[Zope] Subclassing from Custom Python Classes

[Pete Kazmier]

I need some guidance or pointers to more information.  My Zope journey
began last Thursday and I've been glued to zope.org ever since.  I'm
trying to understand the following paragraph from the Zope Developer's
Guide from the section on "Subclassing from Custom Python Classes":

>> You should also note that your subclass must obey the ZODB
>> persistence rules, which basically state that mutable sub-objects
>> should be treated immutably. For example,
>>
>> self.bird='parrot' # OK
>> self.map['bird']='parrot' # NOT OK, will not trigger persistence
>>                             machinery
>> m=self.map
>> m['bird']='parrot'
>> self.map=m # OK, treats a mutable object immutably

What are these "persistence rules" and how come the second example is
ok?  I thought in Python, when you assign "m = self.map" that m is
simply pointing to the same object as self.map and that any changes to
m affect self.map too?  Does the "self.map = m" have any effect?
Don't they point to the same object?

I ask because I've already built my first product based on a custom
python class I wrote (its a router object that has methods to query
various snmp oids) based on a ZClass and I want to make sure that I am
not doing anything wrong.

In addition to the methods that query the oids, I store the results in
an instance variable of my Python class so I can cache the information
for a specified amount of time.  Is this safe to do?  Can I store
these values in my Python instance (self.xxxx), not a ZClass property?

Also, What happens if multiple people activate my query method, do I
need to worry about them stomping over each other as the method writes
the results to the instance variables?  Is there any record locking of
objects?

Lastly, since I couldn't really find any pointers regarding a builtin
Zope scheduler that is bug free, I need to write a small python script
that I can place in a cron job to run every ten minutes to call the
query methods of these objects so they update their cache instances.
Is there a link on how to get a regular Python script access to the
ZODB?

Thanks in advance!!  Zope is great!!

Pete

-- 
Peter Kazmier                                 http://www.kazmier.com
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