Import packages depending on Python version

While working on py-backwards and it’s setuptools integration I found an interesting problem, how to transparently allow people with different Python versions to use different packages. So, imagine we have a package with a structure:

package
  __init__.py
  _compiled_2_7
    package
      __init__.py
      main.py
  _compiled_3_3
    package
      __init__.py
      main.py
  _compiled_3_4
    package
      __init__.py
      main.py
  _compiled_3_5
    package
      __init__.py
      main.py
  _compiled_3_6
    package
      __init__.py
      main.py

And, for example, on Python 2.7 we need to use the package from _compiled_2_7, on 3.3 from _compiled_3_3 and etc.

First of all, we know that if we import package.main, package.__init__ would be imported first:

# __init__.py
print('init')

# main.py
print('main')

# REPL
>>> import package.main
init
main

>>> from package import main
init
main

# shell
➜ python -m package.main
init
main

So it works all the time. And now if we put special bootstrap script, which would change sys.path depending on current Python version, we can easily import the package that we need:

import sys
import os

VERSIONS = {'_compiled_2_7': (2, 7),
            '_compiled_3_0': (3, 0),
            '_compiled_3_1': (3, 1),
            '_compiled_3_2': (3, 2),
            '_compiled_3_3': (3, 3),
            '_compiled_3_4': (3, 4),
            '_compiled_3_5': (3, 5),
            '_compiled_3_6': (3, 6)}
root = os.path.abspath(os.path.dirname(__file__))


def _get_available_versions():
    for version in os.listdir(root):
        if version in VERSIONS:
            yield version


def _get_version():
    available_versions = sorted(_get_available_versions())[::-1]
    for version in available_versions:
        if VERSIONS[version] <= sys.version_info:
            return version


# We should pass `__name__` as an argument, because
# we can't access `__name__` after module deletion
def _import_module(name):
    version = _get_version()
    version_path = os.path.join(root, version)
    sys.path.insert(0, version_path)
    del sys.modules[name]
    __import__(name)


_import_module(__name__)

Let’s try it on our package, but first, we need to fill _compiled_*/package/__init__.py with:

print('init 2.7')  # replace with required version

And _compiled_*/package/main.py with:

print('main 2.7')  # replace with required version

So now we can try it with Python 2.7:

# REPL
>>> import package
init 2.7

>>> import package.main
init 2.7
main 2.7

>>> from package import main
init 2.7
main 2.7

# shell
➜ python2 -m package.main
init 2.7
main 2.7

And with Python 3.5:

# REPL
>>> import package
init 3.5

>>> import package.main
init 3.5
main 3.5

>>> from package import main
init 3.5
main 3.5

# shell
➜ python3 -m package.main
init 3.5
main 3.5

It works!