Question

I need to set association for .py files to be executed with specific python version. But I need to make this association only for single cmd.exe session (parallel sessions should not be affected). Does Windows allow this?

I suspect the answer is no, but I’d like to see some proof before throwing out the idea to get such feature into virtualenv.

Answer 1

Sure you can. You were very close to answer in your comment to Jakob’s answer –

If it is possible to change file association with environment
variables – it will help, but it doesn’t seem possible.

It is possible. All you have to do is to use REG_EXPAND_SZ type of registry key and environment variable in the key’s value. For example putting
%python_home%\python %1 %*
as the (Default) value of
HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Python.File\shell\open\command
key and setting its value to REG_EXPAND_SZ makes it possible to define what Python your Python files will be opened with. You decide by setting python_home environment variable and you can do this per command line session of course. Take a look at my answer to the question where in the registry does Windows store, with which program to open certain file types?

Having said that there is now special tool for solving exactly this problem which I highly recommend. It’s called pylauncher. From the docs:

PEP 397 compatible launcher for Python under Windows. See
http://www.python.org/dev/peps/pep-0397/ for PEP,
http://www.red-dove.com/screencasts/launcher/launcher.html for screencast

You can even skip the .py extension if you add it to the PATHEXT environment variable. See the question What environment variables will be used when calling an EXE from command line?