO [文档](http://docs.python.org/library/stringio.html) |