Make distinct parameter dictionaries for _TemplateAtomData by default (#5091)

wrappers/python/openmm/app/forcefield.py

@@ -744,11 +744,11 @@ class ForceField(object):
 
     class _TemplateAtomData(object):
         """Inner class used to encapsulate data about an atom in a residue template definition."""
-        def __init__(self, name, type, element, parameters={}):
+        def __init__(self, name, type, element, parameters=None):
             self.name = name
             self.type = type
             self.element = element
-            self.parameters = parameters
+            self.parameters = {} if parameters is None else parameters
             self.bondedTo = []
             self.externalBonds = 0
 

wrappers/python/tests/TestForceField.py

@@ -804,6 +804,9 @@ class TestForceField(unittest.TestCase):
         forcefield = ForceField('amber99sb.xml', 'tip3p.xml', StringIO(simple_ffxml_contents))
         # Get list of unique unmatched residues.
         [templates, residues] = forcefield.generateTemplatesForUnmatchedResidues(pdb.topology)
+        # Make sure template atom parameter dictionaries are distinct objects.
+        parameters = [atom.parameters for template in templates for atom in template.atoms]
+        self.assertEqual(len(set(map(id, parameters))), len(parameters))
         # Add residue templates to forcefield.
         for template in templates:
             # Replace atom types.