Using different versions of python with virtualenvwrapper

I've got various versions of python installed on my Mac using Macports. When I've selected python 2.7 via $ port select python python27, virtualenvwrapper works perfectly.

But if I select another version of python, i.e. 2.6, virtualenvwrapper generates an error message: ImportError: No module named virtualenvwrapper.hook_loader

I checked my .profile and it's setting VIRTUALENVWRAPPER_PYTHON to /opt/local/bin/python, so it seems to me virtualenvwrapper should work regardless of which python I've selected.

Any idea what would cause virtualenvwrapper to generate a .hook_loader error when I switch python versions?

You can select the python version explicitly

mkvirtualenv -p python3 venvname

or

mkvirtualenv -p python2.7 venvname

I know this is pretty much solved in your comments, but it's mac only,

and even more I think the correct way should be to set VIRTUALENVWRAPPER_PYTHON to the real python you are using on the command line.

To be sure you can do which python.

Actually, you can even do:

export VIRTUALENVWRAPPER_PYTHON=`which python`

On linux I do this in my .bashrc, so all in all, assuming you installed virtualenv and created your first "virtual environment" virtualenv (how original)

. virtualenv/bin/activate
export WORKON_HOME=$HOME/.virtualenvs # or whatever else you want
export VIRTUALENVWRAPPER_PYTHON=`which python`
export PROJECT_HOME=SOMETHING
source $HOME/virtualenv/bin/virtualenvwrapper.sh # or wherever else you got that installed

(and by the way, you wrote:

I checked my .profile and it's setting VIRTUALENVWRAPPER_PYTHON to /opt/local/bin/python, so it seems to me virtualenvwrapper should work regardless of which python I've selected

which is actually the opposite - virtualenv relies on using the correct python (and the packages that go with it) so it's very important to set the python path accordingly.

Even running a py file with a "#!/bin/python" might bring trouble once you are virtualenved!

None of these worked. I installed Python3 first when setting up my osx machine, and pip and all default to that.

First, check which python you have installed:

$ `which python` -V

If this returns "Python 2.7.12", then you are set to run:

$ mkvirtualenv -p `which python` api_server
Running virtualenv with interpreter /usr/local/bin/python
New python executable in /Users/eric/.virtualenvs/api_server/bin/python2.7
Also creating executable in /Users/eric/.virtualenvs/api_server/bin/python
Installing setuptools, pip, wheel...done.
virtualenvwrapper.user_scripts creating /Users/eric/.virtualenvs/api_server/bin/predeactivate
virtualenvwrapper.user_scripts creating /Users/eric/.virtualenvs/api_server/bin/postdeactivate
virtualenvwrapper.user_scripts creating /Users/eric/.virtualenvs/api_server/bin/preactivate
virtualenvwrapper.user_scripts creating /Users/eric/.virtualenvs/api_server/bin/postactivate
virtualenvwrapper.user_scripts creating /Users/eric/.virtualenvs/api_server/bin/get_env_details

This will also activate the api_server workon, which changes your python executable:

$ which python
/Users/eric/.virtualenvs/api_server/bin/python
$ python -V
Python 2.7.12

What does which python actually do? It outputs the directory of the python executables found in your PATH:

$ which python
/usr/local/bin/python

By using which python, you are basically passing in /usr/local/bin/python to the -p option in the mkvirtualenv directory.

What happens when you have more than one python executable returned in which python? Just find the one you want and pass it in:

$ mkvirtualenv -p /usr/local/bin/python3 api_server

And virtualenvwrapper will end up using that python executable instead.

Confirmed the use of two similarly-named environment variables:

VIRTUALENVWRAPPER_PYTHON -- which Python version is used by the virtualenvwrapper utility itself. In other words, which version of Python executes virtualenvwrapper, as if that Python version had been explicitly named in the #! line of the virtualenvwrapper script file.

VIRTUALENV_PYTHON -- which Python version will be installed by virtualenv when you create a new virtual environment. Equivalent to -p / --python option on the virtualenv command line.

And perhaps obviously :) the version of Python run in a virtual environment is the version you install for that environment -- has no relation to the above environment variables after the env is created.

See https://stackoverflow.com/a/24724360/763269 for how to upgrade Python within a virtualenv.