restore backwards compatibility in BuildRawModel function

As of 3.1.0, BuildRawModel only accepts a list of strings as chain
names if the alignments have been passed as AlignmentList. This
commit restores the behaviour that we can also pass a string where
each chain gets named according to the single letters in that string.

modelling/pymod/_raw_model.py

@@ -77,8 +77,11 @@ def BuildRawModel(aln, chain_names = None, include_ligands = False,
                           If *aln* is of type :class:`ost.seq.AlignmentHandle`, 
                           *chain_names* is expected to be a :class:`str`.
                           If *aln* is of type :class:`ost.seq.AlignmentList`,
-                          chain_names is expected to be a :class:`list` of 
-                          :class:`str` of same size as *aln*. 
+                          *chain_names* is expected to be a :class:`list` of 
+                          :class:`str` of same size as *aln* or a :class:`str`.
+                          For the latter case, chains will consecutively named
+                          according to characters in *chain_names*.
+ 
       :type chain_names:  :class:`str` / :class:`list`
     
       :param spdbv_style: True, if we need a model in the old SPDBV style.
@@ -130,9 +133,15 @@ def BuildRawModel(aln, chain_names = None, include_ligands = False,
                 raise RuntimeError('Number of alns and chain_names must be '\
                                    'consistent')
             name_list = chain_names
+        elif isinstance(chain_names, str):
+            if len(chain_names) < len(aln_list):
+                raise RuntimeError('If you provide a string as chain_names, '\
+                                   'it must be at least as long as the '\
+                                   'AlignmentList in aln.')
+            name_list = [n for n in chain_names[:len(aln_list)]]
         else:
-            raise RuntimeError('chain_names must be list if aln is of type '\
-                               'ost.seq.AlignmentList') 
+            raise RuntimeError('chain_names must be list of str or str if aln '\
+                               'is of type ost.seq.AlignmentList') 
     else:
         raise RuntimeError('aln must be of type ost.seq.AlignmentHandle or '\
                            'ost.seq.AlignmentList')

modelling/tests/test_modelling.py

@@ -76,24 +76,28 @@ class ModellingTests(unittest.TestCase):
         result = modelling.BuildRawModel(aln_lst, ['cheese', 'steak'])
         self.assertEqual(result.model.chains[0].GetName(), 'cheese')
         self.assertEqual(result.model.chains[1].GetName(), 'steak')
+        result = modelling.BuildRawModel(aln_lst, 'ch')
+        self.assertEqual(result.model.chains[0].GetName(), 'c')
+        self.assertEqual(result.model.chains[1].GetName(), 'h')
+        result = modelling.BuildRawModel(aln_lst, 'cheese')
+        self.assertEqual(result.model.chains[0].GetName(), 'c')
+        self.assertEqual(result.model.chains[1].GetName(), 'h')
 
         # we only accept a string as chain_names if aln is AlignmentHandle
         self.assertRaises(RuntimeError, modelling.BuildRawModel, aln, 1)
         self.assertRaises(RuntimeError, modelling.BuildRawModel, aln, ['A'])
 
-        # we only accept a list as chain_names if aln is AlignmentList
+        # we only accept a list or str as chain_names if aln is AlignmentList
         self.assertRaises(RuntimeError, modelling.BuildRawModel, aln_lst, 1)
-        self.assertRaises(RuntimeError, modelling.BuildRawModel, aln_lst, 'A')
         # size also matters...
         self.assertRaises(RuntimeError, modelling.BuildRawModel, aln_lst, ['A'])
+        self.assertRaises(RuntimeError, modelling.BuildRawModel, aln_lst, 'A')
 
         # increase size of aln_list => at some point we should run out of 
         # default chain_names
         aln_lst = 100*[aln]
         self.assertRaises(RuntimeError, modelling.BuildRawModel, aln_lst)
 
-
-
     def testModeledSequence(self):
         # test if the model has the sequence we want.
         tpl = io.LoadPDB('data/gly.pdb')