Deprecated categories

[radarhere]

Images have a category property. It is set to Image.NORMAL

Pillow/src/PIL/Image.py

Line 538 in aa35f6b

self.category = NORMAL

Unless the image format is MIC and there are multiple frames, in which case it is Image.CONTAINER.

Pillow/src/PIL/MicImagePlugin.py

Lines 70 to 71 in aa35f6b

	if len(self.images) > 1:
	self.category = Image.CONTAINER

There is also the unused attribute Image.SEQUENCE.

Pillow/src/PIL/Image.py

Lines 190 to 193 in aa35f6b

	# categories
	NORMAL = 0
	SEQUENCE = 1
	CONTAINER = 2

This minimal support across formats shows the feature has clearly not kept up with our other changes around multi-frame images, and can be deprecated in favour of is_animated.

[hugovk]

I'm fine with deprecating it.

It was a pain when we initally removed Image.PILLOW_VERSION without active deprecation warnings (only warnings in docs).

I expect calls to im.category are less common, but shall we nevertheless include deprecation warnings?

[radarhere]

Ok, sure. I've added deprecation warnings.

However, not for Python 3.6, since __getattr__ was only added in Python 3.7 - but Python 3.6 reaches EOL at the end of this year, so even if a user switched across to Python 3.7 at the start of 2022, they would still have a full year of seeing the warnings before the feature was removed.

[hugovk]

Sounds good, thank you!