code: def getFilename(self,signature):
          
  
          filename=constructFilename(signature)
  
          if(fileutil.checkFileExists(filename)):
              return filename
          else:
              print("\nmMask file for ", str(signature), " does not exist on disk", file=sys.stderr)
              return None
docs: Returns the name of the output mask file that
          should reside on disk for the given signature.