20.8. "xml.dom.pulldom" --- Support for building partial DOM trees
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**Source code:** Lib/xml/dom/pulldom.py

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

The "xml.dom.pulldom" module provides a "pull parser" which can also
be asked to produce DOM-accessible fragments of the document where
necessary. The basic concept involves pulling "events" from a stream
of incoming XML and processing them. In contrast to SAX which also
employs an event-driven processing model together with callbacks, the
user of a pull parser is responsible for explicitly pulling events
from the stream, looping over those events until either processing is
finished or an error condition occurs.

警告: The "xml.dom.pulldom" module is not secure against maliciously
  constructed data.  If you need to parse untrusted or unauthenticated
  data see XML 漏洞.

3.6.7 版更變: The SAX parser no longer processes general external
entities by default to increase security by default. To enable
processing of external entities, pass a custom parser instance in:

   from xml.dom.pulldom import parse
   from xml.sax import make_parser
   from xml.sax.handler import feature_external_ges

   parser = make_parser()
   parser.setFeature(feature_external_ges, True)
   parse(filename, parser=parser)

示例:

   from xml.dom import pulldom

   doc = pulldom.parse('sales_items.xml')
   for event, node in doc:
       if event == pulldom.START_ELEMENT and node.tagName == 'item':
           if int(node.getAttribute('price')) > 50:
               doc.expandNode(node)
               print(node.toxml())

"event" is a constant and can be one of:

* "START_ELEMENT"

* "END_ELEMENT"

* "COMMENT"

* "START_DOCUMENT"

* "END_DOCUMENT"

* "CHARACTERS"

* "PROCESSING_INSTRUCTION"

* "IGNORABLE_WHITESPACE"

"node" is an object of type "xml.dom.minidom.Document",
"xml.dom.minidom.Element" or "xml.dom.minidom.Text".

Since the document is treated as a "flat" stream of events, the
document "tree" is implicitly traversed and the desired elements are
found regardless of their depth in the tree. In other words, one does
not need to consider hierarchical issues such as recursive searching
of the document nodes, although if the context of elements were
important, one would either need to maintain some context-related
state (i.e. remembering where one is in the document at any given
point) or to make use of the "DOMEventStream.expandNode()" method and
switch to DOM-related processing.

class xml.dom.pulldom.PullDom(documentFactory=None)

   Subclass of "xml.sax.handler.ContentHandler".

class xml.dom.pulldom.SAX2DOM(documentFactory=None)

   Subclass of "xml.sax.handler.ContentHandler".

xml.dom.pulldom.parse(stream_or_string, parser=None, bufsize=None)

   Return a "DOMEventStream" from the given input. *stream_or_string*
   may be either a file name, or a file-like object. *parser*, if
   given, must be an "XMLReader" object. This function will change the
   document handler of the parser and activate namespace support;
   other parser configuration (like setting an entity resolver) must
   have been done in advance.

If you have XML in a string, you can use the "parseString()" function
instead:

xml.dom.pulldom.parseString(string, parser=None)

   Return a "DOMEventStream" that represents the (Unicode) *string*.

xml.dom.pulldom.default_bufsize

   Default value for the *bufsize* parameter to "parse()".

   The value of this variable can be changed before calling "parse()"
   and the new value will take effect.


20.8.1. DOMEventStream Objects
==============================

class xml.dom.pulldom.DOMEventStream(stream, parser, bufsize)

   getEvent()

      Return a tuple containing *event* and the current *node* as
      "xml.dom.minidom.Document" if event equals "START_DOCUMENT",
      "xml.dom.minidom.Element" if event equals "START_ELEMENT" or
      "END_ELEMENT" or "xml.dom.minidom.Text" if event equals
      "CHARACTERS". The current node does not contain information
      about its children, unless "expandNode()" is called.

   expandNode(node)

      Expands all children of *node* into *node*. Example:

         from xml.dom import pulldom

         xml = '<html><title>Foo</title> <p>Some text <div>and more</div></p> </html>'
         doc = pulldom.parseString(xml)
         for event, node in doc:
             if event == pulldom.START_ELEMENT and node.tagName == 'p':
                 # Following statement only prints '<p/>'
                 print(node.toxml())
                 doc.expandNode(node)
                 # Following statement prints node with all its children '<p>Some text <div>and more</div></p>'
                 print(node.toxml())

   reset()
