Hi Michael,
Thanks for the reply.
On Thu, 19 May 2011 10:59:18 -0400, Michael Bayer <mike...@zzzcomputing.com>
wrote:
> I'm sure I mentioned earlier, one of the reasons you're finding this
> difficult is because of this type of pattern, where you're calling
> mapper() and Table at a different scope than where you make your
> classes i.e. within a callable versus module level. At the very
> least a function like make_tables() would ensure that its internal
> implementation is called only once, such as:
> _table_dict = None
> def make_tables():
> global _table_dict
> if _table_dict is not None:
> return _table_dict
> _table_dict = {.... }
> otherwise, you're creating new MetaData objects each time,
> rebuilding a who= le new set of Tables in it each time, i.e. its a
> total mess. Engine, Session registry, MetaData, your set of classes
> and their mappers, these should all be global objects, created once
> per application. If you want to use functions to initiate their
> existence that is fine but you should be maintaining singleton
> behavior in any case.
You make good points. This certainly makes sense if I am only dealing
with one set of database tables at a time. However, I'm working with
multiple datasets, sometimes within a single Python script. So I guess
some extra "wipe the slate clean" stuff when switching between
different datasets is needed here.
A couple of comments/questions.
I see you are using _table_dict as an indicator. I'm not sure what
_table_dict = {.... }
is intended to indicate. Do you just mean initialize _table_dict with
some non-None value before proceeding with the function?
So if I am switching data sets, I guess I would need to recreate the
tables, presumably by creating a new MetaData. Since the classes
don't change, they don't need to be removed. So, would it sufice to
have
*********************************************************************
do stuff with dataset 1
clear_mappers()
_table_dict = None # so the make_tables will be rerun and MetaData recreated
do stuff with dataset 2
*********************************************************************
Hmm. maybe clear_mappers should be inside make_tables?
Am I missing anything?
Faheem
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