code: def is_unique(keys, *, scope=None, comparison_operators=None, present_optional=False,
                  message=None):
      
      def check(pname, validator):
        """Checks that a value is unique in its column, with an optional scope."""
        # pylint: disable=too-many-branches
        model = validator.model
        data_access = validator.data_access
        pkname = model.primary_key_name
        pkey = model.primary_key
  
        if isinstance(keys, str):
          key = getattr(model, keys)
  
          if present_optional and key is None:
            return True
  
          if comparison_operators:
            if isinstance(comparison_operators, str):
              op = comparison_operators
            else:
              op = comparison_operators[0]
          else:
            op = " = "
  
          constraints = [(keys, key, op)]
        else:
          if comparison_operators:
            ops = comparison_operators
          else:
            ops = [" = "] * len(keys)
  
          constraints = list(zip(keys, [getattr(model, key) for key in keys], ops))
  
        if scope:
          if comparison_operators:
            ops = comparison_operators[len(constraints):]
          else:
            ops = [" = "] * len(scope)
  
          constraints.extend(zip(scope, [getattr(model, col) for col in scope], ops))
  
        dupe = data_access.find(model.table_name, constraints, columns=pkname)
  
        if dupe is None:
          return True
        if isinstance(pkname, str):
          return dupe[0] == pkey
        else:
          return tuple(dupe) == tuple(pkey)
  
      return Validation(check, keys, message or "is already taken", is_simple=False)
docs: Returns a Validation that makes sure the given value is unique for a table and optionally a
      scope.