[Python-Dev] Unloading modules (regrtest.py mystery) (original) (raw)

M.-A. Lemburg mal@lemburg.com
Mon, 17 Dec 2001 22:25:56 +0100

Ka-Ping Yee wrote:

On Mon, 17 Dec 2001, Tim Peters wrote: > "A fix" seems to amount to treating > > import M > > as if it were > > try: > import M > except: > if M in sys.modules: > del sys.modules[M] > raise I remember suggesting exactly that some time ago (i think it was motivated at the time by the extreme pain that broken modules were causing for webserver-mode pydoc in its attempt to update loaded modules if the source files had changed on disk). Guido rejected it because you can't guarantee that the refcount on M is 1 at the point where you attempt to 'del sys.modules[M]' above. (For example, some other module imported by M could have imported M again, and so hold a reference to it while M is running its startup commands. This is why the entry is added to sys.modules before the body of M starts to run.) He deemed the situation where M is loaded-but-missing-from-sys.modules to be even worse than for M to be left loaded-but-broken therein. If you allow M to stay in sys.modules, then you can at least maintain the guarantee that there is a one-to-one association between loaded module names and loaded module objects. If you remove M from sys.modules but it lingers in memory, referenced elsewhere, you lose even that -- there can be many modules loaded all with the same name and it's a nightmare. The argument is compelling and i am forced to agree, but i still think that we should look for a better solution. I'll ponder this and post any ideas i come up with.

How about only deleting it from sys.modules iff the ref count is 1 ?!

Note that you cannot currently remove extension modules that have failed to import correctly. Would be nice if we could come up with a reasonable way of unloading modules, but I guess this requires some hard PEP work...

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