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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 url(self, request):
        '''
            Return absolute URL for assignment description.
        '''
        if self.pk:
            if self.has_description():
                return request.build_absolute_uri(reverse('assignment_description_file', args=[self.pk]))
            else:
                return self.download
        else:
            return None