20.5. "xml.etree.ElementTree" ---  ElementTree XML API
******************************************************

**源代码：** Lib/xml/etree/ElementTree.py

======================================================================

"xml.etree.ElementTree" 模块实现了一个简单高效的API，用于解析和创建XML
数据。

在 3.3 版更改: 只要有可能，这个模块将使用快速实现。
"xml.etree.cElementTree" 模块已弃用。

警告:

  "xml.etree.ElementTree" 模块对于恶意构建的数据是不安全的。如果需要解
  析不可信或未经身份验证的数据，请参见 XML 漏洞 。


20.5.1. 教程
============

这是一个使用 "xml.etree.ElementTree" （简称 "ET" ）的简短教程。目标是
演示模块的一些构建块和基本概念。


20.5.1.1. XML树和元素
---------------------

XML是一种固有的分层数据格式，最自然的表示方法是使用树。为此， "ET" 有
两个类 "ElementTree" 将整个XML文档表示为一个树， "Element" 表示该树中
的单个节点。与整个文档的交互（读写文件）通常在 "ElementTree" 级别完成
。与单个XML元素及其子元素的交互是在 "Element" 级别完成的。


20.5.1.2. 解析XML
-----------------

我们将使用以下XML文档作为本节的示例数据：

   <?xml version="1.0"?>
   <data>
       <country name="Liechtenstein">
           <rank>1</rank>
           <year>2008</year>
           <gdppc>141100</gdppc>
           <neighbor name="Austria" direction="E"/>
           <neighbor name="Switzerland" direction="W"/>
       </country>
       <country name="Singapore">
           <rank>4</rank>
           <year>2011</year>
           <gdppc>59900</gdppc>
           <neighbor name="Malaysia" direction="N"/>
       </country>
       <country name="Panama">
           <rank>68</rank>
           <year>2011</year>
           <gdppc>13600</gdppc>
           <neighbor name="Costa Rica" direction="W"/>
           <neighbor name="Colombia" direction="E"/>
       </country>
   </data>

我们可以通过从文件中读取来导入此数据：

   import xml.etree.ElementTree as ET
   tree = ET.parse('country_data.xml')
   root = tree.getroot()

或直接从字符串中：

   root = ET.fromstring(country_data_as_string)

"fromstring()" 将XML从字符串直接解析为 "Element" ，该元素是已解析树的
根元素。其他解析函数可能会创建一个 "ElementTree" 。确切的信息请检查文
档。

作为一个 "Element" ， "root" 有一个标记和一个属性字典:

   >>> root.tag
   'data'
   >>> root.attrib
   {}

它还有我们可以迭代的子节点：

   >>> for child in root:
   ...     print(child.tag, child.attrib)
   ...
   country {'name': 'Liechtenstein'}
   country {'name': 'Singapore'}
   country {'name': 'Panama'}

子级是可以嵌套的，我们可以通过索引访问特定的子级节点：

   >>> root[0][1].text
   '2008'

注解:

  并非XML输入的所有元素都将作为解析树的元素结束。目前，此模块跳过输入
  中的任何XML注释、处理指令和文档类型声明。然而，使用这个模块的API而不
  是从XML文本解析构建的树可以包含注释和处理指令，生成XML输出时同样包含
  这些注释和处理指令。可以通过将自定义 "TreeBuilder" 实例传递给
  "XMLParser" 构造函数来访问文档类型声明。


20.5.1.3. Pull API进行非阻塞解析
--------------------------------

Most parsing functions provided by this module require the whole
document to be read at once before returning any result.  It is
possible to use an "XMLParser" and feed data into it incrementally,
but it is a push API that calls methods on a callback target, which is
too low-level and inconvenient for most needs.  Sometimes what the
user really wants is to be able to parse XML incrementally, without
blocking operations, while enjoying the convenience of fully
constructed "Element" objects.

The most powerful tool for doing this is "XMLPullParser".  It does not
require a blocking read to obtain the XML data, and is instead fed
with data incrementally with "XMLPullParser.feed()" calls.  To get the
parsed XML elements, call "XMLPullParser.read_events()".  Here is an
example:

   >>> parser = ET.XMLPullParser(['start', 'end'])
   >>> parser.feed('<mytag>sometext')
   >>> list(parser.read_events())
   [('start', <Element 'mytag' at 0x7fa66db2be58>)]
   >>> parser.feed(' more text</mytag>')
   >>> for event, elem in parser.read_events():
   ...     print(event)
   ...     print(elem.tag, 'text=', elem.text)
   ...
   end

The obvious use case is applications that operate in a non-blocking
fashion where the XML data is being received from a socket or read
incrementally from some storage device.  In such cases, blocking reads
are unacceptable.

