Most efficient property to hash for numpy array

I need to be able to store a numpy array in a dict for caching purposes. Hash speed is important.

The array represents indicies, so while the actual identity of the object is not important, the value is. Mutabliity is not a concern, as I'm only interested in the current value.

What should I hash in order to store it in a dict?

My current approach is to use str(arr.data), which is faster than md5 in my testing.

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I've incorporated some examples from the answers to get an idea of relative times:

In [121]: %timeit hash(str(y))
10000 loops, best of 3: 68.7 us per loop

In [122]: %timeit hash(y.tostring())
1000000 loops, best of 3: 383 ns per loop

In [123]: %timeit hash(str(y.data))
1000000 loops, best of 3: 543 ns per loop

In [124]: %timeit y.flags.writeable = False ; hash(y.data)
1000000 loops, best of 3: 1.15 us per loop

In [125]: %timeit hash((b*y).sum())
100000 loops, best of 3: 8.12 us per loop

It would appear that for this particular use case (small arrays of indicies), arr.tostring offers the best performance.

While hashing the read-only buffer is fast on its own, the overhead of setting the writeable flag actually makes it slower.

You can simply hash the underlying buffer, if you make it read-only:

>>> a = random.randint(10, 100, 100000)
>>> a.flags.writeable = False
>>> %timeit hash(a.data)
100 loops, best of 3: 2.01 ms per loop
>>> %timeit hash(a.tostring())
100 loops, best of 3: 2.28 ms per loop

For very large arrays, hash(str(a)) is a lot faster, but then it only takes a small part of the array into account.

>>> %timeit hash(str(a))
10000 loops, best of 3: 55.5 us per loop
>>> str(a)
'[63 30 33 ..., 96 25 60]'

You can try xxhash via its Python binding. For large arrays this is much faster than hash(x.tostring()).

Example IPython session:

>>> import xxhash
>>> import numpy
>>> x = numpy.random.rand(1024 * 1024 * 16)
>>> h = xxhash.xxh64()
>>> %timeit hash(x.tostring())
1 loops, best of 3: 208 ms per loop
>>> %timeit h.update(x); h.intdigest(); h.reset()
100 loops, best of 3: 10.2 ms per loop

And by the way, on various blogs and answers posted to Stack Overflow, you'll see people using sha1 or md5 as hash functions. For performance reasons this is usually not acceptable, as those "secure" hash functions are rather slow. They're useful only if hash collision is one of the top concerns.

Nevertheless, hash collisions happen all the time. And if all you need is implementing __hash__ for data-array objects so that they can be used as keys in Python dictionaries or sets, I think it's better to concentrate on the speed of __hash__ itself and let Python handle the hash collision[1].

[1] You may need to override __eq__ too, to help Python manage hash collision. You would want __eq__ to return a boolean, rather than an array of booleans as is done by numpy.

Coming late to the party, but for large arrays, I think a decent way to do it is to randomly subsample the matrix and hash that sample:

def subsample_hash(a):
    rng = np.random.RandomState(89)
    inds = rng.randint(low=0, high=a.size, size=1000)
    b = a.flat[inds]
    b.flags.writeable = False
    return hash(b.data)

I think this is better than doing hash(str(a)), because the latter could confuse arrays that have unique data in the middle but zeros around the edges.

If your np.array() is small and in a tight loop, then one option is to skip hash() completely and just use np.array().data.tobytes() directly as your dict key:

grid  = np.array([[True, False, True],[False, False, True]])
hash  = grid.data.tobytes()
cache = cache or {}
if hash not in cache:
    cache[hash] = function(grid)
return cache[hash]

What kind of data do you have?

• array-size
• do you have an index several times in the array

If your array only consists of permutation of indices you can use a base-convertion

(1, 0, 2) -> 1 * 3**0 + 0 * 3**1 + 2 * 3**2 = 10(base3)

and use '10' as hash_key via

import numpy as num

base_size = 3
base = base_size ** num.arange(base_size)
max_base = (base * num.arange(base_size)).sum()

hashed_array = (base * array).sum()

Now you can use an array (shape=(base_size, )) instead of a dict in order to access the values.