Changing palette's of 8-bit .png images using python PIL

Question

I'm looking for a fast way to apply a new palette to an existing 8-bit .png image. How can I do that? Is the .png re-encoded when I save the image? (Own answer: it seems so)

What I have tried (edited):

import Image, ImagePalette
output = StringIO.StringIO()
palette = (.....) #long palette of 768 items
im = Image.open('test_palette.png') #8 bit image
im.putpalette(palette) 
im.save(output, format='PNG')

With my testimage the save function takes about 65 millis. My thought: without the decoding and encoding, it can be a lot faster??

Answer 1

im.palette is not callable -- it's an instance of the ImagePalette class, in mode P, otherwise None. im.putpalette(...) is a method, so callable: the argument must be a sequence of 768 integers giving R, G and B value at each index.

Answer 2

Changing palette's without decoding and (re)encoding does not seem possible. The method in the question seems best (for now). If performance is important, encoding to GIF seems a lot faster.