How can I create a dict that sets a flag if it's been modified

[sandravandale]

I can think of several messy ways of making a dict that sets a flag if
it's been altered, but I have a hunch that experienced python
programmers would probably have an easier (well maybe more Pythonic)
way of doing this.

It's important that I can read the contents of the dict without
flagging it as modified, but I want it to set the flag the moment I add
a new element or alter an existing one (the values in the dict are
mutable), this is what makes it difficult. Because the values are
mutable I don't think you can tell the difference between a read and a
write without making some sort of wrapper around them.

Still, I'd love to hear how you guys would do it.

Thanks,
-Sandra

[Paul Rubin]

Steve Holden <st***@holdenwe b.com> writes:

Make a subclass of dict, or an object containing a dictionary, that
has a special __setattr__ method that traps updates and sets that

/__setattr__/__setitem__/ ?

Yes, thinkographical error. Thanks.

[Mike Meyer]

>> here's one attempt. (I'm no expert, so wait for better :-)

>>> class ModFlagDict(dic t):

def __init__(self, *args, **kwargs):
super(ModFlagDi ct, self).__init__( *args, **kwargs)
self.modified = False
def __setitem__(sel f, key, value):
self.modified = True
super(ModFlagDi ct, self).__setitem __(key, value)

You also need to catch __delitem__.

You may need to catch pop, update, clear and setdefault, depending on
whether or not they call the __setitem__/__delitem__. Personally, I'd
catch them all and make sure the flag got set appropriately, because
whether or not they use those methods is an implementation detail that
may change in the future.

<mike
--
Mike Meyer <mw*@mired.or g> http://www.mired.org/home/mwm/
Independent WWW/Perforce/FreeBSD/Unix consultant, email for more information.