survey: is shelve broken? should it be fixed?

[Alex Martelli]

Just checking if I'm the only one to feel that shelve seriosly violates the 
principle of least astonishment when you shelve modifiable values:

[alex at lancelot Lib]$ python
Python 2.2.1 (#1, Apr 15 2002, 17:55:14)
[GCC 2.96 20000731 (Mandrake Linux 8.1 2.96-0.62mdk)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import shelve
>>> s=shelve.open('ciao','c')
>>> s['ciao']=range(4)
>>> s['ciao']
[0, 1, 2, 3]
>>> s['ciao'].append(23)
>>> s['ciao']
[0, 1, 2, 3]

The access to s['ciao'] returns a temporary and promptly forgets about
it.  The .append call is done on the temporary object and thus 'silently
ignored' from the point of view of the shelf.  I must have helped people
out of this trap a dozen times (plus, fallen into it myself two or three
times:-) -- Python so rarely violates the principle of least astonishment
that it's *really* astonishing when it does:-).

We probably can't change this default (mis-)behavior due to backward
compatibility needs, but it wouldn't be too hard to at least give a simple 
_optional_ way out, even though off-by-default:

>>> s.close()
>>> s=shelve.open('ciao','c',smart=1)
>>> s['ciao']
[0, 1, 2, 3]
>>> s['ciao'].append(23)
>>> s['ciao']
[0, 1, 2, 3, 23]
>>> s.close()
>>> s=shelve.open('ciao','c')
>>> s['ciao']
[0, 1, 2, 3, 23]
>>>

http://sourceforge.net/tracker/index.php?func=detail&aid=553171&group_id=5470&atid=305470

is a Q&D fix enabling this optional smart=1 parameter (doing it the RIGHT
way would no doubt involve weak references, but I didn't try that yet -- it
takes a bit more care because the values may or may not be weakly 
referenceable etc, I'd guess).

But, implementation issues apart, what do people think of the general issue
of functionality and interface to it?  Am I the only silly Pythonista who
worries about this, and everybody else once they've learned the trap just
deftly avoids shelving mutables or uses other workarounds (or eschews shelve
altogether I guess:-)...?

I think that having the 'smart=1' option in the docs would help people be
_aware_ that the trap exists, and thus would be beneficial anyway.  Maybe
we should just document the trap, but, as fixing it (at least optionally)
isn't that hard, that doesn't strike me as ideal.

Opinions welcome -- that's why I called this a 'survey'.  Thanks!


Alex