Python: Image resizing: keep proportion - add white background

Question

I would like to create a Python script to resize images, but not changing its proportions, just by adding a white background

(So, a : 500*700 px im

Answer 1

The other answer didn't work for me, I rewrote it and this worked:

def resize_with_pad(im, target_width, target_height):
    '''
    Resize PIL image keeping ratio and using white background.
    '''
    target_ratio = target_height / target_width
    im_ratio = im.height / im.width
    if target_ratio > im_ratio:
        # It must be fixed by width
        resize_width = target_width
        resize_height = round(resize_width * im_ratio)
    else:
        # Fixed by height
        resize_height = target_height
        resize_width = round(resize_height / im_ratio)

    image_resize = im.resize((resize_width, resize_height), Image.ANTIALIAS)
    background = Image.new('RGBA', (target_width, target_height), (255, 255, 255, 255))
    offset = (round((target_width - resize_width) / 2), round((target_height - resize_height) / 2))
    background.paste(image_resize, offset)
    return background.convert('RGB')