I love the combination of virtualenv and pip requirements files. This is a great 95% solution to the problem of deploying and distributing known-good sets of python packages, safely (semi-)isolated from other python environments on the same system.
--no-site-packages option is great because it makes your
installation isolated from everything except the globally-installed
standard python library.
Here's the 5% where virtualenv falls painfully flat:
- You can't always use
--no-site-packages, or rather you can't always document that as the safest way for people to install your software, because you might have dependencies on C extensions (for me it's always lxml, gdal, and psycopg2), and
--no-site-packagesis tantamount to saying "you need a working C development toolchain and all the libraries and headers those C extensions are going to link against, and you better hope your system has working versions of all that, which sometimes it doesn't."
Using pre-built binary distributions of the C extensions generally
removes a world of installation pain for people using your
code, and that means you can't use
--no-site-packages, you can't depend on entry points working properly. For example, http://trac.pythonpaste.org/pythonpaste/ticket/458 https://github.com/pypa/virtualenv/issues/124 https://github.com/pypa/pip/issues/17
--no-site-packages, you can't depend on console scripts working properly, because these rely on entry points.
Now, I can now understand what's going on in those cases, and work around it. Unfortunately I am often distributing my software to people who are not python experts. Good luck explaining any of the above.
I wonder if
zc.buildout has these or similar issues. I have no doubt
that it gives the software distributor more power and more control,
but it's also a hell of a lot more kool-aid to swallow, and I've never
been able to bring myself to do it.
Meanwhile there are workarounds to the entry point / console script problems. If you specify a URL instead of a package version in a pip requirements file, it will get installed in your virtualenv. Checking out editable source works too. For example, if you depend on PasteScript 1.7.3:
# This will only get installed locally if it's *not* installed # globally. Yikes. pastescript==1.7.3 # This will definitely get installed. http://pypi.python.org/packages/source/P/PasteScript/PasteScript-1.7.3.tar.gz # So would this (note the @1.7.3 syntax to request a tag or branch) -e hg+https://firstname.lastname@example.org#egg=PasteScript-1.7.3
Caveat: I haven't done this in a long time but I vaguely recall that
--editable) is different in at least one
important way - it fails to install console scripts in the installed package.