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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 invoke_function(self, function_name, **function_params):
 """ Invokes an Excel Function """

        url = self.build_url(self._endpoints.get('function').format(function_name))
        response = self.session.post(url, data=function_params)
        if not response:
            return None
        data = response.json()

        error = data.get('error')
        if error is None:
            return data.get('value')
        else:
            raise FunctionException(error)