Zato blog post: A successful Python 3 migration story

[Terry Reedy]

The migration was from 2.7 to 2.7 and 3.x, rather than 3.x only.
I think it worth reading for anyone interested in the subject.
https://zato.io/blog/posts/python-3-migration-success-story.html

60,000 lines of Python and Cython, 130 external dependencies (but only 
10 not already 3.x ready) took 2 people 80 hours total.

Their head start was to write the 2.7 modules, from the beginning, with 
the following at the top.
from __future__ import absolute_import, division, print_function, 
unicode_literals


-- 
Terry Jan Reedy