[Python-Dev] Unload a module written in C

Fri Mar 25 03:03:40 CET 2011

Hi,
I am trying to understand why I am unable to unload my faulthandler
module (implemented in C). Antoine pointed me
_PyImport_FixupExtensionObject() comment which gave me a first clue:
 Modules which do support multiple initialization set their m_size
 field to a non-negative number (indicating the size of the
 module-specific state). They are still recorded in the extensions
 dictionary, to avoid loading shared libraries twice.
Ok, so I changed the size from -1 to 0, and so the m_free callback was
called at exit. Nice. This is thanks to PyImport_Cleanup() which clears
my module attributes.
--
But if I do
 import faulthandler
 del sys.modules['faulthandler']
 del faulthandler
the module is never unloaded. There is another secret reference in the
interpreter state: state->modules_by_index. This list is cleared at exit
(by PyInterpreterState_Clear), but not my module attributes, and some of
them are functions pointing to the module.
My module attribute are not cleared at exit because PyImport_Cleanup()
clears only modules from sys.modules, and my module is no more
referenced in sys.modules.
The workaround to unload the module is to explicitly clear its
attributes:
 import faulthandler
 del sys.modules['faulthandler']
 faulthandler.__dict__.clear()
 del faulthandler
--
Is there a bug somewhere, or do I misunderstood something important?
Note: I implemented m_traversal, but it is not revelant here (you can
consider that my module only contains functions).
Victor