class Element:

     """An XML element.

     This class is the reference implementation of the Element interface.

     An element's length is its number of subelements.  That means if you

     want to check if an element is truly empty, you should check BOTH

     its length AND its text attribute.

     The element tag, attribute names, and attribute values can be either

     bytes or strings.

     *tag* is the element name.  *attrib* is an optional dictionary containing

     element attributes. *extra* are additional element attributes given as

     keyword arguments.

     Example form:

         <tag attrib>text<child/>...</tag>tail

     """

     当前节点的标签名

     tag = None

     """The element's name."""

     当前节点的属性

     attrib = None

     """Dictionary of the element's attributes."""

     当前节点的内容

     text = None

     """

     Text before first subelement. This is either a string or the value None.

     Note that if there is no text, this attribute may be either

     None or the empty string, depending on the parser.

     """

     tail = None

     """

     Text after this element's end tag, but before the next sibling element's

     start tag.  This is either a string or the value None.  Note that if there

     was no text, this attribute may be either None or an empty string,

     depending on the parser.

     """

     def __init__(self, tag, attrib={}, **extra):

         if not isinstance(attrib, dict):

             raise TypeError("attrib must be dict, not %s" % (

                 attrib.__class__.__name__,))

         attrib = attrib.copy()

         attrib.update(extra)

         self.tag = tag

         self.attrib = attrib

         self._children = []

     def __repr__(self):

         return "<%s %r at %#x>" % (self.__class__.__name__, self.tag, id(self))

     def makeelement(self, tag, attrib):

         创建一个新节点

         """Create a new element with the same type.

         *tag* is a string containing the element name.

         *attrib* is a dictionary containing the element attributes.

         Do not call this method, use the SubElement factory function instead.

         """

         return self.__class__(tag, attrib)

     def copy(self):

         """Return copy of current element.

         This creates a shallow copy. Subelements will be shared with the

         original tree.

         """

         elem = self.makeelement(self.tag, self.attrib)

         elem.text = self.text

         elem.tail = self.tail

         elem[:] = self

         return elem

     def __len__(self):

         return len(self._children)

     def __bool__(self):

         warnings.warn(

             "The behavior of this method will change in future versions.  "

             "Use specific 'len(elem)' or 'elem is not None' test instead.",

             FutureWarning, stacklevel=2

             )

         return len(self._children) != 0 # emulate old behaviour, for now

     def __getitem__(self, index):

         return self._children[index]

     def __setitem__(self, index, element):

         # if isinstance(index, slice):

         #     for elt in element:

         #         assert iselement(elt)

         # else:

         #     assert iselement(element)

         self._children[index] = element

     def __delitem__(self, index):

         del self._children[index]

     def append(self, subelement):

         为当前节点追加一个子节点

         """Add *subelement* to the end of this element.

         The new element will appear in document order after the last existing

         subelement (or directly after the text, if it's the first subelement),

         but before the end tag for this element.

         """

         self._assert_is_element(subelement)

         self._children.append(subelement)

     def extend(self, elements):

         为当前节点扩展 n 个子节点

         """Append subelements from a sequence.

         *elements* is a sequence with zero or more elements.

         """

         for element in elements:

             self._assert_is_element(element)

         self._children.extend(elements)

     def insert(self, index, subelement):

         在当前节点的子节点中插入某个节点，即：为当前节点创建子节点，然后插入指定位置

         """Insert *subelement* at position *index*."""

         self._assert_is_element(subelement)

         self._children.insert(index, subelement)

     def _assert_is_element(self, e):

         # Need to refer to the actual Python implementation, not the

         # shadowing C implementation.

         if not isinstance(e, _Element_Py):

             raise TypeError('expected an Element, not %s' % type(e).__name__)

     def remove(self, subelement):

         在当前节点在子节点中删除某个节点

         """Remove matching subelement.

         Unlike the find methods, this method compares elements based on

         identity, NOT ON tag value or contents.  To remove subelements by

         other means, the easiest way is to use a list comprehension to

         select what elements to keep, and then use slice assignment to update

         the parent element.

         ValueError is raised if a matching element could not be found.

         """

         # assert iselement(element)

         self._children.remove(subelement)

     def getchildren(self):

         获取所有的子节点（废弃）

         """(Deprecated) Return all subelements.

         Elements are returned in document order.

