Animation in matplotlib keeps crashing when trying to work with random values within the update function

So the idea is that I want to reposition the inner point of a circle randomly after each animation (so after the 128th frame in this case). Since Repeat=True, this should yield an ongoing animation where new circles keep appearing randomly after the other ones disappeared (due to my choice of the gist_gray colormap). So I thought doing something like

if i == 0:
   x = randint(-2, 2)
   y = randint(-2, 2)

at the beginning of my update function should work, but the animation just keeps crashing. It works when I initalise the variables outside of the update function though. There might be an error concerning the start of the animation, because when I put print(i) at the beginning of the update function, it gives

0
0
0
1
2

so for one reason or another, FuncAnimation seems to work with three zeros at the beginning. Does anybody have an idea how to get around that? The whole code:

import matplotlib.pyplot as plt
from matplotlib.animation import FuncAnimation
import numpy as np
from random import randint

colors = plt.cm.gist_gray(np.linspace(0, 1, 256))

fig, ax = plt.subplots()
fig.set_size_inches(5, 5)

ax.set_xlim(-5, 5)
ax.set_ylim(-5, 5)

ax.axis('off')

def update(i, fig, ax):
    if i == 0:
        x = randint(-2, 2)
        y = randint(-2, 2)

    circle_1 = plt.Circle((x, y), i / 100, color=colors[2*i])
    circle_1.set_facecolor('white')

    circle_2 = plt.Circle((x, y), i / 200, color=colors[2*i])
    circle_2.set_facecolor('white')

    circle_3 = plt.Circle((x, y), i / 400, color=colors[2*i])
    circle_3.set_facecolor('white')

    ax.add_artist(circle_1)
    ax.add_artist(circle_2)
    ax.add_artist(circle_3)

    return [circle_1, circle_2, circle_3]

anim = FuncAnimation(fig, update, frames=128, interval=1, blit=True, repeat=True, fargs=(fig, ax))

plt.show()