Anchorer

Plugin for virtualenvwrapper that extends mkvirtualenv behaviour to add code that is loaded by the python interpreter for every run. The loaded code resolves symlinks in discovered site-package directories, allowing symlinks to virtualenvs to be updated while scripts/services are running.

Example problem anchorer solves

# assuming you have the virtualenvwrapper python package installed, and have sourced virtualenvwrapper.sh
mkvirtualenv env-v1
mkvirtualenv env-v2

# create a pseudo-virtualenv which is a symlink to a particular version
ln -s "$WORKON_HOME/env-v1" "$WORKON_HOME/active-env"

# now use the linked environment to start something in env-v1
workon active-env

# start some imaginary python service which may import modules a long time after starting
python -m my_long_runner &

# update the active symlink, switching what version
ln -sT "$WORKON_HOME/env-v2" "$WORKON_HOME/active-env"

# imagine at this point that my_long_runner tries to import a module, it will be using un-resolved paths which will mean
# the modules will be loaded from an environment that is not the one it started in

Architecture

1. virtualenvwrapper.anchorer.fix.main() resolves paths that are used at runtime
   • current working directory
   • paths used for determining where packages are found
2. virtualenvwrapper runs virtualenvwrapper.anchorer.plugin.pre_mkvirtualenv(...) during calls to mkvirtualenv to modify the virtualenv's site-packages directory:
   1. __anchorer.py is added, it is a copy of the fix module
   2. __anchorer.pth is added, it simply imports __anchorer which causes the main method to run. See site docs for more information on the mechanism