         """

         warnings.warn(

             "This method will be removed in future versions.  "

             "Use 'list(elem)' or iteration over elem instead.",

             DeprecationWarning, stacklevel=2

             )

         return self._children

     def find(self, path, namespaces=None):

         获取第一个寻找到的子节点

         """Find first matching element by tag name or path.

         *path* is a string having either an element tag or an XPath,

         *namespaces* is an optional mapping from namespace prefix to full name.

         Return the first matching element, or None if no element was found.

         """

         return ElementPath.find(self, path, namespaces)

     def findtext(self, path, default=None, namespaces=None):

         获取第一个寻找到的子节点的内容

         """Find text for first matching element by tag name or path.

         *path* is a string having either an element tag or an XPath,

         *default* is the value to return if the element was not found,

         *namespaces* is an optional mapping from namespace prefix to full name.

         Return text content of first matching element, or default value if

         none was found.  Note that if an element is found having no text

         content, the empty string is returned.

         """

         return ElementPath.findtext(self, path, default, namespaces)

     def findall(self, path, namespaces=None):

         获取所有的子节点

         """Find all matching subelements by tag name or path.

         *path* is a string having either an element tag or an XPath,

         *namespaces* is an optional mapping from namespace prefix to full name.

         Returns list containing all matching elements in document order.

         """

         return ElementPath.findall(self, path, namespaces)

     def iterfind(self, path, namespaces=None):

         获取所有指定的节点，并创建一个迭代器（可以被for循环）

         """Find all matching subelements by tag name or path.

         *path* is a string having either an element tag or an XPath,

         *namespaces* is an optional mapping from namespace prefix to full name.

         Return an iterable yielding all matching elements in document order.

         """

         return ElementPath.iterfind(self, path, namespaces)

     def clear(self):

         清空节点

         """Reset element.

         This function removes all subelements, clears all attributes, and sets

         the text and tail attributes to None.

         """

         self.attrib.clear()

         self._children = []

         self.text = self.tail = None

     def get(self, key, default=None):

         获取当前节点的属性值

         """Get element attribute.

         Equivalent to attrib.get, but some implementations may handle this a

         bit more efficiently.  *key* is what attribute to look for, and

         *default* is what to return if the attribute was not found.

         Returns a string containing the attribute value, or the default if

         attribute was not found.

         """

         return self.attrib.get(key, default)

     def set(self, key, value):

         为当前节点设置属性值

         """Set element attribute.

         Equivalent to attrib[key] = value, but some implementations may handle

         this a bit more efficiently.  *key* is what attribute to set, and

         *value* is the attribute value to set it to.

         """

         self.attrib[key] = value

     def keys(self):

         获取当前节点的所有属性的 key

         """Get list of attribute names.

         Names are returned in an arbitrary order, just like an ordinary

         Python dict.  Equivalent to attrib.keys()

         """

         return self.attrib.keys()

     def items(self):

         获取当前节点的所有属性值，每个属性都是一个键值对

         """Get element attributes as a sequence.

         The attributes are returned in arbitrary order.  Equivalent to

         attrib.items().

         Return a list of (name, value) tuples.

         """

         return self.attrib.items()

     def iter(self, tag=None):

         在当前节点的子孙中根据节点名称寻找所有指定的节点，并返回一个迭代器（可以被for循环）。

         """Create tree iterator.

         The iterator loops over the element and all subelements in document

         order, returning all elements with a matching tag.

         If the tree structure is modified during iteration, new or removed

         elements may or may not be included.  To get a stable set, use the

         list() function on the iterator, and loop over the resulting list.

         *tag* is what tags to look for (default is to return all elements)

         Return an iterator containing all the matching elements.

         """

         if tag == "*":

             tag = None

         if tag is None or self.tag == tag:

             yield self

         for e in self._children:

             yield from e.iter(tag)

     # compatibility

     def getiterator(self, tag=None):

         # Change for a DeprecationWarning in 1.4

         warnings.warn(

             "This method will be removed in future versions.  "

             "Use 'elem.iter()' or 'list(elem.iter())' instead.",

             PendingDeprecationWarning, stacklevel=2

         )

         return list(self.iter(tag))

     def itertext(self):

         在当前节点的子孙中根据节点名称寻找所有指定的节点的内容，并返回一个迭代器（可以被for循环）。

         """Create text iterator.

         The iterator loops over the element and all subelements in document

         order, returning all inner text.

         """

         tag = self.tag

         if not isinstance(tag, str) and tag is not None:

             return

         if self.text:

             yield self.text

         for e in self:

             yield from e.itertext()

             if e.tail:

                 yield e.tail