Major Description
The documentation for req.add_handler() says:
"""Optional dir is a string containing the name of the directory to be added to the pythonpath. If no directory is specified, then, if there is already a handler of the same type specified, its directory is inherited, otherwise the directory of the presently executing handler is used. I there is a PythonPath directive in effect, then sys.path will be set exactly according to it (no directories added, the dir argument is ignored)."""
This comment about the directory being inherited from the prior or currently executing handler is actually bogus as the code does not do anything specific at all to try and implement such behaviour. If it works this way at all it is partly by luck as what will actually dictate where the module specified to the req.add_handler() method is found is the current order of directories specified in sys.path. Since additional directories added into sys.path by the old importer can be performed in effectively random order, behaviour could actually be quite random if the same module name were used in multiple directories.
Because the new importer doesn't add directories into sys.path for Python*Handler directives, a problem will currently arise if no directory is supplied to req.add_handler(). Specifically, a module may not be able to be found. This is because it can no longer fall back on to fact that with old module importer, the directory corresponding to the Python*Handler directive would be listed in sys.path somewhere.
Thus, the documented behaviour for req.add_handler() when the directory hasn't been set needs to actually be implemented as described with an appropriate directory being calculated at the time that req.add_handler() is called with that directory being recorded as needing to be searched for the module. In changing the code though, if old and new importers are going to be supported during a transition phase, it must detect when the new module importer is being used and only do this when it is, as otherwise it will screw up how modules are found for the old importer.