Fix returns for simple Python functions for optional ws. Refs #5701

Also add two extra tests for optional workspace properties.

Code/Mantid/Framework/PythonInterface/inc/MantidPythonInterface/api/WorkspacePropertyMacro.h

@@ -125,6 +125,7 @@ namespace Mantid
           .def("__init__", make_constructor(&createPropertyWithLockFlag,
                                             default_call_policies(),
                                             args("name","defaultValue", "direction", "optional", "locking", "validator")))
+          .def("isOptional", &TypedWorkspaceProperty::isOptional, "Returns true if the property has been marked as optional")
        ;
 
       }

Code/Mantid/Framework/PythonInterface/mantid/simpleapi.py

@@ -386,19 +386,27 @@ def ignore_property(name, ignore_regex):
         # Parent algorithms store their workspaces in the ADS
         # Child algorithms store their workspaces in the property
         if not algm_obj.isChild() and _is_workspace_property(prop):
-            retvals.append(_ads[prop.valueAsStr])
+            value = prop.valueAsStr
+            if value == '':
+                if not prop.isOptional():
+                    raise RuntimeError("Logical error. Output workspace property '%s' on '%s' is mandatory but has no value."
+                                       "Please contact development team." % (name,  algm_obj.name()))
+            else:
+                retvals.append(_ads[value])
         else:
             if not hasattr(prop, 'value'):
                 print ('Unknown property type encountered. "%s" on "%s" is not understood by '
                        'Python. Return value skipped. Please contact development team' % (name, algm_obj.name()))
             else:
                 retvals.append(prop.value)
+
     nvals = len(retvals)
     nlhs = lhs[0]
     if nlhs > 1 and nvals != nlhs:
         # There is a discrepancy in the number are unpacking variables
         # Let's not have the more cryptic unpacking error raised
-        raise RuntimeError("%s is trying to return %d output(s) but you have provided %d variable(s). These numbers must match." % (func_name, nvals, nlhs))
+        raise RuntimeError("%s is trying to return %d output(s) but you have provided %d variable(s). "
+                           "These numbers must match." % (func_name, nvals, nlhs))
     if nvals > 1:
         return tuple(retvals) # Create a tuple
     elif nvals == 1:

Code/Mantid/Framework/PythonInterface/test/python/SimpleAPITest.py

@@ -1,6 +1,6 @@
 import unittest
 import sys
-from mantid.api import AlgorithmFactory, mtd
+from mantid.api import AlgorithmFactory, mtd, ITableWorkspace
 import mantid.simpleapi as simpleapi
 
 class SimpleAPITest(unittest.TestCase):
@@ -98,6 +98,34 @@ def test_function_uses_OutputWorkspace_keyword_over_lhs_var_name_if_provided(sel
         wkspace = simpleapi.CreateWorkspace(data,data,OutputWorkspace=wsname,NSpec=1,UnitX='Wavelength')
         self.assertTrue( wsname in mtd )
 
+    def test_function_returns_only_mandatory_workspace_when_optional_output_is_not_given(self):
+        _demo = simpleapi.CreateMDHistoWorkspace(SignalInput='1,2,3,4,5',ErrorInput='1,1,1,1,1',
+                                                Dimensionality='1',Extents='-1,1',NumberOfBins='5',Names='A',Units='U')
+        wsname = 'test_function_returns_only_mandatory_workspace_when_optional_output_is_not_given'
+        query = simpleapi.QueryMDWorkspace(InputWorkspace=_demo,OutputWorkspace=wsname,MaximumRows='500',
+                                           Normalisation='volume')
+
+        self.assertTrue( isinstance(query, ITableWorkspace) )
+        self.assertTrue( wsname in mtd )
+
+    def test_function_returns_both_mandatory_and_optional_workspaces_when_optional_output_is_given(self):
+        _demo = simpleapi.CreateMDWorkspace(Dimensions='2',EventType='MDEvent',Extents='1,10,1,10',Names='a,b',
+                                            Units='MomentumTransfer,MomentumTransfer',SplitInto='4')
+        wsname = 'test_function_returns_only_mandatory_workspace_when_optional_output_is_not_given'
+        wsname_box = wsname + '_box'
+        query = simpleapi.QueryMDWorkspace(InputWorkspace=_demo,OutputWorkspace=wsname,MaximumRows='500',
+                                           Normalisation='volume',BoxDataTable=wsname_box)
+
+        self.assertTrue( wsname in mtd )
+        self.assertTrue( wsname_box in mtd )
+
+        self.assertTrue( type(query) == tuple )
+        self.assertEquals( 2, len(query) )
+
+        self.assertTrue( isinstance(query[0], ITableWorkspace) )
+        self.assertTrue( isinstance(query[1], ITableWorkspace) )
+
+
     def test_that_dialog_call_raises_runtime_error(self):
         try:
             simpleapi.LoadEventNexusDialog()