This is an old one that has just biten again and cost a
lot of debugging time again: the dict lookup function
swallows all errors during key comparison.  This is bad
in itself, but if the current stack depth is just wrong
then *any* comparison will raise a RuntimeError and
lookup will pretend that the dictionary is essentially
empty.  The attached sample program shows this and
crashes with a KeyError instead of a RuntimeError.

While at the C level there is a basic compatibility
problem involved (PyDict_GetItem() not allowed to raise
any exception -- see
http://thread.gmane.org/gmane.comp.python.devel/62427),
I think it should be possible to improve the situation
on the Python interface.  Unless someone points me to
something I missed, I plan to come up with a patch that
changes the internal dict functions (including
lookdict()) to let exceptions pass through, and have
the exceptions swallowed only by the PyDict_*() API
functions that require it.

The possible (remote) incompatibility here is existing
code relying on the exceptions being swallowed --
either Python code, or C code using PyObject_GetItem()
or PyMapping_GetItemString().  Such code deserves to be
shot down in my opinion.

Assigned to mwh for his feedback (not because I want
him to write the patch!).