python persistence

[Gabriel Genellina]

En Tue, 01 Apr 2008 00:48:35 -0300, <castironpi at gmail.com> escribió:

> We do have, but on Windows, file is not locked to multi-tasking,
> shelve.  But why don't we have types in there, or non-string
> primitives in keys?
>
>>>> c= shelve.open( 'temp', 'c' )
>>>> c['0']= 'a'
>>>> c.sync()
>>>> del c
>>>> c= shelve.open( 'temp', 'c' )
>>>> c['0']
> 'a'
>
>>>> c['0'].append( 0 )
>>>> c['0']
> []

The above session doesn't make sense unless it's somewhat related to "on  
Windows, file is not locked to multi-tasking," and another process has  
modified the database in-between. I don't think the problem is restricted  
to Windows only. There exist file locking mechanisms.

> And why don't primitive mutations modify contents of disk?  A
> metaquestion.

They do, if you pass writeback=True to the shelve constructor, but read  
the docs.
shelve is a simple class; if it can't fulfill your needs, you may want to  
use a relational database (perhaps with an ORM like SQLObjects or  
SQLAlchemy) or an object database like ZODB or Durus.

>>>> c['0']= type('None',(),{})
> Traceback (most recent call last):
> pickle.PicklingError: Can't pickle <class '__main__.None'>: it's not
> found as __main__.None

Don't do that then. Or use the available pickle hooks to customize how  
such classes may be pickled. All persistence mechanisms have limitations.

-- 
Gabriel Genellina