def length(
            cls, request,
            vector: (Ptypes.body,
                     Vector('The vector to analyse.'))) -> [
            (200, 'Ok', Float),
            (400, 'Wrong vector format')]:
        '''Return the modulo of a vector.'''
        log.info('Computing the length of vector {}'.format(vector))
        try:
            Respond(200, sqrt(vector['x'] ** 2 +
                              vector['y'] ** 2 +
                              vector.get('z', 0) ** 2))
        except ValueError:
            Respond(400)
Return the modulo of a vector.