anchorer 0.4.0

Avoid breakages that would occur from updating symlinks to running virtualenvs

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