In Python 3.2 and earlier:
>>> str(ImportError(b'foo'))
"b'foo'"

In Python 3.3:
>>> str(ImportError(b'foo'))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: __str__ returned non-string (type bytes)

This appears to be due to commit 76272.

I'm attaching a patch which fixes it, though perhaps ImportError should type-check the arguments

Motivation:
This leads to a failure of the test suite for "docutils" when run under Python 3.3 betas, specifically:

======================================================================
ERROR: test_unicode (test_error_reporting.ErrorStringTests)
----------------------------------------------------------------------
Traceback (most recent call last):
File "/home/david/coding/python3.3/docutils/trunk/docutils/test3/test_error_reporting.py", line 154, in test_unicode
str(ErrorString(ImportError(self.bs))))
File "/home/david/coding/python3.3/docutils/trunk/docutils/build/lib/docutils/utils/error_reporting.py", line 124, in __str__
super(ErrorString, self).__str__())
File "/home/david/coding/python3.3/docutils/trunk/docutils/build/lib/docutils/utils/error_reporting.py", line 74, in __str__
return str(self.data)
TypeError: __str__ returned non-string (type bytes)

See http://docutils.svn.sourceforge.net/viewvc/docutils/trunk/docutils/test/test_error_reporting.py?revision=7464&view=markup for the test code.  Arguably docutils could just pick a different exception subclass.

> This appears to be due to commit 76272.

Could you give the changeset id? Revision numbers are not portable from one repository clone to another.

Sorry, it's 6825fd9b00ed

(patch added)

New changeset 91909962d7f5 by Brett Cannon in branch 'default':
Issue #15778: Coerce ImportError.args to a string when it isn't
http://hg.python.org/cpython/rev/91909962d7f5