hashkey/digest for a complex object

geremy condra debatem1 at gmail.com
Wed Oct 6 21:28:30 CEST 2010


On Wed, Oct 6, 2010 at 11:58 AM, kj <no.email at please.post> wrote:
>
>
> The short version of this question is: where can I find the algorithm
> used by the tuple class's __hash__ method?

>From Objects/tuple.c, line 315 in Python3.2:

static long
tuplehash(PyTupleObject *v)
{
    register long x, y;
    register Py_ssize_t len = Py_SIZE(v);
    register PyObject **p;
    long mult = 1000003L;
    x = 0x345678L;
    p = v->ob_item;
    while (--len >= 0) {
        y = PyObject_Hash(*p++);
        if (y == -1)
            return -1;
        x = (x ^ y) * mult;
        /* the cast might truncate len; that doesn't change hash stability */
        mult += (long)(82520L + len + len);
    }
    x += 97531L;
    if (x == -1)
        x = -2;
    return x;
}

> Now, for the long version of this question, I'm working with some
> complext Python objects that I want to be able to compare for
> equality easily.
>
> These objects are non-mutable once they are created, so I would
> like to use a two-step comparison for equality, based on the
> assumption that I can compute (either at creation time, or as needed
> and memoized) a hashkey/digest for each object.  The test for
> equality of two of these objects would first compare their hashkeys.
> If they are different, the two objects are declared different; if
> they match, then a more stringent test for equality is performed.
>
> So the problem is to come up with a reasonable hashkey for each of
> these objects.  The objects have two significant attributes, and
> two of these objects should be regarded as equal if these attributes
> are "the same" in both.  The first attribute is a simple dictionary
> whose keys are integers and values are strings.  The second attribute
> is more complicated.  It is a tree structure, represented as a
> dictionary of dictionaries of dictionaries... until we get to the
> leaf elements, which are frozensets of strings.  The keys at every
> level of this structure are strings.  E.g. a simple example of such
> an attribute would look like:
>
> {'A': {'a': set(['1', '2', '3']),
>       'b': set(['4', '5'])},
>  'B': set(['6', '7', '8'])}
>
> I'm looking for a good algorithm for computing a hash key for
> something like this?  (Basically I'm looking for a good way to
> combine hashkeys.)

Sounds like you're trying to hash mutable data structures, which is a
no-no, but assuming you've got that part all figured out then the easy
way out would be to print the whole thing and take the hash of the
resulting string. A (possibly) better way would be to do a Schwartzian
transform[1] and freeze everything. Other approaches may be better,
but those are the first two out of my head.

Geremy Condra

[1] http://en.wikipedia.org/wiki/Schwartzian_transform



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