def before_get(self, parser, section, option, value, vars):
        rawval = value
        depth = MAX_INTERPOLATION_DEPTH
        while depth:                    # Loop through this until it's done
            depth -= 1
            if value and "%(" in value:
                replace = functools.partial(self._interpolation_replace,
                                            parser=parser)
                value = self._KEYCRE.sub(replace, value)
                try:
                    value = value % vars
                except KeyError as e:
                    raise InterpolationMissingOptionError(
                        option, section, rawval, e.args[0])
            else:
                break
        if value and "%(" in value:
            raise InterpolationDepthError(option, section, rawval)
        return value