Python中将字典转换为XML以及相关的命名空间解析
尽管 xml.etree.ElementTree 库通常用来做解析工作，其实它也可以创建XML文档。 例如，考虑如下这个函数：
from xml.etree.ElementTree import Element def dict_to_xml(tag, d): ''' Turn a simple dict of key/value pairs into XML ''' elem = Element(tag) for key, val in d.items(): child = Element(key) child.text = str(val) elem.append(child) return elem
下面是一个使用例子：
>>> s = { 'name': 'GOOG', 'shares': 100, 'price':490.1 }
>>> e = dict_to_xml('stock', s)
>>> e
<Element 'stock' at 0x1004b64c8>
>>>
转换结果是一个 Element 实例。对于I/O操作，使用 xml.etree.ElementTree 中的 tostring() 函数很容易就能将它转换成一个字节字符串。例如：
>>> from xml.etree.ElementTree import tostring >>> tostring(e) b'<stock><price>490.1</price><shares>100</shares><name>GOOG</name></stock>' >>>
如果你想给某个元素添加属性值，可以使用 set() 方法：
>>> e.set('_id','1234')
>>> tostring(e)
b'<stock _id="1234"><price>490.1</price><shares>100</shares><name>GOOG</name>
</stock>'
>>>
如果你还想保持元素的顺序，可以考虑构造一个 OrderedDict 来代替一个普通的字典。当创建XML的时候，你被限制只能构造字符串类型的值。例如：
def dict_to_xml_str(tag, d):
'''
Turn a simple dict of key/value pairs into XML
'''
parts = ['<{}>'.format(tag)]
for key, val in d.items():
parts.append('<{0}>{1}</{0}>'.format(key,val))
parts.append('</{}>'.format(tag))
return ''.join(parts)
问题是如果你手动的去构造的时候可能会碰到一些麻烦。例如，当字典的值中包含一些特殊字符的时候会怎样呢？
>>> d = { 'name' : '<spam>' }
>>> # String creation
>>> dict_to_xml_str('item',d)
'<item><name><spam></name></item>'
>>> # Proper XML creation
>>> e = dict_to_xml('item',d)
>>> tostring(e)
b'<item><name><spam></name></item>'
>>>
注意到程序的后面那个例子中，字符 ‘<' 和 ‘>' 被替换成了 < 和 >
下面仅供参考，如果你需要手动去转换这些字符， 可以使用 xml.sax.saxutils 中的 escape() 和 unescape() 函数。例如：
>>> from xml.sax.saxutils import escape, unescape
>>> escape('<spam>')
'<spam>'
>>> unescape(_)
'<spam>'
>>>
除了能创建正确的输出外，还有另外一个原因推荐你创建 Element 实例而不是字符串， 那就是使用字符串组合构造一个更大的文档并不是那么容易。 而 Element 实例可以不用考虑解析XML文本的情况下通过多种方式被处理。 也就是说，你可以在一个高级数据结构上完成你所有的操作，并在最后以字符串的形式将其输出。
利用命名空间解析XML文档
如果你解析这个文档并执行普通的查询，你会发现这个并不是那么容易，因为所有步骤都变得相当的繁琐。
>>> # Some queries that work
>>> doc.findtext('author')
'David Beazley'
>>> doc.find('content')
<Element 'content' at 0x100776ec0>
>>> # A query involving a namespace (doesn't work)
>>> doc.find('content/html')
>>> # Works if fully qualified
>>> doc.find('content/{http://www.w3.org/1999/xhtml}html')
<Element '{http://www.w3.org/1999/xhtml}html' at 0x1007767e0>
>>> # Doesn't work
>>> doc.findtext('content/{http://www.w3.org/1999/xhtml}html/head/title')
>>> # Fully qualified
>>> doc.findtext('content/{http://www.w3.org/1999/xhtml}html/'
... '{http://www.w3.org/1999/xhtml}head/{http://www.w3.org/1999/xhtml}title')
'Hello World'
>>>
你可以通过将命名空间处理逻辑包装为一个工具类来简化这个过程：
class XMLNamespaces:
def __init__(self, **kwargs):
self.namespaces = {}
for name, uri in kwargs.items():
self.register(name, uri)
def register(self, name, uri):
self.namespaces[name] = '{'+uri+'}'
def __call__(self, path):
return path.format_map(self.namespaces)
通过下面的方式使用这个类：
>>> ns = XMLNamespaces(html='http://www.w3.org/1999/xhtml')
>>> doc.find(ns('content/{html}html'))
<Element '{http://www.w3.org/1999/xhtml}html' at 0x1007767e0>
>>> doc.findtext(ns('content/{html}html/{html}head/{html}title'))
'Hello World'
>>>
讨论
解析含有命名空间的XML文档会比较繁琐。 上面的 XMLNamespaces 仅仅是允许你使用缩略名代替完整的URI将其变得稍微简洁一点。
很不幸的是，在基本的 ElementTree 解析中没有任何途径获取命名空间的信息。 但是，如果你使用 iterparse() 函数的话就可以获取更多关于命名空间处理范围的信息。例如：
>>> from xml.etree.ElementTree import iterparse
>>> for evt, elem in iterparse('ns2.xml', ('end', 'start-ns', 'end-ns')):
... print(evt, elem)
...
end <Element 'author' at 0x10110de10>
start-ns ('', 'http://www.w3.org/1999/xhtml')
end <Element '{http://www.w3.org/1999/xhtml}title' at 0x1011131b0>
end <Element '{http://www.w3.org/1999/xhtml}head' at 0x1011130a8>
end <Element '{http://www.w3.org/1999/xhtml}h1' at 0x101113310>
end <Element '{http://www.w3.org/1999/xhtml}body' at 0x101113260>
end <Element '{http://www.w3.org/1999/xhtml}html' at 0x10110df70>
end-ns None
end <Element 'content' at 0x10110de68>
end <Element 'top' at 0x10110dd60>
>>> elem # This is the topmost element
<Element 'top' at 0x10110dd60>
>>>
最后一点，如果你要处理的XML文本除了要使用到其他高级XML特性外，还要使用到命名空间， 建议你最好是使用 lxml 函数库来代替 ElementTree 。 例如，lxml 对利用DTD验证文档、更好的XPath支持和一些其他高级XML特性等都提供了更好的支持。 这一小节其实只是教你如何让XML解析稍微简单一点。
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