How to make an animated gif that shows each picture only once

I want to make an animated gif with one second per picture that shows each picture exactly once. Here is a MWE:

from PIL import Image
import seaborn as sns
import random
import matplotlib.pyplot as plt

# Make three random heatmaps and save them as "pic0.png", "pic1.png" and "pic2.png".
# You can use any three pictures you have instead.
for i in range(3):
    new_list = [random.sample(range(80), 10)]*6
    sns.heatmap(new_list)
    plt.savefig("pic"+str(i)+".png")
    plt.clf()

# Add the images to a list
images = []
for i in range(3):
    images.append(Image.open("pic"+str(i)+".png"))

# Create the animated GIF. I was hoping loop=1 would show each image once but it doesn't.
images[0].save("out.gif", save_all=True, duration=1000,loop=1, append_images=images[1:])

The result of this is the following:

• How do I make it show each image exactly once? Currently it shows them all twice.

According to the following post it's a known bug in Pillow.

>loop=1 actually loops twice!

We may use FFmpeg CLI instead of Pillow:

import subprocess as sp

...

sp.run(['ffmpeg', '-y', '-r', '1', '-i', 'pic%1d.png', '-vf', 'split[s0][s1];[s0]palettegen
;[s1]
paletteuse', '-loop' '-1' 'out.gif'])

The FFmpeg command is based on the following answer.

• pic%1d.png Uses all png files that ends with one digit - consider storing the PNG images in a "clean" foder.
• -r 1 Sets the frame rate to 1fps
• -loop -1 Applies no looping

Updated code sample (assume FFmpeg CLI is in the executable path):

from PIL import Image
import seaborn as sns
import random
import matplotlib.pyplot as plt
import subprocess as sp

# Make three random heatmaps and save them as "pic0.png", "pic1.png" and "pic2.png".
# You can use any three pictures you have instead.
for i in range(3):
    new_list = [random.sample(range(80), 10)]*6
    sns.heatmap(new_list)
    plt.savefig("pic"+str(i)+".png")
    plt.clf()

# Add the images to a list
images = []
for i in range(3):
    images.append(Image.open("pic"+str(i)+".png"))

# Create the animated GIF. I was hoping loop=1 would show each image once but it doesn't.
#images[0].save("out.gif", save_all=True, duration=1000, loop=1, append_images=images[1:])

# Use FFmpeg CLI instead of using Pillow
sp.run(['ffmpeg', '-y', '-r', '1', '-i', 'pic%1d.png', '-vf', 'split[s0][s1];[s0]palettegen
;[s1]
paletteuse', '-loop', '-1', 'out.gif'])

A cleaner solution is writing the images to stdin pipe of FFmpeg subprocess:

from PIL import Image
import seaborn as sns
import random
import matplotlib.pyplot as plt
import subprocess as sp

p = sp.Popen(['ffmpeg', '-y', '-r', '1', '-f', 'image2pipe', '-i', 'pipe:', '-vf', 'split[s0][s1];[s0]palettegen
;[s1]
paletteuse', '-loop', '-1', 'out.gif'], stdin=sp.PIPE)

# Make three random heatmaps and save them as "pic0.png", "pic1.png" and "pic2.png".
# You can use any three pictures you have instead.
for i in range(3):
    new_list = [random.sample(range(80), 10)]*6
    sns.heatmap(new_list)
    #plt.savefig("pic"+str(i)+".png")
    plt.savefig(p.stdin, format='png')  # Write image to stdin pipe of FFmpeg subprocess.
    plt.clf()

p.stdin.close()

We may also try using matplotlib animation.