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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 run(self, x=None):
 r""" Builds the A and b matrices, and calls the solver specified in the ``settings`` attribute. Parameters x : ND-array Initial guess of unknown variable """

        logger.info('Running ReactiveTransport')
        # Create S1 & S1 for 1st Picard's iteration
        if x is None:
            x = np.zeros(shape=[self.Np, ], dtype=float)
        self[self.settings['quantity']] = x
        self._update_physics()

        x = self._run_reactive(x=x)
        self[self.settings['quantity']] = x