Check if input is in exlusion list before testing it

Author: ablachair

pydra/engine/specs.py

@@ -164,7 +164,8 @@ def check_fields_input_spec(self):
             if getattr(self, fld.name) is attr.NOTHING:
                 if mdata.get("mandatory"):    
                     # checking if the mandatory field is provided elsewhere in the xor list
-                    alreday_populated = [getattr(self, el) for el in mdata["xor"] if (getattr(self, el) is not attr.NOTHING)]
+                    in_exclusion_list = mdata.get("xor") is not None
+                    alreday_populated = in_exclusion_list and [getattr(self, el) for el in mdata["xor"] if (getattr(self, el) is not attr.NOTHING)]
                     if alreday_populated: #another input satisfies mandatory attribute via xor condition
                         continue
                     else: