read all ffxmls together by section, rather than all sections in one file first

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

@@ -120,21 +120,27 @@ class ForceField(object):
         self._forces = []
         self._scripts = []
         self._templateGenerators = []
-        for file in files:
-            self.loadFile(file)
+        self.loadFile(files)
 
-    def loadFile(self, file):
+    def loadFile(self, files):
         """Load an XML file and add the definitions from it to this ForceField.
 
         Parameters
         ----------
-        file : string or file
-            An XML file containing force field definitions.  It may be either an
-            absolute file path, a path relative to the current working
+        files : string or file or tuple
+            An XML file or tuple of XML files containing force field definitions.
+            Each entry may be either an absolute file path, a path relative to the current working
             directory, a path relative to this module's data subdirectory (for
             built in force fields), or an open file-like object with a read()
             method from which the forcefield XML data can be loaded.
         """
+
+        if not isinstance(files, tuple):
+            files = (files,)
+
+        trees = []
+
+        for file in files:
             try:
                 # this handles either filenames or open file-like objects
                 tree = etree.parse(file)
@@ -152,18 +158,21 @@ class ForceField(object):
                 msg += "ForceField.loadFile() encountered an error reading file '%s'\n" % filename
                 raise Exception(msg)
 
-        root = tree.getroot()
+            trees.append(tree)
+
 
         # Load the atom types.
 
+        for tree in trees:
             if tree.getroot().find('AtomTypes') is not None:
                 for type in tree.getroot().find('AtomTypes').findall('Type'):
                     self.registerAtomType(type.attrib)
 
         # Load the residue templates.
 
+        for tree in trees:
             if tree.getroot().find('Residues') is not None:
-            for residue in root.find('Residues').findall('Residue'):
+                for residue in tree.getroot().find('Residues').findall('Residue'):
                     resName = residue.attrib['name']
                     template = ForceField._TemplateData(resName)
                     atomIndices = {}
@@ -194,12 +203,14 @@ class ForceField(object):
 
         # Load force definitions
 
-        for child in root:
+        for tree in trees:
+            for child in tree.getroot():
                 if child.tag in parsers:
                     parsers[child.tag](child, self)
 
         # Load scripts
 
+        for tree in trees:
             for node in tree.getroot().findall('Script'):
                 self.registerScript(node.text)