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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def render_string(self, source: str, **vars) -> str: """ Render the template contained in the given string. The current context will be available to the template as the ``ctx`` variable. :param source: content of the template to render :param vars: extra variables made available to the template :return: the rendered results """

        vars.setdefault('ctx', self._ctx)
        return self._renderer.render_string(source, **vars)