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Archive: Whack

Relocatable Python virtualenvs using Whack

Saturday 7 September 2013 17:25

One of the uses for Whack is creating relocatable (aka path-independent) Python virtualenvs. Normally, a virtualenv is tied to a specific absolute path, meaning that moving the virtualenv causes errors:

$ virtualenv venv
$ venv/bin/pip install glances
(Snipping pip output)
$ mv venv venv2
$ venv2/bin/glances -v
bash: venv2/bin/glances: /tmp/venv/bin/python: bad interpreter: No such file or directory

Copying the entire virtualenv has similar but subtler problems. Rather than getting a straightforward error, the scripts in the new virtualenv will use the Python interpreter and libraries in the original virtualenv.

Whack allows virtualenvs to be created, and then moved to any other location:

$ whack install \
    git+https://github.com/mwilliamson/whack-package-python-virtualenv-env.git \
$ venv/bin/pip install glances
(Snipping pip output)
$ whack deploy venv --in-place
$ # Now we can copy the virtualenv to any other path,
$ # and it will continue to work
$ mv venv venv2
$ venv2/bin/glances -v
Glances version 1.7.1 with PsUtil 1.0.1

The whack deploy command is necessary to add any newly-installed scripts in the virtualenv to the bin directory.

One question is: why not use the --relocatable argument that virtualenv itself provides? This works in many cases, and doesn't require installation of Whack, but it also comes with a warning from virtualenv's documentation:

The --relocatable option currently has a number of issues, and is not guaranteed to work in all circumstances. It is possible that the option will be deprecated in a future version of virtualenv.

Topics: Python, Whack, Programs