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

Question

How do I  in the main py module  presumably   tell Python which interpreter to use  What I mean is  if I want a particular script to use version 3 of Python to interpret the entire program  how do I do that   Bonus  How would this affect a virtualenv  Am I right in thinking that if I create a virtualenv for my program and then tell it to use a different version of Python  then I may encounter some conflicts

User · Answer

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   lt  3      raise Exception  Python 3 or a more recent version is required

User · Answer

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  quot within my script quot  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  Python25 python exe mymodule py or D  Python27 python exe mymodule py However you d 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 2 python exe mymodule Yet  the environment variables may point to the wrong version as such  PYTHONPATH   D  Python27 PYTHONLIB   D  Python27 lib 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  quot start quot  or  quot cmd exe quot  command to  quot save and replace the  quot console quot  environment quot  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  quot within quot  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

User · Answer

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   dont 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

User · Answer

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

User · Answer

You can add a shebang line the to the top of the script      usr bin env python2 7   But that will only work when executing as   my program py   If you execute as python my program py  then the whatever Python version that which python returns will be used   In re  to virtualenv use  virtualenv -p  usr bin python3 2 or whatever to set it up to use that Python executable

User · Answer

put at the start of my programs its use full for work with python  import sys  if sys version info 0   lt  3      raise Exception  Python 3 or a more recent version is required      This code will help full for the progress

User · Answer

For those using pyenv to control their virtual environments  I have found this to work in a script     home  lt user gt   pyenv versions  lt virt name gt  bin python  DO STUFF

User · Answer

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

User · Answer

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

[python] How do I tell a Python script to use a particular version

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