Adaptive resize for avatars using Python and PIL/Pillow, square crop from center plus an optional resizing

adaptive_resize.py

def adaptive_resize(img, size=None):  #img is a PIL/Pillow Image object, size is a tuple of desired dimensions
    if size is not None:  # resize image
        if size[0] != size[1]:
            raise Exception("Provided target dimensions must be equal")
        smallest_side = min(img.size[0], img.size[1])
        if size[0] < smallest_side:  # make sure we actually need to resize
            resize_ratio = float(size[0]) / float(smallest_side)
            resize_x = int(math.floor(img.size[0] * resize_ratio))
            resize_y = int(math.floor(img.size[1] * resize_ratio))
            img = img.resize((resize_x, resize_y))
    if img.size[0] < img.size[1]:  # if y is greater than x
        top, bottom = crop_dimensions_math(img.size)
        crop_dimensions = (0, img.size[1] - top, img.size[0], img.size[1] - bottom)  # coordinates are measured from the top boundry
    elif img.size[0] > img.size[1]:  # if x is greater than y
        left, right = crop_dimensions_math(img.size)
        crop_dimensions = (img.size[0] - left, 0, img.size[0] - right, img.size[1])  # coordinates are measured from the left boundry
    else:  # image is already a square, no need for cropping
        crop_dimensions = None
    if crop_dimensions is not None:  # crop (if neccessary)
        img = img.crop(crop_dimensions)
    return img

function crop_dimensions_math ?