Pickle vs XML for file I/O

[crystalattice]

Marc 'BlackJack' Rintsch wrote:
> In <op.tdk6cd0pqaun6z at emac.local>, crystalattice wrote:
>
> > On Mon, 31 Jul 2006 14:35:39 -1000, Simon Forman <rogue_pedro at yahoo.com>
> > wrote:
> >
> >> What kind of trouble were you having with pickle?
> >
> > It's mostly a combination of things (I hope you can follow my logic).
> > First, to use "good programming practice", I want to implement a
> > try/except block for opening the pickle file.  But I can't figure out if
> > this block should be included outside the classes, included in just the
> > base class, or if the base and subclasses need it; I'm leaning to putting
> > outside the classes as a global method.
>
> That's not a problem with pickle but where you think a potential IO error
> is handled best.  Ask yourself if you can do something sensible at the
> point where you catch an exception.  If you don't know what to do with it,
> don't catch it and just let it propagate.
>
> > When pickling a class instance, is the pickle statement placed within the
> > class (say, at the end of the class) or is it where the instance is
> > created, such as a test() method?
>
> `pickle.dump()` and `pickle.load()` are not statements but just functions.
> And you should place a call where you actually want to save or load
> instances in your program flow.
>
> > Plus, to modify data in a class, do I have to unpickle the whole thing
> > first or is there a way to modify the data while it's pickled?  Actually,
> > I think I can answer that last question:  a character instance, having
> > been created, will stay resident in memory until the player quits,
> > character dies, or otherwise is no longer needed.  At that point the
> > character instance should be pickled (if necessary) to disk; pickle
> > shouldn't be used while data modification is required and the class
> > instance is "active", correct?
>
> Yes that's correct.  It wouldn't be different with XML.
>
> > I also remember reading on a thread here that pickle sometimes has issues
> > when used with classes, which makes me hesitant to pickle an entire class
> > instance.  That's why I thought XML may be safer/better.
>
> You can't pickle everything.  Files for example are unpickleable and you
> can't pickle code so pickling classes is not that useful.
>
> An advantage of `pickle` is that it's in the standard library and ready to
> use for serialization.  To get an XML solution you must either write your
> own serialization code or use a 3rd party library.
>
> What are the problems you fear when using `shelve` by the way?
>
> Ciao,
> 	Marc 'BlackJack' Rintsch
The ideas I got about shelve are mostly due to this thread:
http://tinyurl.com/lueok.  There weren't any other threads
contradicting the information so I figured it has merit.  Actually, not
many people seem to use shelve very much; pickle is used more often so
I decided to give it a try and see how it works for me.