code: def input_variables(self, exclude_specials=True):
          
  
          def has_write_access(accesses):
              return any(acc for acc in accesses if acc.access_type == 'write')
  
          def has_read_access(accesses):
              return any(acc for acc in accesses if acc.access_type == 'read')
  
          input_variables = [ ]
  
          for variable, accesses in self._variable_accesses.items():
              if not has_write_access(accesses) and has_read_access(accesses):
                  if not exclude_specials or not variable.category:
                      input_variables.append(variable)
  
          return input_variables
docs: Get all variables that have never been written to.
  
          :return: A list of variables that are never written to.