Re: [lxml] Isoschematron.Schematron not working as expected

Feb. 25, 2015

      Hi Pierpaolo,
...
Finally I went back to a very simple example included in
isoschematron.Schematron docstring:
...
...
...
from lxml import isoschematron
schematron = isoschematron.Schematron(etree.XML('''
... <schema xmlns="http://purl.oclc.org/dsdl/schematron" >
...   <pattern id="id_only_attribute">
...     <title>id is the only permitted attribute name</title>
...     <rule context="*">
...       <report test="@*[not(name()='id')]">Attribute
...         <name path="@*[not(name()='id')]"/> is forbidden<name/>
...       </report>
...     </rule>
...   </pattern>
... </schema>
... '''))
...
...
...
xml = etree.XML('''
... <AAA name="aaa">
...   <BBB id="bbb"/>
...   <CCC color="ccc"/>
... </AAA>
... ''')
...
...
...
schematron.validate(xml)
0
...
...
...
xml = etree.XML('''
... <AAA id="aaa">
...   <BBB id="bbb"/>
...   <CCC/>
... </AAA>
... ''')
...
...
...
schematron.validate(xml)
1
Now if I run the above code I always get True, even with the invalid
xml input. Same situation: validation_report is correct, but return
value is True and error_log is empty.
I'm running python 3.4.1 with lxml 3.5dev0. Same result with lxml 3.4.2.
Am I doing something horribly wrong without realizing or is there
actually a bug here?
It's definitely a bug in the documentation, i.e. the docstring is wrong,
probably since the dawn of lxml isoschematron times
(and I wonder why this isn't noticed by doctest...).

Anyway: The current implementation only counts failed asserts as errors,
not reports that have been triggered. This XPath is used to look for error
elements in the svrl result report:

isoschematron/__init__.py:
[...]
# svrl result accessors
svrl_validation_errors = _etree.XPath(
'//svrl:failed-assert', namespaces={'svrl': SVRL_NS})
[...]

I *think* the rationale was that reports should be usable additionally
to the validation results, but I'd need to look a bit more into this. It's
been
a while.

That said you can easily change behaviour by using a custom schematron
validator class.

Inheriting from class Schematron and overriding the _validation_errors
class attribute
in your subclass with an etree.XPath object that detects whatever you deem
an error
in the result validation report document should do the trick:

class Schematron(_etree._Validator):

[...]
# etree.XPath object that determines input document validity when applied
to
# the svrl result report; must return a list of result elements (empty if
# valid)
_validation_errors = svrl_validation_errors

There's a customization example in src/lxml/tests/test_isoschematron.py,
too.

Holger

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Re: [lxml] Isoschematron.Schematron not working as expected

Holger Joukl