Because it's so flexible, "XMLPullParser" can be inconvenient to use
for simpler use-cases.  If you don't mind your application blocking on
reading XML data but would still like to have incremental parsing
capabilities, take a look at "iterparse()".  It can be useful when
you're reading a large XML document and don't want to hold it wholly
in memory.


20.5.1.4. 寻找有趣的元素
------------------------

"Element" 有一些很有效的方法，可帮助递归遍历其下的所有子树（包括子级，
子级的子级，等等）。例如 "Element.iter()":

   >>> for neighbor in root.iter('neighbor'):
   ...     print(neighbor.attrib)
   ...
   {'name': 'Austria', 'direction': 'E'}
   {'name': 'Switzerland', 'direction': 'W'}
   {'name': 'Malaysia', 'direction': 'N'}
   {'name': 'Costa Rica', 'direction': 'W'}
   {'name': 'Colombia', 'direction': 'E'}

"Element.findall()" 仅查找当前元素的直接子元素中带有指定标签的元素。
"Element.find()" 找带有特定标签的 *第一个* 子级，然后可以用
"Element.text" 访问元素的文本内容。 "Element.text" 访问元素的属性:

   >>> for country in root.findall('country'):
   ...     rank = country.find('rank').text
   ...     name = country.get('name')
   ...     print(name, rank)
   ...
   Liechtenstein 1
   Singapore 4
   Panama 68

通过使用 XPath ，可以更精确地指定要查找的元素。


20.5.1.5. 修改XML文件
---------------------

"ElementTree" 提供了一种构建XML文档并将其写入文件的简单方法。
"ElementTree.write()" 方法可达到此目的。

创建后可以直接操作 "Element" 对象。例如：使用 "Element.text" 修改文本
字段，使用 "Element.set()" 方法添加和修改属性，以及使用
"Element.append()" 添加新的子元素。

假设我们要在每个国家/地区的中添加一个排名，并在rank元素中添加一个
"updated" 属性：

   >>> for rank in root.iter('rank'):
   ...     new_rank = int(rank.text) + 1
   ...     rank.text = str(new_rank)
   ...     rank.set('updated', 'yes')
   ...
   >>> tree.write('output.xml')

生成的XML现在看起来像这样：

   <?xml version="1.0"?>
   <data>
       <country name="Liechtenstein">
           <rank updated="yes">2</rank>
           <year>2008</year>
           <gdppc>141100</gdppc>
           <neighbor name="Austria" direction="E"/>
           <neighbor name="Switzerland" direction="W"/>
       </country>
       <country name="Singapore">
           <rank updated="yes">5</rank>
           <year>2011</year>
           <gdppc>59900</gdppc>
           <neighbor name="Malaysia" direction="N"/>
       </country>
       <country name="Panama">
           <rank updated="yes">69</rank>
           <year>2011</year>
           <gdppc>13600</gdppc>
           <neighbor name="Costa Rica" direction="W"/>
           <neighbor name="Colombia" direction="E"/>
       </country>
   </data>

可以使用 "Element.remove()" 删除元素。假设我们要删除排名高于50的所有国
家/地区:

   >>> for country in root.findall('country'):
   ...     rank = int(country.find('rank').text)
   ...     if rank > 50:
   ...         root.remove(country)
   ...
   >>> tree.write('output.xml')

生成的XML现在看起来像这样：

   <?xml version="1.0"?>
   <data>
       <country name="Liechtenstein">
           <rank updated="yes">2</rank>
           <year>2008</year>
           <gdppc>141100</gdppc>
           <neighbor name="Austria" direction="E"/>
           <neighbor name="Switzerland" direction="W"/>
       </country>
       <country name="Singapore">
           <rank updated="yes">5</rank>
           <year>2011</year>
           <gdppc>59900</gdppc>
           <neighbor name="Malaysia" direction="N"/>
       </country>
   </data>


20.5.1.6. 构建XML文档
---------------------

"SubElement()" 函数还提供了一种便捷方法来为给定元素创建新的子元素:

   >>> a = ET.Element('a')
   >>> b = ET.SubElement(a, 'b')
   >>> c = ET.SubElement(a, 'c')
   >>> d = ET.SubElement(c, 'd')
   >>> ET.dump(a)
   <a><b /><c><d /></c></a>


20.5.1.7. 使用命名空间解析XML
-----------------------------

If the XML input has namespaces, tags and attributes with prefixes in
the form "prefix:sometag" get expanded to "{uri}sometag" where the
*prefix* is replaced by the full *URI*. Also, if there is a default
namespace, that full URI gets prepended to all of the non-prefixed
tags.

