code: def _validate_label(cls, name, value):
      
      # Rules for labels are described in:
      #  https://cloud.google.com/compute/docs/labeling-resources#restrictions
  
      # * Keys and values cannot be longer than 63 characters each.
      # * Keys and values can only contain lowercase letters, numeric characters,
      #   underscores, and dashes.
      # * International characters are allowed.
      # * Label keys must start with a lowercase letter and international
      #   characters are allowed.
      # * Label keys cannot be empty.
      cls._check_label_name(name)
      cls._check_label_value(value)
  
      # Ensure that reserved labels are not being used.
      if not cls._allow_reserved_keys and name in RESERVED_LABELS:
        raise ValueError('Label flag (%s=...) must not use reserved keys: %r' %
                         (name, list(RESERVED_LABELS)))
docs: Raise ValueError if the label is invalid.