[Zope] Global variables in ExternalMethod modules

[Ben Last (Zope)]

Dear Zopistas

I'm trying to work out what the rules are for global (that is, 
module-level) variables in a module that contains external methods.

The scenario: we have a single module (.py file) in the Extensions 
subdirectory of the instance directory, which contains all our external 
methods plus many helper methods.  Amongst other things, this module 
contains a lot of methods that perform database queries to an associated 
MySQL database.  The same module is also used by non-Zope Python tasks, 
so the queries are all direct, not via Zope SQL products.

As part of the ongoing optimization of the code, I've been looking at 
having a single MySQLdb connection per Zope thread, and re-using this 
for all SQL queries.  My understanding was that if I declare a 
connection variable at module level, like so:

databaseConnection = None

I could then initialize it in a method that's called at the start of 
handling every request, but re-use the value between requests.  However, 
it *appears* that the value of the databaseConnection variable is 
(nearly) always None when any new request starts, as though the module 
were being reloaded afresh for every one.  Thus there seems to be no way 
to keep per-thread module-level variables.

This is on our test platform, where the Zope instance is running in 
debug-mode.

We've also noticed an odd connection leak on our production server, 
where Zope is not running in debug-mode.  It *appears* as though the 
__del__ methods of MySQLdb.Connection objects are not being called (and 
thus the connections are remaining open indefinitely).  We've recently 
started wrapping Connection objects in a class of our own, and tracing 
__init__ and __del__ calls for that also appears to bear out that theory.

All this on Zope 2.7.3, Python 2.3.4, Linux.

So; is there a quick-n-easy summary of how module-level variables behave 
in ExtensionMethod modules?  Is there any reason that, in a non-debug 
environment, the __del__ methods of simple Python objects may not be called?

Regards to all

Ben Last