changes to shelf items fail silently?

Alex Martelli aleax at
Tue Mar 18 08:53:27 CET 2003

eichin at wrote:

>> They're mutating the returned list (to which you retain no
>> references, and which therefore is immediately discarded).
> Ah, so I'm missing the distinction that it's returning a list, and not
> being a reference to one.  (Knowable only by looking at the code to
> shelve, since the syntax hides it, right?)

Right.  I consider this a bug in shelve, myself.

>> That's pretty hard in general, since the shelve-like thing can't
>> know a priori what operations mutate an object and what operations
>> don't.  (It could make an effort to detect changes after the fact
> Or just pass back wrapped-objects, whose methods are "like" the input
> type but commit themselves on __setitem__ calls?

Unfortunately insufficient -- there are a zillion more ways to mutate
shelved objects in addition to __setitem__ calls.  For lists, you could
go through the list of operations and methods and religiously wrap
every single one that's a mutator.  But then you'd have to do it all
over again for dicts, and (the kicker...) for every mutable type (an
infinity thereof, mostly instances of classic and new-style classes).

> In the meantime, I've worked around it with explicit stores, but
> eventually will want something cleaner.

It _would_ be nice if there was a general way for objects to expose
an "I'm about to mutate" hook -- it would make such wrapping quite
feasible and also allow a general solution to the issue of making
immutable snapshots of something (wrap that thing with a wrapper that
raises a suitable exception when mutation would be about to happen).

Unfortunately, so far we have no design yet for such a feature:-(.


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