What is the standard way to recommend "Python 3 only" compatibility for a Python module?

Question

There is a python code, which is supposed to support Python 3, but may or may not run in Python 2.7. For example, this snippet can run in both Python 2.7 and Python 3. What is the standard way to enforce and recommend Python 3 compatibility in strict mode, even if the code runs fine on Python 2.7 ?

print('This file works in both')
print('How to throw an exception,and suggest recommendation of python 3 only ?')

Python 2.7 : https://ideone.com/bGnbvd

Python 3.5 : https://ideone.com/yrTi3p

There can be multiple hacks and exceptions, which work in Python 3, and not in Python 2.7, which can be used to achieve this. I am looking for most recommended way of doing this in the beginning of files/modules/projects.

Answer 1

If it is a proper Python package with the setup.py, you can use several things:

• python_requires classifier

  If your project only runs on certain Python versions, setting the python_requires argument to the appropriate PEP 440 version specifier string will prevent pip from installing the project on other Python versions.

  Sample: python_requires='>=3',

• Since the support for python_requires classifier has been added relatively recently, you should account for users installing your package with older versions of pip and setuptools. In this case you can check the sys.version_info inside the setup.py file like Django does:

  import sys
  
  CURRENT_PYTHON = sys.version_info[:2]
  REQUIRED_PYTHON = (3, 5)
  
  # This check and everything above must remain compatible with Python 2.7.
  if CURRENT_PYTHON < REQUIRED_PYTHON:
      sys.stderr.write("""...""")
      sys.exit(1)
  

• Programming Language Python version classifiers:

  'Programming Language :: Python',
  'Programming Language :: Python :: 3',
  'Programming Language :: Python :: 3.5',
  'Programming Language :: Python :: 3.6',
  'Programming Language :: Python :: 3 :: Only',
  

And, as a bonus, if the package is distributed through PyPI package index, python_requires and other classifiers are going to be displayed on the package home page.

Answer 2

You could simply check the sys.version_info:

import sys
if sys.version_info[0] < 3:
    raise SystemExit("Use Python 3 (or higher) only")