Validate a potential list.

def Validate(self, value):
    """Validate a potential list."""
    if isinstance(value, string_types):
      raise TypeValueError("Value must be an iterable not a string.")

    elif not isinstance(value, (list, tuple)):
      raise TypeValueError("%r not a valid List" % value)

    # Validate each value in the list validates against our type.
    return [self.validator.Validate(val) for val in value]