Here is an XML example that incorporates two namespaces, one with the
prefix "fictional" and the other serving as the default namespace:

   <?xml version="1.0"?>
   <actors xmlns:fictional="http://characters.example.com"
           xmlns="http://people.example.com">
       <actor>
           <name>John Cleese</name>
           <fictional:character>Lancelot</fictional:character>
           <fictional:character>Archie Leach</fictional:character>
       </actor>
       <actor>
           <name>Eric Idle</name>
           <fictional:character>Sir Robin</fictional:character>
           <fictional:character>Gunther</fictional:character>
           <fictional:character>Commander Clement</fictional:character>
       </actor>
   </actors>

One way to search and explore this XML example is to manually add the
URI to every tag or attribute in the xpath of a "find()" or
"findall()":

   root = fromstring(xml_text)
   for actor in root.findall('{http://people.example.com}actor'):
       name = actor.find('{http://people.example.com}name')
       print(name.text)
       for char in actor.findall('{http://characters.example.com}character'):
           print(' |-->', char.text)

A better way to search the namespaced XML example is to create a
dictionary with your own prefixes and use those in the search
functions:

   ns = {'real_person': 'http://people.example.com',
         'role': 'http://characters.example.com'}

   for actor in root.findall('real_person:actor', ns):
       name = actor.find('real_person:name', ns)
       print(name.text)
       for char in actor.findall('role:character', ns):
           print(' |-->', char.text)

These two approaches both output:

   John Cleese
    |--> Lancelot
    |--> Archie Leach
   Eric Idle
    |--> Sir Robin
    |--> Gunther
    |--> Commander Clement


20.5.1.8. 其他资源
------------------

See http://effbot.org/zone/element-index.htm for tutorials and links
to other docs.


20.5.2. XPath支持
=================

This module provides limited support for XPath expressions for
locating elements in a tree.  The goal is to support a small subset of
the abbreviated syntax; a full XPath engine is outside the scope of
the module.


20.5.2.1. 示例
--------------

Here's an example that demonstrates some of the XPath capabilities of
the module.  We'll be using the "countrydata" XML document from the
Parsing XML section:

   import xml.etree.ElementTree as ET

   root = ET.fromstring(countrydata)

   # Top-level elements
   root.findall(".")

   # All 'neighbor' grand-children of 'country' children of the top-level
   # elements
   root.findall("./country/neighbor")

   # Nodes with name='Singapore' that have a 'year' child
   root.findall(".//year/..[@name='Singapore']")

   # 'year' nodes that are children of nodes with name='Singapore'
   root.findall(".//*[@name='Singapore']/year")

   # All 'neighbor' nodes that are the second child of their parent
   root.findall(".//neighbor[2]")


20.5.2.2. 支持的XPath语法
-------------------------

+-------------------------+--------------------------------------------------------+
| 语法                    | 含义                                                   |
|=========================|========================================================|
| "tag"                   | Selects all child elements with the given tag. For     |
|                         | example, "spam" selects all child elements named       |
|                         | "spam", and "spam/egg" selects all grandchildren named |
|                         | "egg" in all children named "spam".                    |
+-------------------------+--------------------------------------------------------+
| "*"                     | Selects all child elements.  For example, "*/egg"      |
|                         | selects all grandchildren named "egg".                 |
+-------------------------+--------------------------------------------------------+
| "."                     | 选择当前节点。这在路径的开头非常有用，用于指示它是相对 |
|                         | 路径。                                                 |
+-------------------------+--------------------------------------------------------+
| "//"                    | Selects all subelements, on all levels beneath the     |
|                         | current  element. For example, ".//egg" selects all    |
|                         | "egg" elements in the entire tree.                     |
+-------------------------+--------------------------------------------------------+
| ".."                    | Selects the parent element.  Returns "None" if the     |
|                         | path attempts to reach the ancestors of the start      |
|                         | element (the element "find" was called on).            |
+-------------------------+--------------------------------------------------------+
| "[@attrib]"             | 选择具有给定属性的所有元素。                           |
+-------------------------+--------------------------------------------------------+
| "[@attrib='value']"     | 选择给定属性具有给定值的所有元素。该值不能包含引号。   |
+-------------------------+--------------------------------------------------------+
| "[tag]"                 | 选择所有包含 "tag" 子元素的元素。只支持直系子元素。    |
+-------------------------+--------------------------------------------------------+
| "[tag='text']"          | 选择所有包含名为 "tag" 的子元素的元素，这些子元素（包  |
|                         | 括后代）的完整文 本内容等于给定的 "text" 。            |
+-------------------------+--------------------------------------------------------+
| "[position]"            | Selects all elements that are located at the given     |
|                         | position.  The position can be either an integer (1 is |
|                         | the first position), the expression "last()" (for the  |
|                         | last position), or a position relative to the last     |
|                         | position (e.g. "last()-1").                            |
+-------------------------+--------------------------------------------------------+

