python之类的赋值及默认值报错：TypeError: object of type 'int' has no len()

在做分子热运动模拟时发现的问题：

import matplotlib.pyplot as plt
from random import choice as choice

class Molecular_thermal_motion():
    def __init__(self,x_values=[0],y_values=[0],lentgh_move=5000):
        self.x_values = x_values   #为什么这里只能写[0]？？？
        self.y_values = y_values   #同上
        self.lentgh_move = lentgh_move #为什么这里又可以了？

    def molecular_move(self):
        while len(self.x_values) < self.lentgh_move:
            x_direction = choice([-1,1])
            x_length = choice([0,2,4,6,8])
            x_step = x_direction * x_length
            y_direction = choice([-1,1])
            y_length = choice(