Python matplotlib animate arc - python-2.7

Question

I am trying to animate arcs and circles. The circles are moving every frame. While the arcs are changing radius, position and disappearing as functions of the positions of the circles.
I am trying to animate these arcs , but they are not changing.
Below is the code sample:
import matplotlib
import matplotlib.pyplot as plt
import matplotlib.animation as animation
from matplotlib import patches
import numpy as np
particle_one = np.zeros((10,2)) #10 times steps and x,y positions
particle_two = np.zeros((10,2)) #10 times steps and x,y positions
#the two particles are moving away from each other in the x direction
for i in range(0,10):
particle_one[i,0] = i
particle_two[i,0] = 2-i
particle_one[i,1] = 2
particle_two[i,1] = -2
particle_One_Radius = 1
particle_Two_Radius = 1.5
arc_Center = np.zeros((10,2))
for i in range(0,10):
arc_Center[i,0] = (particle_one[i,0] + particle_two[i,0])/2
#the arc should disappear for frame 5
arc_Center[5,0] = 0
arc_Center[5,1] = 0
fig = plt.figure()
plt.axis([-20,20, -5,5]) #axis that I like
ax = plt.gca()
circle_One = plt.Circle([particle_one[0,0],particle_one[0,1]],particle_One_Radius)
circle_Two = plt.Circle([particle_two[0,0],particle_two[0,1]],particle_Two_Radius)
circles = []
circles.append(circle_One)
circles.append(circle_Two)
arcs = []
#defines the arc
arc_one = patches.Arc([arc_Center[0,0],arc_Center[0,1]],5,3,angle =0 ,theta1 = 0,theta2= 270)
arcs.append(arc_one)
def init():
ax.add_patch(circles[0])
ax.add_patch(circles[1])
ax.add_patch(arcs[0])
return ax
#draw every frame by frame
def animate(m):
circles[0].center=((particle_one[m,0],particle_one[m,1]))
circles[1].center=((particle_two[m,0],particle_two[m,1]))
#the arcs does not change
arcs[0] =patches.Arc([arc_Center[m,0],arc_Center[m,1]],5+m,3+m,angle =0 ,theta1 = 0,theta2= 270)
return ax
#animation function that draws 10 frames
anim = animation.FuncAnimation(fig,animate , init_func= init , frames = 10 , interval = 20)
plt.show()
The circles animate correctly , but the arc does not change shape or location

Answer 1

Your problem is that instead of modifying your Arc patch as you do the circles, you create a new one at each step, but do not add it to the axes after it's created.
I've checked briefly, but I don't know how to modify the properties of an Arc instance, although I'm sure it's possible.
In the mean time, I've modified your function to remove the previous Arc from the patches list, create a new arc, and add it back to the Axes
#draw every frame by frame
def animate(m):
circles[0].center=((particle_one[m,0],particle_one[m,1]))
circles[1].center=((particle_two[m,0],particle_two[m,1]))
ax.patches.remove(arcs[0])
arcs[0] = patches.Arc([arc_Center[m,0],arc_Center[m,1]],5+m,3+m,angle =0 ,theta1 = 0,theta2= 270)
ax.add_patch(arcs[0])
print "step %d: arc = %s" % (m, arcs[0])
return circles,arcs

Answer 2

I have also ran into the problem of my arc not moving. Trying to remove the arc as Diziet suggested generated the
error: "x not in list".
However, what seems to work is to do both the instantiation of the arc and adding it to the axis within the animate function, but without the call to remove it - essentially, Diziet's solution minus the line "ax.patches.remove(arcs[0])".