do-release-upgrade crashed with UnboundLocalError in doDistUpgradeFetching(): local variable 'e' referenced before assignment

e is set to None in line 1031 so I'm not certain how this happens.

I don't think it can -- with the current Raring or Quantal branch. Setting e = None on that line should do the trick, and that was added by cjwatson in revno 2.1.7 for 1:0.181. If they've got:

Package: ubuntu-release-upgrader-core 1:0.192.4

I can't see how they'd be hitting this. OTOH, note that the reference to 'e' on the current trunk branch is actually at line 1062, so it certainly seems like they are using an out of date version.

Downloading the source from https://launchpad.net/ubuntu/+source/ubuntu-release-upgrader/1:0.192.4 the line number for this error message is 1060 and as e is set to None before it is used.

Found it!

Here's the problem: in Python 3, the targets of except clauses are deleted from the current namespace after the exception is handled. This is so the circular references created during the exception handling are eliminated. This is detailed in the Python 3 language reference: http://docs.python.org/3/reference/compound_stmts.html#except

The tricky thing here is that `e = None` set before the try/except doesn't help because if one of the excepts get triggered, the equivalent of `del e` is executed, which removes it from the namespace and causes the UnboundLocalError. FWIW, the `del e` behavior does not occur in Python 2, so this wouldn't crash. It's certainly an odd corner case of the language.

The fix is to use a different name to capture the exception for later printing than is used in the except clause. Should be an easy fix and I'll update trunk. Attached is a boiled down example.

This bug was fixed in the package ubuntu-release-upgrader - 1:0.192.9

---------------
ubuntu-release-upgrader (1:0.192.9) raring; urgency=low

  [ Barry Warsaw ]
  * Fix UnboundLocalError caused by corner case in the Python 3 language
    spec. (LP: #1102593)

  [ Jonathan Riddell ]
  * Use correct argument for frontend in kubuntu-devel-release-upgrade

  [ Brian Murray ]
  * Remove cups-pdf from ForcedObsoletes in DistUpgrade.cfg (LP: #457221)
 -- Brian Murray <email address hidden> Thu, 18 Apr 2013 13:10:08 -0700