I'm getting a crash when running coverage with django 1.4 under eclipse, i.e. under the debugger. At issue seems to be this line:
collectory.py:256: traces0 = getattr(tracer0, 'traces', None)
In my case, there appears to be no traces, so traces0 is None, but then we try to iterate over it in the next section:
(261): for args in traces0:
Seems the getattr should be getattr(tracer0, 'traces', ), instead of None.
However, it's also entirely possible that running under pydb is not supported.
File "/Library/Python/2.7/site-packages/coverage-3.5.3-py2.7-macosx-10.8-intel.egg/coverage/control.py", line 383, in start
File "/Library/Python/2.7/site-packages/coverage-3.5.3-py2.7-macosx-10.8-intel.egg/coverage/collector.py", line 261, in start
for args in traces0:
TypeError: 'NoneType' object is not iterable
Hmm, i don't know pydb, and it's likely that you can't debug with coverage running anyway, since both want to use trace functions. But, you are right, the default of  seems correct, and None seems bad.