Image.register_open called multiple times

[bigcat88]

What did you do?

Calling Image.register_open multiply times should not lead to growth of Image.ID list.

What did you expect to happen?

After second call to Image.register_open Image.ID should not contain similar values.

What are your OS, Python and Pillow versions?

• OS: not OS dependent
• Python: not python version dependent
• Pillow: 9.4.0

Maybe it is invalid usage, I know that applications should not call Image.register_open, but this very small problem present.

def register_open(id, factory, accept=None):
    """
    Register an image file plugin.  This function should not be used
    in application code.

    :param id: An image format identifier.
    :param factory: An image file factory method.
    :param accept: An optional function that can be used to quickly
       reject images having another format.
    """
    id = id.upper()
    ID.append(id)  # growth of the list here.
    OPEN[id] = factory, accept

Image.ID used only in a few places:

1. PIL.Image.open

    if formats is None:
        formats = ID

2. PIL.FitsStubImagePlugin

    # Override FitsImagePlugin with this handler
    # for backwards compatibility
    try:
        Image.ID.remove(FITSStubImageFile.format)
    except ValueError:
        pass

    Image.register_open(
        FITSStubImageFile.format, FITSStubImageFile, FitsImagePlugin._accept
    )

3. PIL.features

        for i in sorted(Image.ID):
            line = f"{i}"
            if i in Image.MIME:
                line = f"{line} {Image.MIME[i]}"
            print(line, file=out)

Probably in 1 and 3 places this can be replaced with OPEN.keys() that contain the same values, if I understand code correct, and in place with number 2 just remove Image.ID.remove lines.
These changes should not affect performance.

[radarhere]

To be clear - you didn't actually have code that you'd written that you found calling Image.register_open multiple times, right? This is just a theoretical concern?

I'm reluctant to make ID redundant by replacing it with OPEN.keys(), since ID is part of the public API.

I've created PR #6917 to simply not append existing ids to ID.

[bigcat88]

To be clear - you didn't actually have code that you'd written that you found calling Image.register_open multiple times, right? This is just a theoretical concern?

Yes, it is more theoretical, you are right.

I've created PR #6917 to simply not append existing ids to ID.

Thanks.