谓词（方括号内的表达式）之前必须带有标签名称，星号或其他谓词。
"position" 谓词前必须有标签名称。


20.5.3. 参考
============


20.5.3.1. 函数
--------------

xml.etree.ElementTree.Comment(text=None)

   Comment element factory.  This factory function creates a special
   element that will be serialized as an XML comment by the standard
   serializer.  The comment string can be either a bytestring or a
   Unicode string.  *text* is a string containing the comment string.
   Returns an element instance representing a comment.

   Note that "XMLParser" skips over comments in the input instead of
   creating comment objects for them. An "ElementTree" will only
   contain comment nodes if they have been inserted into to the tree
   using one of the "Element" methods.

xml.etree.ElementTree.dump(elem)

   Writes an element tree or element structure to sys.stdout.  This
   function should be used for debugging only.

   The exact output format is implementation dependent.  In this
   version, it's written as an ordinary XML file.

   *elem* is an element tree or an individual element.

xml.etree.ElementTree.fromstring(text)

   Parses an XML section from a string constant.  Same as "XML()".
   *text* is a string containing XML data.  Returns an "Element"
   instance.

xml.etree.ElementTree.fromstringlist(sequence, parser=None)

   Parses an XML document from a sequence of string fragments.
   *sequence* is a list or other sequence containing XML data
   fragments.  *parser* is an optional parser instance.  If not given,
   the standard "XMLParser" parser is used.  Returns an "Element"
   instance.

   3.2 新版功能.

xml.etree.ElementTree.iselement(element)

   Checks if an object appears to be a valid element object.
   *element* is an element instance.  Returns a true value if this is
   an element object.

xml.etree.ElementTree.iterparse(source, events=None, parser=None)

   Parses an XML section into an element tree incrementally, and
   reports what's going on to the user.  *source* is a filename or
   *file object* containing XML data.  *events* is a sequence of
   events to report back.  The supported events are the strings
   ""start"", ""end"", ""start-ns"" and ""end-ns"" (the "ns" events
   are used to get detailed namespace information).  If *events* is
   omitted, only ""end"" events are reported. *parser* is an optional
   parser instance.  If not given, the standard "XMLParser" parser is
   used.  *parser* must be a subclass of "XMLParser" and can only use
   the default "TreeBuilder" as a target.  Returns an *iterator*
   providing "(event, elem)" pairs.

   Note that while "iterparse()" builds the tree incrementally, it
   issues blocking reads on *source* (or the file it names).  As such,
   it's unsuitable for applications where blocking reads can't be
   made.  For fully non-blocking parsing, see "XMLPullParser".

   注解:

     "iterparse()" only guarantees that it has seen the ">" character
     of a starting tag when it emits a "start" event, so the
     attributes are defined, but the contents of the text and tail
     attributes are undefined at that point.  The same applies to the
     element children; they may or may not be present.If you need a
     fully populated element, look for "end" events instead.

   3.4 版后已移除: *parser* 参数。

xml.etree.ElementTree.parse(source, parser=None)

   Parses an XML section into an element tree.  *source* is a filename
   or file object containing XML data.  *parser* is an optional parser
   instance.  If not given, the standard "XMLParser" parser is used.
   Returns an "ElementTree" instance.

xml.etree.ElementTree.ProcessingInstruction(target, text=None)

   PI element factory.  This factory function creates a special
   element that will be serialized as an XML processing instruction.
   *target* is a string containing the PI target.  *text* is a string
   containing the PI contents, if given.  Returns an element instance,
   representing a processing instruction.

   Note that "XMLParser" skips over processing instructions in the
   input instead of creating comment objects for them. An
   "ElementTree" will only contain processing instruction nodes if
   they have been inserted into to the tree using one of the "Element"
   methods.

xml.etree.ElementTree.register_namespace(prefix, uri)

   Registers a namespace prefix.  The registry is global, and any
   existing mapping for either the given prefix or the namespace URI
   will be removed. *prefix* is a namespace prefix.  *uri* is a
   namespace uri.  Tags and attributes in this namespace will be
   serialized with the given prefix, if at all possible.

