How do I resize an image using PIL and maintain its aspect ratio?

Define a maximum size. Then, compute a resize ratio by taking min(maxwidth/width, maxheight/height).

The proper size is oldsize*ratio.

There is of course also a library method to do this: the method Image.thumbnail.
Below is an (edited) example from the PIL documentation.

import os, sys
import Image

size = 128, 128

for infile in sys.argv[1:]:
    outfile = os.path.splitext(infile)[0] + ".thumbnail"
    if infile != outfile:
        try:
            im = Image.open(infile)
            im.thumbnail(size, Image.Resampling.LANCZOS)
            im.save(outfile, "JPEG")
        except IOError:
            print "cannot create thumbnail for '%s'" % infile

This script will resize an image (somepic.jpg) using PIL (Python Imaging Library) to a width of 300 pixels and a height proportional to the new width. It does this by determining what percentage 300 pixels is of the original width img.size[0] and then multiplying the original height img.size[1] by that percentage. Change base_width to any other number to change the default width of your images.

from PIL import Image

base_width = 300
img = Image.open('somepic.jpg')
wpercent = (base_width / float(img.size[0]))
hsize = int((float(img.size[1]) * float(wpercent)))
img = img.resize((base_width, hsize), Image.Resampling.LANCZOS)
img.save('somepic.jpg')

I also recommend using PIL's thumbnail method, because it removes all the ratio hassles from you.

One important hint, though: Replace

im.thumbnail(size)

with

im.thumbnail(size,Image.ANTIALIAS)

by default, PIL uses the Image.NEAREST filter for resizing which results in good performance, but poor quality.

If you are trying to maintain the same aspect ratio, then wouldn't you resize by some percentage of the original size?

For example, half the original size

half = 0.5
out = im.resize( [int(half * s) for s in im.size] )