Austin Bingham wrote:
Yes, I can construct a dict as you
specify, where all of the keys map to values with name attributes
equal to the key. My point is that dict doesn't really help me enforce
that beyond simply letting me set it up; it doesn't care about the
values at all, just the keys. All that I'm asking for, and I think
it's been made pretty clear, is a set that let's me define a
uniqueness criteria function other than hash(). As has been thrashed
out already, this is not as straightforward as I might have liked.
To put it in code, I want this:
s = set(hash_func = lambda obj: hash(obj.name), eq_func = ...)
...
x.name = 'foo'
y.name = 'foo'
s.add(x)
s.add(y) # no-op because of uniqueness criteria
assert len(s) == 1
Austin
I'm feeling really dense about now... What am I missing?
class obj(object):
def __init__(self, id, value):
self.id = id
self.value = value
def __eq__(self, other):
if isinstance(other, obj):
return self.id == other.id
raise NotImplemented
def __hash__(self):
return hash(self.id)
def __repr__(self):
return "obj(%s, %s)" % (self.id, self.value)
Python 2.5.4 (r254:67916, Dec 23 2008, 15:10:54) [MSC v.1310 32 bit
(Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
--> from obj import obj
--> s1 = set()
--> d1 = dict()
--> o1 = obj(17, 'first')
--> o2 = obj(18, 'second')
--> o3 = obj(17, 'third')
--> o1, o2, o3
(obj(17, first), obj(18, second), obj(17, third))
--> s1.add(o1)
--> s1.add(o2)
--> s1.add(o3)
--> d1[o1.id] = o1
--> d1[o2.id] = o2
--> d1[o3.id] = o3
--> s1
set([obj(17, first), obj(18, second)])
--> d1
{17: obj(17, third), 18: obj(18, second)}
-->
Ah ha! No-op indeed! If a set already has an item, the new item is
completely ignored, whereas if a dict already has a key, the new key's
value replaces the current value already in the dict.
Learn something new everyday!
I'm still not sure I understand your concern about the values in a set,
though. Sets keep the first object of a given key, dicts keep the last
object of a given key; in both cases, all other objects with the same
key are lost.
So is that the behavior you're wanting, keeping the first object and
discarding all others? Or is there something else I'm still missing?
~Ethan~
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