9. ImageOps contain

See: https://pillow.readthedocs.io/en/stable/reference/ImageOps.html#PIL.ImageOps.contain

9.1. Contain

Use the ImageOps.contain(image, size, method=Resampling.BICUBIC) method to return a resized version of the image, set to the maximum width and height within the requested size, while maintaining the original aspect ratio.
The resultant image may not be the same size as requested, since the preservation of the aspect radio may lead to the extra length or width not being added.
The image below is resized from 256, 256 to 128, 128.
Specifying 128, 256 caused it to be resized to fit the width. The extra height was ignored.
from PIL import Image, ImageOps

with Image.open("test_images/shapes.png") as im:
    im1 = ImageOps.contain(im, size=(128, 256))
    im1.save("imageOps/contain.png")
../_images/compare_contain.png