Python: Retrieve items from a set

Question

In general, Python sets don\'t seem to be designed for retrieving items by key. That\'s obviously what dictionaries are for. But is there anyway that, given a key, you can

Answer 1

I'd definitely use a dictionary here. Reusing the firstname instance variable as a dictionary key won't copy it -- the dictionary will simply use the same object. I doubt a dictionary will use significantly more memory than a set.

To actually save memory, add a __slots__ attribute to your classes. This will prevent each of you 10,000,000 instances from having a __dict__ attribute, which will save much more memory than the potential overhead of a dict over a set.

Edit: Some numbers to back my claims. I defined a stupid example class storing pairs of random strings:

def rand_str():
    return str.join("", (chr(random.randrange(97, 123))
                         for i in range(random.randrange(3, 16))))

class A(object):
    def __init__(self):
        self.x = rand_str()
        self.y = rand_str()
    def __hash__(self):
        return hash(self.x)
    def __eq__(self, other):
        return self.x == other.x

The amount of memory used by a set of 1,000,000 instances of this class

random.seed(42)
s = set(A() for i in xrange(1000000))

is on my machine 240 MB. If I add

    __slots__ = ("x", "y")

to the class, this goes down to 112 MB. If I store the same data in a dictionary

def key_value():
    a = A()
    return a.x, a

random.seed(42)
d = dict(key_value() for i in xrange(1000000))

this uses 249 MB without __slots__ and 121 MB with __slots__.