handle xsl transform that returns an empty result

CHANGELOG

@@ -11,6 +11,14 @@ any necessary information about installation or upgrade notes.
 
 * Corrected a minor bug where schema validation errors were not cleared between
   multiple validations.
+* To avoid permission denied warning for auto-generated parser files,
+  parsetab files are now created in python tempdir if the running user
+  doesn't have write permission in the package installation directory.
+  [`Issue 1 <https://github.com/emory-libraries/eulxml/issues/1>`_]
+* When an XSLT transformation results in an empty document,
+  :meth:`eulxml.xmlap.XmlObject.xsl_transform` now returns None.
+  [`Issue 6 <https://github.com/emory-libraries/eulxml/issues/6>`_]
+*
 
 
 0.18.0 - Formset Ordering and DateTime

eulxml/xmlmap/core.py

@@ -23,12 +23,12 @@
 from lxml import etree
 from lxml.builder import ElementMaker
 
-from eulxml.xmlmap.fields import Field, NodeList
+from eulxml.xmlmap.fields import Field
 
 logger = logging.getLogger(__name__)
 
-__all__ = [ 'XmlObject', 'parseUri', 'parseString', 'loadSchema',
-    'load_xmlobject_from_string', 'load_xmlobject_from_file' ]
+__all__ = ['XmlObject', 'parseUri', 'parseString', 'loadSchema',
+    'load_xmlobject_from_string', 'load_xmlobject_from_file']
 
 # NB: When parsing XML in this module, we explicitly create a new parser
 #   each time. Without this, lxml 2.2.7 uses a global default parser. When
@@ -374,7 +374,15 @@ def xsl_transform(self, filename=None, xsl=None, return_type=None, **params):
         # - to output xml result, use serialize instead of unicode
         if return_type is None:
             return_type = XmlObject
-        return return_type(transform(self.node))
+
+        result = transform(self.node)
+        # If XSLT returns nothing, transform returns an _XSLTResultTree
+        # with no root node.  Log a warning, and don't generate an
+        # empty xmlobject which will behave unexpectedly.
+        if result is None or result.getroot() is None:
+            logger.warning("XSL transform generated an empty result")
+        else:
+            return return_type(result)
 
     def __unicode__(self):
         if isinstance(self.node, basestring):

test/test_xmlmap/test_core.py

@@ -63,6 +63,12 @@ class TestXsl(unittest.TestCase):
     </xsl:stylesheet>
     '''
 
+    EMPTY_RESULT_XSL = '''
+    <xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0">
+        <xsl:template match="/" />
+    </xsl:stylesheet>
+    '''
+
     def setUp(self):
         # parseString wants a url. let's give it a proper one.
         url = '%s#%s.%s' % (__file__, self.__class__.__name__, 'FIXTURE_TEXT')
@@ -89,6 +95,12 @@ class TestObject(xmlmap.XmlObject):
         self.assertEqual('42', newobj.nobar_baz)
         self.assertEqual(None, newobj.bar_baz)
 
+        # empty result
+        obj = TestObject(self.fixture)
+        self.assertEqual(None, obj.xsl_transform(xsl=self.EMPTY_RESULT_XSL,
+            return_type=TestObject),
+            'xsl transform should return None for an empty XSLT result')
+
         self.FILE.close()
         # not yet tested: xsl with parameters