On Nov 8, 2:42 pm, Mick Krippendorf <mad.m...@gmx.de> wrote: > Wells wrote: > > I'm not quite understanding why a tuple is hashable but a list is not. > > The short answer has already been given. Here is the long answer: > > For objects p and q, p==q implies hash(p)==hash(q). It is essential for > dicts and sets that objects used as keys/elements uphold this law, and > also that, for an object o, hash(o) doesn't change during the lifetime > of o. What if you write a class that doesn't? Let's see: > > >>> class Thing(object): > > ... def __init__(self, value): > ... self.value = value > ... def __eq__(self, other): > ... return self.value == other.value > ... def __hash__(self): > ... return hash(self.value) > ... def __repr__(self): > ... return "Thing(%s)" % self.value > ... > > >>> thinglist = [Thing(i) for i in xrange(10)] > > >>> thingdict = dict((y,x) for x,y in enumerate(thinglist)) > > >>> print thingdict[thinglist[5]] > 5 > > >>> thinglist[5].value = 99 > > >>> print thingdict[thinglist[5]] > > Traceback (most recent call last): > File "<stdin>", line 1, in <module> > KeyError: Thing(99) > > What happened? __eq__ and __hash__ both depend on a mutable attribute, > and when that attribute was changed, their outcome changed as well. If a > Thing is stored as key in a dict and later it's value attribute is > changed, it cannot be found anymore. Too bad. > > BTW, this doesn't work either: > > >>> print thingdict[Thing(5)] > > Traceback (most recent call last): > File "<stdin>", line 1, in <module> > KeyError: Thing(5) > > wheras this works: > > >>> print thingdict[Thing(6)] > > 6 > > What has this got to do with lists and tuples? Well, two tuples x and y > are considered equal, iff: > > >>> all(a==b for a,b in zip(x,y)) > > Also, by the law above, hash(x)==hash(y). Since tuples are immutable, x > and y (and hash(x) and hash(y)) will be equal as long as they live. > > Lists have the same properties regarding equality, but are mutable. > If we'd use a list as key in a dict and append an element to the list, > __eq__ and __hash__ would return something different than before the > append. The list couldn't be found anymore, just like the instance of > the broken Thing class above. Lists are not hashable, because it would > lead to untraceable bugs otherwise, and it would confuse beginners. > > This also teaches a lesson on how to implement __eq__ and __hash__, if > you must. Just make sure your objects always do uphold the law above, > and do not change in respect to __hash__ during their lifetime. OTOH it > is possible to do otherwise, as long as you don't try to use these > objects as elements of a set or keys in a dictionary. But then, why > would you bother to implement your own __hash__ method? > > Regards, > Mick.
Thanks Mick - this was very enlightening! -- http://mail.python.org/mailman/listinfo/python-list
