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If a config_error is thrown in the C++ code, it gets turned into a Python exception via PyErr_SetString() [source:trunk/bindings/python/mapnik_python.cpp#L218]. But the class that is specified is PyExc_UserWarning, which means that in the python code it's raised as an error but is of a warning type. Confusing much?
Since config_error isn't actually a warning (ie. there are no alternative code paths), it should be changed into RuntimeError/ValueError/MapnikConfigError or something else that doesn't imply to the user that everything is okay.
[springmeyer] fixed in r1456, thanks rcoup.