Difference of Image.rotate on PIL/Pillow 2.8.1 and 3.1.1

Question

I was having a bad time with Image.rotate() on PIL until I decided to check on a terminal what was going on. This is what I did:

Python 2, Pillow (2.8.1)

from PIL import Image
im = Image.new('RGB', (800, 500))
im.size
out = im.rotate(90)
out.size

Output (as expected):

>> (800, 500)
>> (500, 800)

Python 3, Pillow (3.1.1)

from PIL import Image
im = Image.new('RGB', (800, 500))
im.size
out = im.rotate(90)
out.size

Output:

>> (800, 500)
>> (800, 500)

And I think that's the right way to call rotate in Pillow 3X. Here's the documentation for the function:
Image.Rotate() Am I missing something?

Answer 1

It was a bug in Pillow<=2.9.0, fixed in 3.0.0. If you want the image to change size when rotated, you need to include the expand argument, eg img.rotate(-90, expand=1). See this issue for details.