code: def normalize_constraints(constraints, flds):
      
      local_constraints = constraints or []
      local_constraints = [dict(**c) for c in local_constraints]
      local_constraints = [
          c for c in local_constraints
          if c.get('field') in flds or
          not c.get('optional')
      ]
      return local_constraints
docs: This method renders local constraints such that return value is:
        * a list, not None
        * a list of dicts
        * a list of non-optional constraints or optional with defined field
  
      .. note:: We use a new variable 'local_constraints' because the constraints
                parameter may be a mutable collection, and we do not wish to
                cause side-effects by modifying it locally