def _format_value(self, operation, key, indent):
        """A value that exists in the operation but has value None is displayed.

        A value that does not exist in the operation is left out entirely. The value
        name in the operation must match the value name in the template, but the
        location does not have to match.

        """
        v = self._find_value(operation, key)
        if v == "NOT_FOUND":
            return []
        if not isinstance(v, list):
            v = [v]
        if not len(v):
            v = [None]
        key = key + ":"
        lines = []
        for s in v:
            # Access control rules are stored in tuples.
            if isinstance(s, tuple):
                s = "{}: {}".format(*s)
            lines.append(
                "{}{}{}{}".format(
                    " " * indent, key, " " * (TAB - indent - len(key) - 1), s
                )
            )
            key = ""
        return lines
A value that exists in the operation but has value None is displayed.

        A value that does not exist in the operation is left out entirely. The value
        name in the operation must match the value name in the template, but the
        location does not have to match.