Bug report

Bug summary

I was trying to use FuncAnimation's save method to save an animation that I'd created to a .mp4 file, but it threw a TypeError when I ran my code (the animation displays fine with plt.show()).

Code for reproduction

import matplotlib.pyplot as plt
import matplotlib.animation as animation
import random

fig = plt.figure()
ax1 = fig.add_subplot(1, 1, 1)
x = [random.randint(0, 10) for _ in range(5)]
y = [random.randint(0, 10) for _ in range(5)]


def ani_func(i):
    x.append(random.randint(0, 10))
    y.append(random.randint(0, 10))
    ax1.clear()
    ax1.plot(x, y)

ani = animation.FuncAnimation(fig, ani_func, frames=10, repeat=False)
plt.show()
ani.save('test.mp4')

Actual outcome

The animation itself works fine in plt.show() producing this image as the last frame as expected:

However, the animation doesn't save to test.mp4 and instead throws a TypeError. I have included the entire traceback below.

Traceback (most recent call last):
  File ".\disease_simulation\graphout.py", line 19, in <module>
    ani.save('test.mp4')
  File "C:\Users\menon\Desktop\disease-simulation\env\lib\site-packages\matplotlib\animation.py", line 1102, in save
    alt_writer = next(writers, None)
TypeError: 'MovieWriterRegistry' object is not an iterator

Matplotlib version

• Operating system: Windows 10
• Matplotlib version: 3.2.1
• Matplotlib backend (print(matplotlib.get_backend())): TkAgg
• Python version: 3.6.8

Matplotlib was installed via pip in a venv.