No documented way to programmatically request single iteration of gif animation

[jagerber48]

What did you do?

Generate a .gif file using loop=1.

What did you expect to happen?

I expected this to cause the .gif animation to play through once and then not repeat.

What actually happened?

On some gif viewers the gif played once, on others it played twice.

What are your OS, Python and Pillow versions?

• OS: Windows 11
• Python: 3.9
• Pillow: 9.1.1

Discussion

See this issue which spells out the problem in more detail: #4575

I was able to resolve my issue by omitting the loop argument from my gif save call altogether. In this case 1 iteration of the animation was reliably played on all gif players I tried. But how can I access the omitted argument behavior while still including the argument? Suppose I have code like:

def save_images_to_gif(images, gif_path, tot_duration, loop):
    duration = tot_duration / len(images)
    images[0].save(gif_path, save_all=True, append_iamges=images[1:], duration=duration, loop=loop)

What argument can a user of this function pass in for loop to get the same behavior as if no loop argument was supplied to save? 0 doesn't work, 1 doesn't work, None doesn't work. I don't know of any argument that works and as far as I can tell there is no such argument in the documentation.

[Yay295]

There isn't one. The code just checks whether or not loop has been set at all.

Pillow/src/PIL/GifImagePlugin.py

Lines 940 to 950 in c98a799

	if "loop" in info:
	header.append(
	b"!"
	+ o8(255) # extension intro
	+ o8(11)
	+ b"NETSCAPE2.0"
	+ o8(3)
	+ o8(1)
	+ o16(info["loop"]) # number of loops
	+ o8(0)
	)

[jagerber48]

Ok, thanks for the info and clarifying there is no way to do this. Would it be reasonable to request a way to be able to do this? For example using None, or some other sentinel, to realize this behavior?

[Yay295]

I think None currently raises an exception, right? If so, it could probably check that loop isn't None without breaking anyone's code.

[jagerber48]

Yes, if I pass loop=None I get error: required argument is not an integer.

[radarhere]

I've created PR #7329 to resolve this.