I am learning to use 'pillow 5.0' following book 'Automate the boring stuff with python'

The info about the image object

In [79]: audacious = auda
In [80]: print(audacious.format, audacious.size, audacious.mode)
PNG (1094, 960) RGBA

When I tried to convert filetype, it report error.

In [83]: audacious.save('audacious.jpg')
OSError: cannot write mode RGBA as JPEG

There's no such a n error in book.

JPG does not support transparency - RGBA means Red, Green, Blue, Alpha - Alpha is transparency.

You need to discard the Alpha Channel or save as something that supports transparency - like PNG.

The Image class has a method convert which can be used to convert RGBA to RGB - after that you will be able to save as JPG.

Have a look here: the image class doku

im = Image.open("audacious.png")
rgb_im = im.convert('RGB')
rgb_im.save('audacious.jpg')

Adapted from dm2013's answer to Convert png to jpeg using Pillow