Moved get_child_images() to ImageFile

[radarhere]

This is part of #8362 - I'm hoping to break down that PR into easier-to-review chunks.

get_child_images() relies on self.fp, and since that only exists for an ImageFile and not an Image, I suggest moving it there. This isn't a strictly necessary change, so feel free to disagree.

After that, I've also made changes to fix two mypy errors. One is asserting that fp is not None, like #8617. The other is asserting that an Exif tag is an integer, to fix

src/PIL/ImageFile.py:238: error: Argument 1 to "read" of "IO" has incompatible type "Optional[Any]"; expected "int" [arg-type]
data = self.fp.read(ifd.get(ExifTags.Base.JpegIFByteCount))
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

[hugovk]

get_child_images() relies on self.fp, and since that only exists for an ImageFile and not an Image, I suggest moving it there. This isn't a strictly necessary change, so feel free to disagree.

I wonder, are there some circumstances where this could be a breaking change?

[radarhere]

Images that are not ImageFiles

>>> from PIL import Image
>>> im = Image.new("RGB", (1, 1))
>>> im.get_child_images()
[]

would start failing.

>>> im.get_child_images()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'Image' object has no attribute 'get_child_images'

However, I would suggest that is more helpful than an apparently-successful empty list, since an Image by itself will never return any child images - for example, if I open an image as an ImageFile, and then copy() to turn it into an Image, there is an error.

>>> from PIL import Image
>>> im = Image.open("Tests/images/flower.jpg")
>>> im.get_child_images()
[<PIL.JpegImagePlugin.JpegImageFile image mode=RGB size=160x120 at 0x103ACA970>]
>>> im.copy().get_child_images()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "PIL/Image.py", line 1574, in get_child_images
    current_offset = self.fp.tell()
AttributeError: 'Image' object has no attribute 'fp'

That would now change to

>>> im.copy().get_child_images()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'Image' object has no attribute 'get_child_images'

which again, I think is more helpful.

[hugovk]

Yes okay, if you'd like to go ahead with this, I suggest a deprecation cycle, so we don't break things without warning.

[radarhere]

Ok, I've deprecated the functionality, to be removed in Pillow 13.