Python: get item from set based on key

Question

I have a class with a custom hash method.

class Test(object):

    def __init__(self, key, value):
        self.key = key # key is unique
        self.value = value

    def __hash__(self):
        # 'value' is unhashable, so return the hash of 'key'
        return hash(self.key)

I make a set using objects of this class.

t0, t1, t2 = Test(0, 10), Test(1, 5), Test(2, 10)
s = set([t0, t1, t2])

Now, is there any way to find objects from s using key ? ie I want to do:

find_using_key(s, 1) # should return [t1]

I know I can do this by iterating over the items in the set, but I feel like there should be an O(1) method to do this since key effectively determines the 'position' in the set .

Answer 1

... since key effectively determines the 'position' in the set

That's not really true. Two elements with the same key can coexist in the set:

>>> t0, t1 = Test(1,1), Test(1,2)
>>> len(set((t0,t1)))
2

The hash value does not define equality. That would also not be possible, because you can have hash collisions.

Now as for your question: Don't use a set . It is defined by an abstract interface with the operations insert and find . It does not provide the operation you want. Whether a potential underlying implementation could theoretically support the operation you want is not really relevant. Instead, use a dict , and associate the keys with the instances.

Answer 2

If you don't need to use sets, you can get O(1) lookup using dict. Just use your key as the key for the dict items:

d = {}
d[t0.key] = t0
d[t1.key] = t1
d[t2.key] = t2

You can use dict comprehension to make it cleaner:

d = {t.key: t for t in [t0,t1,t2]}

or in 2.6:

d = dict((t.key,t) for t in [t0,t1,t2])