Question

I’m curious if there are important libraries that support only Python 3, since it appears that many libraries that do support it, also happen to support Python 2.

Answer 1

It appears there isn’t, so I wrote this (with some help):

#!/usr/bin/env python3

import xmlrpc.client

# PyPI classifiers for all Python 3 versions
PY3 = [
    "Programming Language :: Python :: 3",
    "Programming Language :: Python :: 3.0",
    "Programming Language :: Python :: 3.1",
    "Programming Language :: Python :: 3.2",
    "Programming Language :: Python :: 3.3",
    "Programming Language :: Python :: 3.4",
]

# PyPI classifiers for all Python 2 versions
PY2 = [
    "Programming Language :: Python :: 2",
    "Programming Language :: Python :: 2.7",
    "Programming Language :: Python :: 2.6",
    "Programming Language :: Python :: 2.5",
    "Programming Language :: Python :: 2.4",
    "Programming Language :: Python :: 2.3",
]

def main():
    client = xmlrpc.client.ServerProxy('http://pypi.python.org/pypi')
    # name[0] is package name
    # name[1] is package version
    py3names = [
        name[0] for classifier in PY3 for name in client.browse([classifier])
    ]
    py2names = [
        name[0] for classifier in PY2 for name in client.browse([classifier])
    ]
    py3only = [name for name in py3names if name not in py2names]
    template = "Python3-only packages: {} (of {})"
    print(template.format(len(py3only), len(set(py2names + py3names))))

if __name__ == "__main__":
    main()