The other day i was processing an xml tree using lxml.etree. That tree
contained a namespace. During that processing i inserted an element.
Later on
i tried to find that element using xpath. And to my suprise that
element was
not found! Maybe my suprise is just the result of my marginal
knowledge about
xml namespaces.
After some examination i found out that the reason for not finding the
element
was that i did not supply a namespace when inserting the element.
This behavior can be reproduced be the folowing code:
<code>
from lxml import etree
import sys
print "Python: ", sys.version_info
print "lxml.etree:", etree.__version__
string_data = [
'<root xmlns="NameSpace.com"><sometag/></root>',
'<root xmlns="NameSpace.com"></root>',
'<root xmlns="NameSpace.com"></root>'
]
trees = map(etree.fromstring, string_data)
print "\n Before insertion:"
for t in trees:
print etree.tostring(t, pretty_print=True)
trees[1].insert(-1, etree.Element("sometag"))
trees[2].insert(-1, etree.Element("{NameSpace.com}sometag",
nsmap={None : "NameSpace.com"}))
print "\n After insertion:"
for t in trees:
print etree.tostring(t, pretty_print=True)
print "\n Using xpath:"
for t in trees:
elements = t.xpath("//ns:sometag", namespaces={'ns':
'NameSpace.com'})
print len(elements),
if elements:
print [e.tag for e in elements]
else:
print elements
</code>
Its output is:
<output>
Python: (2, 6, 6, 'final', 0)
lxml.etree: 2.2.8
Before insertion:
<root xmlns="NameSpace.com">
<sometag/>
</root>
<root xmlns="NameSpace.com"/>
<root xmlns="NameSpace.com"/>
After insertion:
<root xmlns="NameSpace.com">
<sometag/>
</root>
<root xmlns="NameSpace.com">
<sometag/>
</root>
<root xmlns="NameSpace.com">
<sometag/>
</root>
Using xpath:
1 ['{NameSpace.com}sometag']
0 []
1 ['{NameSpace.com}sometag']
</output>
So my suprise was that in the second case the xpath result is an empty
list.
I have two questions on this:
- Is what i am seeing expected behavior?
- How am i supposed to detect a missing namespace, if there are no
differences
in the serialized representation? (That's what i initially used to
debug the
problem.)
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