Raise a TypeError when Image() is called

[radarhere]

Resolves #7455

When running

from PIL import Image
Image.Image()

this PR will raise a TypeError

Traceback (most recent call last):
  File "test.py", line 2, in <module>
    Image.Image()
  File "PIL/Image.py", line 488, in __init__
TypeError: Images should not be instantiated directly. Use the module new() function instead.

[homm]

Image._init()._new() is quite common and very ugly. In this case we don't want to create a new image second time just to copy empty info and palette properties.

I suggest following:

im._new(core.new()) — creates image like im, copying info and palette. I'm not sure how often this method is used this way.
Image._init(core.new()) — creates empty image using only core image object.

The only place I see where we need to create empty Image object (without core object) is new() function, but it could be easily avoided.

[radarhere]

I've cherry-picked #7460 here, since that helps.

I've pushed a commit with a version of your suggestion. It's not exact, because it turns out that _new() doesn't just copy existing palettes, it also creates new ones. It's not enough to just attach a palette afterwards in new(), because something like Image.linear_gradient("P") also needs a palette.

[homm]

    palette = None
    if mode == "P" and isinstance(color, (list, tuple)) and len(color) in [3, 4]:
        # RGB or RGBA value for a P image
        from . import ImagePalette

        palette = ImagePalette.ImagePalette()
        color = palette.getcolor(color)
    im = Image._init(core.fill(mode, size, color))
    im.palette = palette
    return im

[radarhere]

Per my last comment, _init(im) needs to create a palette for P mode images, which might lead to two ImagePalettes being created here.

However, we don't need getcolor() to know what the color index should be - it is always going to be zero, as this is the first color in the palette. Taking advantage of that, we can just re-use the palette from _init().

[homm]

Why we need to make this assumption rather than just using existing API?

We can rewrite palette if it is exist:

im = Image._init(core.fill(mode, size, color))
if palette:
     im.palette = palette

[radarhere]

Hmm, I understand the desire to use APIs. My original thought was to try and avoid constructing a second imagePalette, but this version of the code seems simpler to me. I don't think it should ever change that a palette starts out empty, and that the first color is at the zero index.

Are you sure using getcolor() is better?

[radarhere]

@homm did my argument persuade you, or no? I can change to your suggestion if you like, I'd rather progress this than let it sit unresolved.

[radarhere]

@hugovk is there any particular problem with this? My guess would be that it is perhaps too complex of a change for an edge case, and the user in question should just use our API as intended?

[hugovk]

I was waiting to see if @homm had more to say (re: #7461 (comment)).

But it does seem like a edge case (they're getting an error calling the API in the wrong way, and this changes it to another error) and I don't think this has come up before, so I'm leaning towards the status quo.