How do I tell a Python script to use a particular version

You can’t do this within the Python program, because the shell decides which version to use if you a shebang line.

If you aren’t using a shell with a shebang line and just type python myprogram.py it uses the default version unless you decide specifically which Python version when you type pythonXXX myprogram.py which version to use.

Once your Python program is running you have already decided which Python executable to use to get the program running.

virtualenv is for segregating python versions and environments, it specifically exists to eliminate conflicts.

I would use the shebang #!/usr/bin/python (first line of code) with the serial number of Python at the end

Then run the Python file as a script, e.g., ./main.py from the command line, rather than python main.py.

It is the same when you want to run Python from a Linux command line.

While the OP may be working on a nix platform this answer could help non-nix platforms. I have not experienced the shebang approach work in Microsoft Windows.

Rephrased: The shebang line answers your question of "within my script" but I believe only for Unix-like platforms. Even though it is the Unix shell, outside the script, that actually interprets the shebang line to determine which version of Python interpreter to call. I am not sure, but I believe that solution does not solve the problem for Microsoft Windows platform users.

In the Microsoft Windows world, the simplify the way to run a specific Python version, without environment variables setup specifically for each specific version of Python installed, is just by prefixing the python.exe with the path you want to run it from, such as C:Python25python.exe mymodule.py or D:Python27python.exe mymodule.py

However you would need to consider the PYTHONPATH and other PYTHON… environment variables that would point to the wrong version of Python libraries.

For example, you might run:

C:Python2.5.2python.exe mymodule

Yet, the environment variables may point to the wrong version as such:

PYTHONPATH = D:Python27

PYTHONLIB = D:Python27lib

Loads of horrible fun!

So a non-virtualenv way, in Windows, would be to use a batch file that sets up the environment and calls a specific Python executable via prefixing the python.exe with the path it resides in. This way has additional details you’ll have to manage though; such as using command line arguments for either of the "start" or "cmd.exe" command to "save and replace the "console" environment" if you want the console to stick around after the application exits.

Your question leads me to believe you have several Python modules, each expecting a certain version of Python. This might be solvable "within" the script by having a launching module which uses the subprocess module. Instead of calling mymodule.py you would call a module that calls your module; perhaps launch_mymodule.py

launch_mymodule.py

import sys
import subprocess
if sys.argv[2] == '272':
  env272 = {
    'PYTHONPATH': 'blabla',
    'PYTHONLIB': 'blabla', }
  launch272 = subprocess.Popen('D:\Python272\python.exe mymodule.py', env=env272)

if sys.argv[1] == '252'
  env252 = {
    'PYTHONPATH': 'blabla',
    'PYTHONLIB': 'blabla', }
  launch252 = subprocess.Popen('C:\Python252\python.exe mymodule.py', env=env252)

I have not tested this.

Perhaps not exactly what you asked, but I find this to be useful to put at the start of my programs:

import sys

if sys.version_info[0] < 3:
    raise Exception("Python 3 or a more recent version is required.")

While working with different versions of Python on Windows,

I am using this method to switch between versions.

I think it is better than messing with shebangs and virtualenvs

1. install python versions you desire

2. go to Environment Variables > PATH

   (I assume that paths of python versions are already added to Env.Vars.>PATH)

3. suppress the paths of all python versions you dont want to use

   (don’t delete the paths, just add a suffix like "_sup")

4. call python from terminal

   (so Windows will skip the wrong paths you changed,
   and will find the python.exe at the path you did not suppressed,
   and will use this version after on)

5. switch between versions by playing with suffixes

I had this problem and just decided to rename one of the programs from python.exe to python2.7.exe. Now I can specify on command prompt which program to run easily without introducing any scripts or changing environmental paths.
So i have two programs: python2.7 and python (the latter which is v.3.8 aka default).

For those using pyenv to control their virtual environments, I have found this to work in a script:

#!/home/<user>/.pyenv/versions/<virt_name>/bin/python

DO_STUFF