In Python, Python Image Library 1.1.6, how can I expand the canvas without resizing?

The ImageOps.expand function will expand the image, but it adds the same amount of pixels in each direction.

The best way is simply to make a new image and paste:

newImage = Image.new(mode, (newWidth,newHeight))
newImage.paste(srcImage, (x1,y1,x1+oldWidth,y1+oldHeight))

If performance is an issue, make your original image bigger than needed and crop it after the drawing is done.

Based on interjays answer:

#!/usr/bin/env python

from PIL import Image
import math


def resize_canvas(old_image_path="314.jpg", new_image_path="save.jpg",
                  canvas_width=500, canvas_height=500):
    """
    Resize the canvas of old_image_path.

    Store the new image in new_image_path. Center the image on the new canvas.

    Parameters
    ----------
    old_image_path : str
    new_image_path : str
    canvas_width : int
    canvas_height : int
    """
    im = Image.open(old_image_path)
    old_width, old_height = im.size

    # Center the image
    x1 = int(math.floor((canvas_width - old_width) / 2))
    y1 = int(math.floor((canvas_height - old_height) / 2))

    mode = im.mode
    if len(mode) == 1:  # L, 1
        new_background = (255)
    if len(mode) == 3:  # RGB
        new_background = (255, 255, 255)
    if len(mode) == 4:  # RGBA, CMYK
        new_background = (255, 255, 255, 255)

    newImage = Image.new(mode, (canvas_width, canvas_height), new_background)
    newImage.paste(im, (x1, y1, x1 + old_width, y1 + old_height))
    newImage.save(new_image_path)

resize_canvas()

You might consider a rather different approach to your image... build it out of tiles of a fixed size. That way, as you need to expand, you just add new image tiles. When you have completed all of your computation, you can determine the final size of the image, create a blank image of that size, and paste the tiles into it. That should reduce the amount of copying you're looking at for completing the task.

(You'd likely want to encapsulate such a tiled image into an object that hid the tiling aspects from the other layers of code, of course.)