this script will resize and pad an image to desired square size and keep its aspect ratio unchanged. Before running the script, please change the size and image path to valid value.

resize_and_pad_image_to_square.py

from PIL import Image, ImageOps
import cv2

desired_size = 368
im_pth = "/home/jdhao/test.jpg"

# im = Image.open(im_pth)
# old_size = im.size  # old_size[0] is in (width, height) format

# ratio = float(desired_size)/max(old_size)
# new_size = tuple([int(x*ratio) for x in old_size])

## using thumbnai() or resize() method to resize the input image and keep its aspect ratio
# im.thumbnail(new_size, Image.ANTIALIAS)
# im = im.resize(new_size, Image.ANTIALIAS) 

## create a new square image with desired size and paste the resized image onto it. 
# new_im = Image.new("RGB", (desired_size, desired_size))
# new_im.paste(im, ((desired_size-new_size[0])//2,
#                       (desired_size-new_size[1])//2))

## or we can expand the resized image by adding borders to its 4 side
# delta_w = desired_size - new_size[0]
# delta_h = desired_size - new_size[1]
# padding = (delta_w//2, delta_h//2, delta_w-(delta_w//2), delta_h-(delta_h//2))
# print(padding)
# new_im = ImageOps.expand(im, padding, fill="black")
# new_im.show()

## opencv has copyMakeBorder() method which is handy for making borders
im = cv2.imread(im_pth)
old_size = im.shape[:2] # old_size is in (height, width) format
ratio = float(desired_size)/max(old_size)
new_size = tuple([int(x*ratio) for x in old_size])

# new_size should be in (width, height) format
im = cv2.resize(im, (new_size[1], new_size[0])) 

delta_w = desired_size - new_size[1]
delta_h = desired_size - new_size[0]
top, bottom = delta_h//2, delta_h-(delta_h//2)
left, right = delta_w//2, delta_w-(delta_w//2)

color = [0, 0, 0]
new_im = cv2.copyMakeBorder(im, top, bottom, left, right, cv2.BORDER_CONSTANT,
    value=color)

cv2.imshow("image", new_im)
cv2.waitKey(0)
cv2.destroyAllWindows()