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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 get_success_url(self):
 """ Returns redirect URL for valid form submittal. :rtype: str. """

        if self.success_url:
            url = force_text(self.success_url)
        else:
            url = reverse('{0}:index'.format(self.url_namespace))

        return url