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I'm trying to create animation about how the value of a quantum bit would be changed by computation called X-gate, in form of an arrow.
Here are the codes that I wrote.

#Import libraries

from mpl_toolkits.mplot3d import Axes3D

import matplotlib.pyplot as plt

from matplotlib import animation

import numpy as np



#Accept input (theta, phi) from a user

print("Put angle theta and phi, 0≤ theta ≤180, 0≤ phi ≤360")

theta = input("theta:")

phi = input("phi:")

theta = np.radians(float(theta))

phi = np.radians(float(phi))



#Calculate x,y,z coordinates

X = np.sin(theta) * np.cos(phi)

Y = np.sin(theta) * np.sin(phi)

Z = np.cos(theta)



#Adjusting the length of an arrow

length = np.sqrt(X**2 + Y**2 + Z**2)

if length > 1:

    X = X/length

    Y = Y/length

    Z = Z/length



# Figure of the animation

fig = plt.figure()

ax = fig.gca(projection='3d')

ax.set_aspect("equal")

u, v = np.mgrid[0:2*np.pi:20j, 0:np.pi:10j]

x = np.cos(u)*np.sin(v)

y = np.sin(u)*np.sin(v)

z = np.cos(v)

ax.set_xlabel('x')

ax.set_ylabel('y')

ax.set_zlabel('z')

ax.plot_wireframe(x,y,z, color="black")



# Calculate x,y,z coordinates in the process of the change

length = 9

xgate_theta = np.linspace(theta,theta+np.pi,length)

xgate_phi = np.linspace(phi,phi,length)



#Array of x,y,z coordinates

xgate= 



# Only x coordinates

xgate_x = 



# Only y coordinates

xgate_y = 



# Only z coordinates

xgate_z = 



for i in range(length):

    xgate_x.append(X)

    xgate_z.append(np.cos(xgate_theta[i]))

    xgate_y.append(np.sqrt(1-np.sqrt(xgate_x[i]**2+xgate_z[i]**2))*(-1))

for j in range(length):        

  xgate.append(plt.quiver(0,0,0,xgate_x[j],xgate_y[j],xgate_z[j],color="red"))





ani = animation.ArtistAnimation(fig,xgate,interval=1000)

plt.show()

Then, I got the following error.

Traceback (most recent call last):

  File "/Users/makotonakai/anaconda3/lib/python3.6/site-     packages/matplotlib/cbook/__init__.py", line 388, in process

proxy(*args, **kwargs)

  File "/Users/makotonakai/anaconda3/lib/python3.6/site-packages/matplotlib/cbook/__init__.py", line 228, in __call__

return mtd(*args, **kwargs)

  File "/Users/makotonakai/anaconda3/lib/python3.6/site-packages/matplotlib/animation.py", line 1026, in _start

self._init_draw()

  File "/Users/makotonakai/anaconda3/lib/python3.6/site-packages/matplotlib/animation.py", line 1556, in _init_draw

for artist in f:

TypeError: 'Line3DCollection' object is not iterable

I cannot tell what line causes this error by just looking this error message. Can anybody tell me how I can fix this error?