How to modify an image's contrast in Python (contrast-stretching)

contrast.py

## Original code from the series on image processing in python
## Read the tutorial at https://thesourcecodenotes.blogspot.com/search/label/contrast-stretching

from PIL import Image, ImageTk
import tkinter as tk

 

def contrast(input_image, value):
    if input_image.mode != 'L' and input_image.mode != 'P':
        return None
    else:
        output_image = Image.new('L', (input_image.width,
                                     input_image.height))

        for x in range(input_image.width):
            for y in range(input_image.height):
                pix = input_image.getpixel((x, y))
                output_image.putpixel((x,y), int((pix-100) *  value))
    
    return output_image    

 
def main():
    root = tk.Tk()
    img = Image.open("retriver_gray.png", formats=['PNG'])

    width = img.width*2+20
    height = img.height
    root.geometry(f'{width}x{height}')
    
    output_im = contrast(img, 2.0)
     
    if output_im != None:
        output_im = ImageTk.PhotoImage(output_im)
        input_im = ImageTk.PhotoImage(img)
     
        canvas = tk.Canvas(root, width=width, height=height, bg="#ffffff")
        canvas.create_image(width/4-1, height/2-1, image=input_im, state="normal")
        canvas.create_image(20+3*img.width/2-1, img.height/2-1, image=output_im, state="normal")
        canvas.place(x=0, y=0)
        canvas.pack()
        
        root.mainloop()
    else:
        print("Input image's mode must be 'L' or 'P'"\
              "Check https://pillow.readthedocs.io/en/"\
              "latest/handbook/concepts.html#concept-modes")
 
if __name__ == "__main__":
    main()

Head for the tutorial at https://thesourcecodenotes.blogspot.com/search/label/contrast-stretching to see how is done with colours images