Add more informative error message when loading ffxml file fails.

wrappers/python/simtk/openmm/app/forcefield.py

@@ -124,7 +124,7 @@ class ForceField(object):
             self.loadFile(file)
 
     def loadFile(self, file):
-        """Load an XML file and add the definitions from it to this FieldField.
+        """Load an XML file and add the definitions from it to this ForceField.
 
         Parameters
         ----------
@@ -140,6 +140,18 @@ class ForceField(object):
             tree = etree.parse(file)
         except IOError:
             tree = etree.parse(os.path.join(os.path.dirname(__file__), 'data', file))
+        except Exception as e:
+            # Fail with a more useful error message about which file could not be read.
+            # TODO: Also handle case where fallback to 'data' directory encounters problems,
+            # but this is much less worrisome because we control those files.
+            msg  = str(e) + '\n'
+            if hasattr(file, 'close'):
+                filename = file.name
+            else:
+                filename = file
+            msg += 'ForceField.loadFile() encountered an error reading file: %s' % filename
+            raise Exception(msg)
+
         root = tree.getroot()
 
         # Load the atom types.

wrappers/python/tests/TestForceField.py

@@ -293,20 +293,20 @@ class TestForceField(unittest.TestCase):
         # Define forcefield parameters used by simpleTemplateGenerator.
         # NOTE: This parameter definition file will currently only work for residues that either have
         # no external bonds or external bonds to other residues parameterized by the simpleTemplateGenerator.
-        simple_ffxml_contents = """
+        simple_ffxml_contents = """\
 <ForceField>
- <AtomTypes>
-  <Type name="XXX" class="XXX" element="C" mass="12"/>
- </AtomTypes>
- <HarmonicBondForce>
-  <Bond type1="XXX" type2="XXX" length="0.1409" k="392459.2"/>
- </HarmonicBondForce>
- <HarmonicAngleForce>
-  <Angle type1="XXX" type2="XXX" type3="XXX" angle="2.09439510239" k="527.184"/>
- </HarmonicAngleForce>
- <NonbondedForce coulomb14scale="0.833333" lj14scale="0.5">
-  <Atom type="XXX" charge="0.000" sigma="0.315" epsilon="0.635"/>
- </NonbondedForce>
+<AtomTypes>
+ <Type name="XXX" class="XXX" element="C" mass="12"/>
+</AtomTypes>
+<HarmonicBondForce>
+ <Bond type1="XXX" type2="XXX" length="0.1409" k="392459.2"/>
+</HarmonicBondForce>
+<HarmonicAngleForce>
+ <Angle type1="XXX" type2="XXX" type3="XXX" angle="2.09439510239" k="527.184"/>
+</HarmonicAngleForce>
+<NonbondedForce coulomb14scale="0.833333" lj14scale="0.5">
+ <Atom type="XXX" charge="0.000" sigma="0.315" epsilon="0.635"/>
+</NonbondedForce>
 </ForceField>"""
         simple_ffxml = StringIO(simple_ffxml_contents)