A note on migrating python virtual environments

The engine that builds this blog is called Nikola it's written in Python. I write the blog and run the Nikola compiler to build the site inside a Python virtual environment.

Up until recently I was doing this work on a machine running the latest Arch Linux and Python 2.7.8, but I have recently rebuilt that machine to run Debian 8, whose default Python interpreter is currently at version 2.7.6 and I copied my virtual environments to the new OS install.

This caused me a problem with my virtual environments that manifested itself as the following error when I tried to run any Python code from within the active virtual environment:

$ pip freeze
Traceback (most recent call last):
  File "/home/jonasg/.virtualenvs/tso/bin/pip", line 7, in <module>
    from pip import main
  File "/home/jonasg/.virtualenvs/tso/local/lib/python2.7/site-packages/pip/__init__.py", line 11, in <module>
    from pip.vcs import git, mercurial, subversion, bazaar  # noqa
  File "/home/jonasg/.virtualenvs/tso/local/lib/python2.7/site-packages/pip/vcs/mercurial.py", line 9, in <module>
    from pip.download import path_to_url
  File "/home/jonasg/.virtualenvs/tso/local/lib/python2.7/site-packages/pip/download.py", line 2, in <module>
    import email.utils
  File "/usr/lib/python2.7/email/utils.py", line 32, in <module>
    from email._parseaddr import quote
  File "/usr/lib/python2.7/email/_parseaddr.py", line 16, in <module>
    import time, calendar
  File "/usr/lib/python2.7/calendar.py", line 9, in <module>
    import datetime
ImportError: No module named datetime

$ nikola build
...
ImportError: No module named datetime

This happens because my virtual environments had a different version of /usr/bin/python2.7 (the one from Arch Linux) that — unlike the new binary — does not include the datetime built-in, and therefore generates an error when it cannot find it on disk anywhere. The new interpreter seems to import it without any file I/O (try running it under strace to check).

The fix for this issue is to activate the virtual environment you are having problems with and run the following:

$ cp /usr/bin/python2.7 $(which python2.7)

There are other ways to fix this issue, but I like the one above rather than re-initializing the virtualenv because I use virtualenvwrapper which does NOT gererate the virtual environment within the present directory.

Hope this helps!