   3.2 新版功能.

xml.etree.ElementTree.SubElement(parent, tag, attrib={}, **extra)

   Subelement factory.  This function creates an element instance, and
   appends it to an existing element.

   The element name, attribute names, and attribute values can be
   either bytestrings or Unicode strings.  *parent* is the parent
   element.  *tag* is the subelement name.  *attrib* is an optional
   dictionary, containing element attributes.  *extra* contains
   additional attributes, given as keyword arguments.  Returns an
   element instance.

xml.etree.ElementTree.tostring(element, encoding="us-ascii", method="xml", *, short_empty_elements=True)

   Generates a string representation of an XML element, including all
   subelements.  *element* is an "Element" instance.  *encoding* [1]
   is the output encoding (default is US-ASCII).  Use
   "encoding="unicode"" to generate a Unicode string (otherwise, a
   bytestring is generated).  *method* is either ""xml"", ""html"" or
   ""text"" (default is ""xml""). *short_empty_elements* has the same
   meaning as in "ElementTree.write()". Returns an (optionally)
   encoded string containing the XML data.

   3.4 新版功能: *short_empty_elements* 形参。

xml.etree.ElementTree.tostringlist(element, encoding="us-ascii", method="xml", *, short_empty_elements=True)

   Generates a string representation of an XML element, including all
   subelements.  *element* is an "Element" instance.  *encoding* [1]
   is the output encoding (default is US-ASCII).  Use
   "encoding="unicode"" to generate a Unicode string (otherwise, a
   bytestring is generated).  *method* is either ""xml"", ""html"" or
   ""text"" (default is ""xml""). *short_empty_elements* has the same
   meaning as in "ElementTree.write()". Returns a list of (optionally)
   encoded strings containing the XML data. It does not guarantee any
   specific sequence, except that "b"".join(tostringlist(element)) ==
   tostring(element)".

   3.2 新版功能.

   3.4 新版功能: *short_empty_elements* 形参。

xml.etree.ElementTree.XML(text, parser=None)

   Parses an XML section from a string constant.  This function can be
   used to embed "XML literals" in Python code.  *text* is a string
   containing XML data.  *parser* is an optional parser instance.  If
   not given, the standard "XMLParser" parser is used.  Returns an
   "Element" instance.

xml.etree.ElementTree.XMLID(text, parser=None)

   Parses an XML section from a string constant, and also returns a
   dictionary which maps from element id:s to elements.  *text* is a
   string containing XML data.  *parser* is an optional parser
   instance.  If not given, the standard "XMLParser" parser is used.
   Returns a tuple containing an "Element" instance and a dictionary.


20.5.3.2. 元素对象
------------------

class xml.etree.ElementTree.Element(tag, attrib={}, **extra)

   Element class.  This class defines the Element interface, and
   provides a reference implementation of this interface.

   The element name, attribute names, and attribute values can be
   either bytestrings or Unicode strings.  *tag* is the element name.
   *attrib* is an optional dictionary, containing element attributes.
   *extra* contains additional attributes, given as keyword arguments.

   tag

      一个标识此元素意味着何种数据的字符串(换句话说，元素类型)。

   text
   tail

      These attributes can be used to hold additional data associated
      with the element.  Their values are usually strings but may be
      any application-specific object.  If the element is created from
      an XML file, the *text* attribute holds either the text between
      the element's start tag and its first child or end tag, or
      "None", and the *tail* attribute holds either the text between
      the element's end tag and the next tag, or "None".  For the XML
      data

         <a><b>1<c>2<d/>3</c></b>4</a>

      the *a* element has "None" for both *text* and *tail*
      attributes, the *b* element has *text* ""1"" and *tail* ""4"",
      the *c* element has *text* ""2"" and *tail* "None", and the *d*
      element has *text* "None" and *tail* ""3"".

      To collect the inner text of an element, see "itertext()", for
      example """.join(element.itertext())".

      Applications may store arbitrary objects in these attributes.

   attrib

      A dictionary containing the element's attributes.  Note that
      while the *attrib* value is always a real mutable Python
      dictionary, an ElementTree implementation may choose to use
      another internal representation, and create the dictionary only
      if someone asks for it.  To take advantage of such
      implementations, use the dictionary methods below whenever
      possible.

   The following dictionary-like methods work on the element
   attributes.

   clear()

      Resets an element.  This function removes all subelements,
      clears all attributes, and sets the text and tail attributes to
      "None".

   get(key, default=None)

      Gets the element attribute named *key*.

