python animation.FuncAnimation error : object is not iterable

Question

I'm new to Python and now studying matplotlib to use animation function. I'm following qutip tutorial because of my study. But when I copied and pasted example code of qutip tutorial, it didn't work Error message was Axes3D object is not iterable.
So, I want to check the code I created but don't know whether the problem is my code of is other thing. I don't know what to do and want to know why the example code doesn't work.

This is the example code from the tutorial:

from qutip import *
from scipy import *
def qubit_integrate(w, theta, gamma1, gamma2, psi0, tlist):

    sx = sigmax(); sy = sigmay(); sz = sigmaz(); sm = sigmam()
    H = w * (cos(theta) * sz + sin(theta) * sx)
    c_op_list = []
    n_th = 0.5 # temperature
    rate = gamma1 * (n_th + 1)
    if rate > 0.0: c_op_list.append(sqrt(rate) * sm)
    rate = gamma1 * n_th
    if rate > 0.0: c_op_list.append(sqrt(rate) * sm.dag())
    rate = gamma2
    if rate > 0.0: c_op_list.append(sqrt(rate) * sz)

    output = mesolve(H, psi0, tlist, c_op_list, [sx, sy, sz])  
    return output.expect[0], output.expect[1], output.expect[2]

    w     = 1.0 * 2 * pi   # qubit angular frequency
    theta = 0.2 * pi       # qubit angle from sigma_z axis (toward sigma_x axis)
    gamma1 = 0.5      # qubit relaxation rate
    gamma2 = 0.2      # qubit dephasing rate

    a = 1.0
    psi0 = (a* basis(2,0) + (1-a)*basis(2,1))/(sqrt(a**2 + (1-a)**2))
    tlist = linspace(0,4,250)
    sx, sy, sz = qubit_integrate(w, theta, gamma1, gamma2, psi0, tlist)
from pylab import *
import matplotlib.animation as animation
from mpl_toolkits.mplot3d import Axes3D

fig = figure()
ax = Axes3D(fig,azim=-40,elev=30)
sphere = Bloch(axes=ax)

def animate(i):
    sphere.clear()
    sphere.add_vectors([np.sin(theta),0,np.cos(theta)])
    sphere.add_points([sx[:i+1],sy[:i+1],sz[:i+1]])
    sphere.make_sphere()
    return ax

def init():
    sphere.vector_color = ['r']
    return ax

ani = animation.FuncAnimation(fig, animate, np.arange(len(sx)),
                            init_func=init, blit=True, repeat=False)
ani.save('bloch_sphere.mp4', fps=20, clear_temp=True)

And this is my own code:

import numpy as np
import qutip as q
import scipy as sp

up=q.basis(2,0)
sx=q.sigmax()
sy=q.sigmay()
sz=q.sigmaz()
bz=0.
by=0.
bx=15.
w=np.pi/20

H=w*(sx*bx+sy*by+sz*bz)

def state(t):
    states=[q.Qobj.expm(-(0+1j)*H*t)*up]
    return states

import matplotlib.pyplot as plt
import matplotlib.animation as ani
from mpl_toolkits.mplot3d import Axes3D

fig=plt.figure()
ax=Axes3D(fig,azim=-40,elev=30)
sphere=q.Bloch(axes=ax)
sphere.add_states(up)

def ini():
    sphere.vector_color=("r")
    return ax

t=np.linspace(0,1,256)

def animate(i):
    sphere.clear()
    sphere.add_states(state[i])
    return ax

ani.FuncAnimation(fig,animate,frames=len(t),init_func=ini,blit=True,repeat=False)

plt.show()

Answer 1

Fixing the tutorial

Remove the blit argument to make the tutorial work:

ani = animation.FuncAnimation(fig, animate, np.arange(len(sx)),
                               init_func=init,  repeat=False)
plt.show()
ani.save('bloch_sphere.mp4', fps=20)

Fixing your example

frames must be iterable.

Change:

frames=len(t)

into:

frames=t

I.e., this line:

ani.FuncAnimation(fig, animate, frames=len(t), init_func=ini, blit=True, repeat=False)

should become this:

ani.FuncAnimation(fig, animate, frames=t, init_func=ini, blit=True, repeat=False)

A few more changes.

1. Call your function stare with parasynthesis state(i) not with square brackets state[i]
2. Keep a reference to the animation ani = ani.FuncAnimation

Full code:

def animate(i):
    sphere.clear()
    sphere.add_states(state(i))
    sphere.make_sphere()
    return ax

ani = ani.FuncAnimation(fig, animate, frames=t, init_func=ini, repeat=False)

plt.show()

This is the end state of the animation: