When/where should I check for the minimum Python version?

Question

This question tells me how to check the version of Python. For my package I require at least Python 3.3:

MIN_VERSION_INFO = 3, 3

import sys
if not sys.version_info >= MIN_VERSION_INFO:
    exit("Python {}.{}+ is required.".format(*MIN_VERSION_INFO))

but where/when should this check occur?

I want to produce the clearest possible error message for users installing via pip (sdist and wheel) or python setup.py install. Something like:

$ pip -V
pip x.x.x from ... (Python 3.2)
$ pip install MyPackage
Python 3.3+ is required.

$ python -V
Python 3.2
$ python setup.py install
Python 3.3+ is required.

Answer 1

The primary point of a compatibility check is to have this check either elegantly handle a compatibility issue or gracefully exit with an explanation before the incompatibility causes problems.

I'd put it near the top of setup.py, or the first script that belongs to you that will be called. Best practice is not to include code in __init__.py (unless you're making a MAJOR package, I'd opine), but if you already have a lot of code there, it's fine.

The quickest point of reference I can find: The old Python 2.6 unittest module had a test for it near the top, naturally, after the module docstring, imports, and __all__.

Another point of reference shows a compatibility check in an __init__.py, again, near the top, though here it is immediately after the docstring and an import of sys, required for the check. There are other similar examples of this usage in __init__.py in this same set of libraries.