      Returns the attribute value, or *default* if the attribute was
      not found.

   items()

      Returns the element attributes as a sequence of (name, value)
      pairs.  The attributes are returned in an arbitrary order.

   keys()

      Returns the elements attribute names as a list.  The names are
      returned in an arbitrary order.

   set(key, value)

      Set the attribute *key* on the element to *value*.

   The following methods work on the element's children (subelements).

   append(subelement)

      Adds the element *subelement* to the end of this element's
      internal list of subelements.  Raises "TypeError" if
      *subelement* is not an "Element".

   extend(subelements)

      Appends *subelements* from a sequence object with zero or more
      elements. Raises "TypeError" if a subelement is not an
      "Element".

      3.2 新版功能.

   find(match, namespaces=None)

      Finds the first subelement matching *match*.  *match* may be a
      tag name or a path.  Returns an element instance or "None".
      *namespaces* is an optional mapping from namespace prefix to
      full name.

   findall(match, namespaces=None)

      Finds all matching subelements, by tag name or path.  Returns a
      list containing all matching elements in document order.
      *namespaces* is an optional mapping from namespace prefix to
      full name.

   findtext(match, default=None, namespaces=None)

      Finds text for the first subelement matching *match*.  *match*
      may be a tag name or a path.  Returns the text content of the
      first matching element, or *default* if no element was found.
      Note that if the matching element has no text content an empty
      string is returned. *namespaces* is an optional mapping from
      namespace prefix to full name.

   getchildren()

      3.2 版后已移除: Use "list(elem)" or iteration.

   getiterator(tag=None)

      3.2 版后已移除: Use method "Element.iter()" instead.

   insert(index, subelement)

      Inserts *subelement* at the given position in this element.
      Raises "TypeError" if *subelement* is not an "Element".

   iter(tag=None)

      Creates a tree *iterator* with the current element as the root.
      The iterator iterates over this element and all elements below
      it, in document (depth first) order.  If *tag* is not "None" or
      "'*'", only elements whose tag equals *tag* are returned from
      the iterator.  If the tree structure is modified during
      iteration, the result is undefined.

      3.2 新版功能.

   iterfind(match, namespaces=None)

      Finds all matching subelements, by tag name or path.  Returns an
      iterable yielding all matching elements in document order.
      *namespaces* is an optional mapping from namespace prefix to
      full name.

      3.2 新版功能.

   itertext()

      Creates a text iterator.  The iterator loops over this element
      and all subelements, in document order, and returns all inner
      text.

      3.2 新版功能.

   makeelement(tag, attrib)

      Creates a new element object of the same type as this element.
      Do not call this method, use the "SubElement()" factory function
      instead.

   remove(subelement)

      Removes *subelement* from the element.  Unlike the find* methods
      this method compares elements based on the instance identity,
      not on tag value or contents.

   "Element" objects also support the following sequence type methods
   for working with subelements: "__delitem__()", "__getitem__()",
   "__setitem__()", "__len__()".

   Caution: Elements with no subelements will test as "False".  This
   behavior will change in future versions.  Use specific "len(elem)"
   or "elem is None" test instead.

      element = root.find('foo')

      if not element:  # careful!
          print("element not found, or element has no subelements")

      if element is None:
          print("element not found")


20.5.3.3. ElementTree 对象
--------------------------

class xml.etree.ElementTree.ElementTree(element=None, file=None)

   ElementTree wrapper class.  This class represents an entire element
   hierarchy, and adds some extra support for serialization to and
   from standard XML.

   *element* is the root element.  The tree is initialized with the
   contents of the XML *file* if given.

   _setroot(element)

      Replaces the root element for this tree.  This discards the
      current contents of the tree, and replaces it with the given
      element.  Use with care.  *element* is an element instance.

   find(match, namespaces=None)

      Same as "Element.find()", starting at the root of the tree.

   findall(match, namespaces=None)

      Same as "Element.findall()", starting at the root of the tree.

   findtext(match, default=None, namespaces=None)

      Same as "Element.findtext()", starting at the root of the tree.

   getiterator(tag=None)

      3.2 版后已移除: Use method "ElementTree.iter()" instead.

   getroot()

      Returns the root element for this tree.

   iter(tag=None)

      Creates and returns a tree iterator for the root element.  The
      iterator loops over all elements in this tree, in section order.
      *tag* is the tag to look for (default is to return all
      elements).

   iterfind(match, namespaces=None)

      Same as "Element.iterfind()", starting at the root of the tree.

