virtualenv -p python2.6 fails with ValueError: zero length field name in format

Note that upstream version 1.11.6 (installed via pip install --user virtualenv) works fine with all pythons from 2.6 to 3.4, as well as with pypy.

This is because in Python 2.6, an index was required inside the .format() field's brace placeholder:

% python2.6 -c "'{}'.format('foo')"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
ValueError: zero length field name in format

Python 2.7 relaxed that constraint so that its .format() syntax is compatible with Python 3.

Our virtualenv hack to use unvendored wheels requires the relaxed syntax.

What doesn't make sense though is why Python 2.6 is trying to execute code out of /usr/lib/python3/dist-packages/virtualenv.py. IIRC, virtualenv supports alternative Python's by re-execing itself using the other Python. This is borderline crazy and has caused other problems in the past. I also believe that upstream virtualenv was recently (or will recently be) rewritten to avoid this error-prone re-exec.

The fix of course would be simple, but it's questionable whether it's worth the effort given that Python 2.6 isn't supported on Ubuntu since Lucid, and we'd probably have to make the change first in Debian, which doesn't support Python 2.6 in Wheezy. Then there are the issues of SRU'ing those changes into the relevant Ubuntu releases.

Maybe a more tractable solution is to add a tweaked virtualenv to the PPA? Since you're getting Python 2.6 out of there, it wouldn't be much additional effort to put a compatible virtualenv in there too.

fwiw, i can verify that:
$ diff -u /usr/lib/python3/dist-packages/virtualenv.py.dist /usr/lib/python3/dist-packages/virtualenv.py
--- /usr/lib/python3/dist-packages/virtualenv.py.dist 2015-07-17 11:34:27.019820883 -0400
+++ /usr/lib/python3/dist-packages/virtualenv.py 2015-07-17 11:34:29.887856748 -0400
@@ -1013,7 +1013,7 @@
             raise
     for project in DEBIAN_WHEEL_DEPS:
         wheel_names = glob.glob(
- '/usr/share/python-wheels/{}-*.whl'.format(project))
+ '/usr/share/python-wheels/{0}-*.whl'.format(project))
         if len(wheel_names) == 0:
             raise RuntimeError('missing dependency wheel %s' % project)
         assert len(wheel_names) == 1, wheel_names

that makes virtualenv seemingly functional for me.
found that at
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=754248

This is a low impact change, so if it makes folks lives easier, I'm happy to apply it. No ETA on that atm, but +1 for making this change to wily and trusty.

 python-virtualenv (15.0.3+ds-3) unstable; urgency=medium
 .
   * d/patches/use-wheels.patch: Use a Python 2.6-friendly format string.
     (Closes: #754248)