Why does PIL.Image.rotate() results in loss of quality despite `resample=Image.BICUBIC` - python-imaging-library

Question

Per https://stackoverflow.com/a/17822099/1639359 using resample=PIL.Image.BICUBIC should retain quality through rotation. This question is follow up from my comment on the above answer that it is not working for me.
What version of Python and PIL do we have:
import sys
print(sys.version)
3.7.3 (default, Mar 27 2019, 22:11:17)
[GCC 7.3.0]
import PIL
import PIL.Image
print(PIL.__version__)
print(PIL.Image.__version__)
6.2.1
6.2.1
Open our test image, and save it. Notice the loss of quality; saved file is much smaller than original:
img = PIL.Image.open('test_image.jpg')
holdexif = img.info['exif']
img.save('testsave.jpg',"jpeg",exif=holdexif)
%ls -l 'test_image.jpg'
%ls -l 'testsave.jpg'
-rwxrwxrwx 1 dino dino 1926859 Dec 24 21:28 test_image.jpg*
-rwxrwxrwx 1 dino dino 452343 Dec 27 13:16 testsave.jpg*
Per Determining JPG quality in Python (PIL) setting quality='keep' prevents the loss of quality on save:
img.save('testsave.jpg',"jpeg",exif=holdexif,quality='keep')
%ls -l 'test_image.jpg'
%ls -l 'testsave.jpg'
-rwxrwxrwx 1 dino dino 1926859 Dec 24 21:28 test_image.jpg*
-rwxrwxrwx 1 dino dino 1926911 Dec 27 13:16 testsave.jpg*
If we rotate the image, two things happen:
img.format disappears
quality is reduced significantly (even though we use resample=PIL.Image.BICUBIC)
print(img.format)
JPEG
rotated_img = img.rotate(-90,resample=PIL.Image.BICUBIC,expand=True)
print(img.format)
print(rotated_img.format)
JPEG
None
Saving the rotated file shows the loss of quality:
rotated_img.format="JPEG" # quality='keep' will raise exception if format != 'JPEG'
rotated_img.save('testsave.jpg',"jpeg",exif=holdexif,quality='keep')
%ls -l 'test_image.jpg'
%ls -l 'testsave.jpg'
-rwxrwxrwx 1 dino dino 1926859 Dec 24 21:28 test_image.jpg*
-rwxrwxrwx 1 dino dino 450997 Dec 27 13:16 testsave.jpg*
Questions:
1. What, if anything, am I doing wrong?
2. Why does img.format disappear on rotate? What else is disappearing?
3. And, just as an aside, is there another way to detect the loss of quality (I only know to save the file and see that it is much smaller).
After #MarkRansom 's comments, I would like to add some color to my question: Although I have been coding for many years, I only last week began playing with images. Without a doubt my knowledge of image processing is minimal at best, with significant gaps and misunderstandings about how things work.
That said, from a coding perspective, I would expect that a method on an object would do pretty much only what that method suggests, with no side effects. For example, I would expect rotate() only to rotate the image, and not clear out meta data at the same time. Even if, let's say for efficiency sake, the default behavior for the PIL package is to always drop the meta data, I would hope there would be an option (either global, or on a per-method basis) for something like preserve_metadata=True
Regarding "the only good way to judge quality" being "to look at the images themselves," I can certainly understand that being the case on an absolute basis. However on a relative basis, most notably when comparing the "same" image before and after some processing, it seems to me there could be ways to physically measure, or at least estimate, the difference in quality before and after the processing. (Of course, I don't yet know enough about images to know what the best method of doing that would be; thus part of the reason for my question). It also seems to me (at least from my beginner perspective) that very often people would want to do whatever processing that want to do with little or no loss of quality at the same time. Thanks for your patience and help filling in my knowledge gaps, and with the answers.

Answer 1

In general, any image rotation is going to result in a loss of image quality simply because the output requires an interpolation of the input for pixels that don't map 1:1 from input to output. For rotations of multiples of 90 degrees, it's possible to get a 1:1 mapping and do a lossless rotation. It's not clear if PIL/Pillow is smart enough to do that, but it might. Your example rotates by -90 degrees so you could luck out.
The metadata is being lost because rotate generates a new image, it doesn't modify the existing one. It's not easy to determine which metadata might be relevant for the new image, so the safest course of action is to simply not copy any of it. As you've discovered, it's possible to manually copy anything you think is important. I don't think there's an option to automatically copy anything.
As noted in the comments, using file size as a judge of image quality is a very poor metric. A smaller size is suggestive of poorer quality, but it doesn't have to be so; the whole premise of JPEG compression is that some image degradations simply won't be visible in normal viewing. A visual comparison is the ultimate test. If you must have an automated comparison there are methods to do so, but I don't think any have emerged as the undisputed best way to compare.

Answer 2

You rotated the image, which creates a new generic image. There is no 'keep' in this instance. When saving to JPEG, default quality is 75, high quality is 95.
Need to copy quality, perhaps tables, and metadata to the new image, just as you are doing with the Exif.