      3.2 新版功能.

   parse(source, parser=None)

      Loads an external XML section into this element tree.  *source*
      is a file name or *file object*.  *parser* is an optional parser
      instance. If not given, the standard "XMLParser" parser is used.
      Returns the section root element.

   write(file, encoding="us-ascii", xml_declaration=None, default_namespace=None, method="xml", *, short_empty_elements=True)

      Writes the element tree to a file, as XML.  *file* is a file
      name, or a *file object* opened for writing.  *encoding* [1] is
      the output encoding (default is US-ASCII). *xml_declaration*
      controls if an XML declaration should be added to the file.  Use
      "False" for never, "True" for always, "None" for only if not US-
      ASCII or UTF-8 or Unicode (default is "None").
      *default_namespace* sets the default XML namespace (for
      "xmlns"). *method* is either ""xml"", ""html"" or ""text""
      (default is ""xml""). The keyword-only *short_empty_elements*
      parameter controls the formatting of elements that contain no
      content.  If "True" (the default), they are emitted as a single
      self-closed tag, otherwise they are emitted as a pair of
      start/end tags.

      The output is either a string ("str") or binary ("bytes"). This
      is controlled by the *encoding* argument.  If *encoding* is
      ""unicode"", the output is a string; otherwise, it's binary.
      Note that this may conflict with the type of *file* if it's an
      open *file object*; make sure you do not try to write a string
      to a binary stream and vice versa.

      3.4 新版功能: *short_empty_elements* 形参。

This is the XML file that is going to be manipulated:

   <html>
       <head>
           <title>Example page</title>
       </head>
       <body>
           <p>Moved to <a href="http://example.org/">example.org</a>
           or <a href="http://example.com/">example.com</a>.</p>
       </body>
   </html>

Example of changing the attribute "target" of every link in first
paragraph:

   >>> from xml.etree.ElementTree import ElementTree
   >>> tree = ElementTree()
   >>> tree.parse("index.xhtml")
   <Element 'html' at 0xb77e6fac>
   >>> p = tree.find("body/p")     # Finds first occurrence of tag p in body
   >>> p
   <Element 'p' at 0xb77ec26c>
   >>> links = list(p.iter("a"))   # Returns list of all links
   >>> links
   [<Element 'a' at 0xb77ec2ac>, <Element 'a' at 0xb77ec1cc>]
   >>> for i in links:             # Iterates through all found links
   ...     i.attrib["target"] = "blank"
   >>> tree.write("output.xhtml")


20.5.3.4. QName Objects
-----------------------

class xml.etree.ElementTree.QName(text_or_uri, tag=None)

   QName wrapper.  This can be used to wrap a QName attribute value,
   in order to get proper namespace handling on output.  *text_or_uri*
   is a string containing the QName value, in the form {uri}local, or,
   if the tag argument is given, the URI part of a QName.  If *tag* is
   given, the first argument is interpreted as a URI, and this
   argument is interpreted as a local name. "QName" instances are
   opaque.


20.5.3.5. TreeBuilder Objects
-----------------------------

class xml.etree.ElementTree.TreeBuilder(element_factory=None)

   Generic element structure builder.  This builder converts a
   sequence of start, data, and end method calls to a well-formed
   element structure.  You can use this class to build an element
   structure using a custom XML parser, or a parser for some other
   XML-like format.  *element_factory*, when given, must be a callable
   accepting two positional arguments: a tag and a dict of attributes.
   It is expected to return a new element instance.

   close()

      Flushes the builder buffers, and returns the toplevel document
      element.  Returns an "Element" instance.

   data(data)

      Adds text to the current element.  *data* is a string.  This
      should be either a bytestring, or a Unicode string.

   end(tag)

      Closes the current element.  *tag* is the element name.  Returns
      the closed element.

   start(tag, attrs)

      Opens a new element.  *tag* is the element name.  *attrs* is a
      dictionary containing element attributes.  Returns the opened
      element.

   In addition, a custom "TreeBuilder" object can provide the
   following method:

   doctype(name, pubid, system)

      Handles a doctype declaration.  *name* is the doctype name.
      *pubid* is the public identifier.  *system* is the system
      identifier.  This method does not exist on the default
      "TreeBuilder" class.

      3.2 新版功能.


20.5.3.6. XMLParser对象
-----------------------

class xml.etree.ElementTree.XMLParser(html=0, target=None, encoding=None)

   This class is the low-level building block of the module.  It uses
   "xml.parsers.expat" for efficient, event-based parsing of XML.  It
   can be fed XML data incrementally with the "feed()" method, and
   parsing events are translated to a push API - by invoking callbacks
   on the *target* object.  If *target* is omitted, the standard
   "TreeBuilder" is used. The *html* argument was historically used
   for backwards compatibility and is now deprecated.  If *encoding*
   [1] is given, the value overrides the encoding specified in the XML
   file.

