How to label a Python package as version 3 only?

Question

What do I have to do to make PyPI identify my package as working on Python 3 only?

Answer 1

You give it a trove classifier:

Programming Language :: Python :: 3

and leave out the Python 2 classifiers.

If you want to prevent your package from being installed on Python 2 systems anyway, you can simply exit the setup.py script after detecting the version:

import sys

if sys.version_info[0] < 3:
    sys.stderr.write('Requires Python 3 or up\n')
    sys.exit(1)