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Re: Replacing the root element

From: Andrew M. Kuchling <akuchlin@cnri.reston.va.us>
Date: Fri, 23 Apr 1999 12:35:04 -0400 (EDT)
To: "'DOM List'" <www-dom@w3.org>
Message-ID: <14112.40590.429997.960649@amarok.cnri.reston.va.us>
Ronald Bourret writes:
>I just tried replacing the root element with:
>
>a) another element
>b) a fragment containing one element
>c) a fragment containing two elements

	The PyDOM code seems to pass this test; the test program below 
produces the output:

amarok /tmp>python t.py
Adding element
<?xml version="1.0"?>
<root />

Adding 1-element fragment
<?xml version="1.0"?>
<elem1 />

Adding 2-element fragment
Traceback (innermost last):
  File "t.py", line 27, in ?
    doc.replaceChild( frag2, doc.documentElement)
  File "/usr/local/lib/python1.5/site-packages/xml/dom/core.py", line 1314, in replaceChild
    raise HierarchyRequestException, \
xml.dom.core.HierarchyRequestException: replaceChild() would result in more than one root document element

	The Document class overrides the basic Node implementation of
replaceChild(), and counts the number of unique Elements in both the
children of the Document element and the list of nodes that will be
inserted; this number must be equal to 1 or 0.  If the total is OK,
the original Node.replaceChild() can be called.  (Similar logic works
for insertBefore().)  Here's what the code looks like:

    def replaceChild(self, newChild, oldChild):
        """Replaces the child node oldChild with newChild in the list of
        children, and returns the oldChild node. If the newChild is
        already in the tree, it is first removed."""

	# Check that this operation wouldn't result in the Document node
	# having more than one children that are Element nodes.
	# This is as in insertBefore, with one change; if the old node being
	# replaced is an element, it shouldn't be counted.
	if newChild._node.type == DOCUMENT_FRAGMENT_NODE:
	    nodelist = newChild._node.children
	else:
	    nodelist = [newChild._node]

	d = {}                         # Dictionary for counting
	for c in nodelist:
            if c.type == ELEMENT_NODE: d[ id(c) ] = None
        for c in self._node.children:
            if c.type == ELEMENT_NODE: d[ id(c) ] = None

	# For a correct count, we should not count the oldChild node, in case
	# it's an Element.
	ocn = oldChild._node
        if ocn.type == ELEMENT_NODE and d.has_key( id(ocn) ):
	    del d[ id(ocn) ]

	if len(d) > 1:
	    raise HierarchyRequestException, \
              "replaceChild() would result in more than one root document element" 

	# Call original version of replaceChild()
	Node.replaceChild(self, newChild, oldChild)


-- 
A.M. Kuchling			http://starship.python.net/crew/amk/
This of course is nothing like the comic book Enigma. This is real life, or as
real as your life gets, anyway.
    -- Violence, in ENIGMA #5: "Lizards and Ghosts"


# Test program for adding DocFragments to the root element
from xml.dom import core

doc = core.createDocument()

# Make single element, 1-element fragment, 2-element fragment
elem = doc.createElement("root")

frag1 = doc.createDocumentFragment()
frag1.appendChild( doc.createElement("elem1") )

frag2 = doc.createDocumentFragment()
frag2.appendChild( doc.createElement("elem1") )
frag2.appendChild( doc.createElement("elem2") )

print 'Adding element'
doc.appendChild( elem )  # Should succeed
print doc.toxml()

# Replace the root element with the 1-element fragment
print '\nAdding 1-element fragment'
doc.replaceChild( frag1, doc.documentElement)
print doc.toxml()

# Replace the root element with the 2-element fragment
print '\nAdding 2-element fragment'
doc.replaceChild( frag2, doc.documentElement)
print doc.toxml()
Received on Friday, 23 April 1999 12:35:16 GMT

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