[Patches] [ python-Patches-553171 ] optionally make shelve less surprising

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Patches item #553171, was opened at 2002-05-07 08:13
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Category: Library (Lib)
Group: Python 2.2.x
Status: Open
Resolution: None
Priority: 5
Submitted By: Alex Martelli (aleax)
Assigned to: Nobody/Anonymous (nobody)
Summary: optionally make shelve less surprising

Initial Comment:
shelve has highly surprising behavior wrt modifiable
values:
    s = shelve.open('she.dat','c')
    s['ciao'] = range(3)
    s['ciao'].append(4)   # doesn't "TAKE"!

Explaining to beginners that s['ciao'] is returning a
temporary object and the modification is done on the
temporary thus "silently ignored" is hard indeed.  It
also makes shelve far less convenient than it could 
be (whenever modifiable values must be shelved).

Having s keep track of all values it has returned may
perhaps break some existing program (due to extra 
memory consumption and/or to lack of "implicit 
copy"/"snapshot" behavior) so I've made the 'caching' 
change optional and by default off.  However it's now 
at least possible to obtain nonsurprising behavior:
    s = shelve.open('she.dat','c',smart=1)
    s['ciao'] = range(3)
    s['ciao'].append(4)   # no surprises any more

I suspect the 'smart=1' should be made the default, 
but, if we at least put it in now, then perhaps we 
can migrate to having it as the default very slowly 
and gradually.


Alex


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>Comment By: Raymond Hettinger (rhettinger)
Date: 2002-05-09 18:43

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Nicely done!  The code is clean and runs in the smart mode 
without problems on my existing programs. I agree that the 
patch solves a real world problem.  The solution is clean, 
but a little expensive.

If there were a way to be able to tell if an entry had been 
altered, it would save the 100% writeback.  Unfortunately, 
I can't think of a way.

The docstring could read more smoothly and plainly.  Also, 
it should be clear that the cost of setting smart=1 is that 
100% of the entries get rewritten on close.

Two microscopically minor thoughts on the coding (feel free 
to disregard). Can some of the try/except blocks be 
replaced by something akin to 'if self.smart:'?  For the 
writeback loop, consider 'for k,v in cache.iteritems()' as 
it takes less memory and saves a lookup.

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Comment By: Martin v. Löwis (loewis)
Date: 2002-05-07 16:38

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Even more important than the backwards compatibility might
be the issue that it writes back all accessed objects on
close, which might be expensive if there have been many
read-only accesses.

So I think the option name could be also 'slow'; although
'writeback' might be more technical.

Also, I wonder whether write-back should be attempted if the
shelve was opened read-only.

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