[Patches] [ python-Patches-553171 ] optionally make shelve less surprising
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Tue, 07 May 2002 09:38:00 -0700
Patches item #553171, was opened at 2002-05-07 10:13
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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: Martin v. Löwis (loewis)
Date: 2002-05-07 18:38
Message:
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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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