   3.4 版后已移除: The *html* argument.  The remaining arguments
   should be passed via keyword to prepare for the removal of the
   *html* argument.

   close()

      Finishes feeding data to the parser.  Returns the result of
      calling the "close()" method of the *target* passed during
      construction; by default, this is the toplevel document element.

   doctype(name, pubid, system)

      3.2 版后已移除: Define the "TreeBuilder.doctype()" method on a
      custom TreeBuilder target.

   feed(data)

      Feeds data to the parser.  *data* is encoded data.

   "XMLParser.feed()" calls *target*'s "start(tag, attrs_dict)" method
   for each opening tag, its "end(tag)" method for each closing tag,
   and data is processed by method "data(data)".  "XMLParser.close()"
   calls *target*'s method "close()". "XMLParser" can be used not only
   for building a tree structure. This is an example of counting the
   maximum depth of an XML file:

      >>> from xml.etree.ElementTree import XMLParser
      >>> class MaxDepth:                     # The target object of the parser
      ...     maxDepth = 0
      ...     depth = 0
      ...     def start(self, tag, attrib):   # Called for each opening tag.
      ...         self.depth += 1
      ...         if self.depth > self.maxDepth:
      ...             self.maxDepth = self.depth
      ...     def end(self, tag):             # Called for each closing tag.
      ...         self.depth -= 1
      ...     def data(self, data):
      ...         pass            # We do not need to do anything with data.
      ...     def close(self):    # Called when all data has been parsed.
      ...         return self.maxDepth
      ...
      >>> target = MaxDepth()
      >>> parser = XMLParser(target=target)
      >>> exampleXml = """
      ... <a>
      ...   <b>
      ...   </b>
      ...   <b>
      ...     <c>
      ...       <d>
      ...       </d>
      ...     </c>
      ...   </b>
      ... </a>"""
      >>> parser.feed(exampleXml)
      >>> parser.close()
      4


20.5.3.7. XMLPullParser对象
---------------------------

class xml.etree.ElementTree.XMLPullParser(events=None)

   A pull parser suitable for non-blocking applications.  Its input-
   side API is similar to that of "XMLParser", but instead of pushing
   calls to a callback target, "XMLPullParser" collects an internal
   list of parsing events and lets the user read from it. *events* is
   a sequence of events to report back.  The supported events are the
   strings ""start"", ""end"", ""start-ns"" and ""end-ns"" (the "ns"
   events are used to get detailed namespace information).  If
   *events* is omitted, only ""end"" events are reported.

   feed(data)

      Feed the given bytes data to the parser.

   close()

      Signal the parser that the data stream is terminated. Unlike
      "XMLParser.close()", this method always returns "None". Any
      events not yet retrieved when the parser is closed can still be
      read with "read_events()".

   read_events()

      Return an iterator over the events which have been encountered
      in the data fed to the parser.  The iterator yields "(event,
      elem)" pairs, where *event* is a string representing the type of
      event (e.g. ""end"") and *elem* is the encountered "Element"
      object.

      Events provided in a previous call to "read_events()" will not
      be yielded again.  Events are consumed from the internal queue
      only when they are retrieved from the iterator, so multiple
      readers iterating in parallel over iterators obtained from
      "read_events()" will have unpredictable results.

   注解:

     "XMLPullParser" only guarantees that it has seen the ">"
     character of a starting tag when it emits a "start" event, so the
     attributes are defined, but the contents of the text and tail
     attributes are undefined at that point.  The same applies to the
     element children; they may or may not be present.If you need a
     fully populated element, look for "end" events instead.

   3.4 新版功能.


20.5.3.8. 异常
--------------

class xml.etree.ElementTree.ParseError

   XML parse error, raised by the various parsing methods in this
   module when parsing fails.  The string representation of an
   instance of this exception will contain a user-friendly error
   message.  In addition, it will have the following attributes
   available:

   code

      A numeric error code from the expat parser. See the
      documentation of "xml.parsers.expat" for the list of error codes
      and their meanings.

   position

      A tuple of *line*, *column* numbers, specifying where the error
      occurred.

-[ 备注 ]-

[1] The encoding string included in XML output should conform to the
    appropriate standards.  For example, "UTF-8" is valid, but "UTF8"
    is not.  See https://www.w3.org/TR/2006/REC-xml11-20060816/#NT-
    EncodingDecl and https://www.iana.org/assignments/character-sets
    /character-sets.xhtml.
