Difference between Python 2 and 3 for shuffle with a given seed

Elaborating on Martijn Pieters excellent answer and comments, and on this discussion, I finally found a workaround, which arguably doesn't answer my very question, but at the same time doesn't require deep changes. To sum up:

• random.seed actually makes every random function deterministic, but doesn't necessarily produces the same output across versions;
• setting PYTHONHASHSEED to 0 disables hash randomization for dictionaries and sets, which by default introduces a factor of non-determinism in Python 3.

So, in the bash script which launches the Python 3 tests, I added:

export PYTHONHASHSEED=0

Then, I temporarily changed my test functions in order to brute-force my way to an integer seed which would reproduces in Python 3 the results expected in Python 2. Lastly, I reverted my changes and replaced the lines:

seed(42)

by something like that:

seed(42 if sys.version_info.major == 2 else 299)

Nothing to brag about, but as the saying goes, sometimes practicality beats purity ;)

This quick workaround may be useful to somebody who wants to test the same stochastic code across different versions of Python!

In Python 3.2 the random module was refactored a little to make the output uniform across architectures (given the same seed), see issue #7889. The shuffle() method was switched to using Random._randbelow().

However, the _randbelow() method was also adjusted, so simply copying the 3.5 version of shuffle() is not enough to fix this.

That said, if you pass in your own random() function, the implementation in Python 3.5 is unchanged from the 2.7 version, and thus lets you bypass this limitation:

random.shuffle(l, random.random)

Note however, than now you are subject to the old 32-bit vs 64-bit architecture differences that #7889 tried to solve.

Ignoring several optimisations and special cases, if you include _randbelow() the 3.5 version can be backported as:

import random
import sys

if sys.version_info >= (3, 2):
    newshuffle = random.shuffle
else:
    try:
        xrange
    except NameError:
        xrange = range

    def newshuffle(x):
        def _randbelow(n):
            "Return a random int in the range [0,n).  Raises ValueError if n==0."
            getrandbits = random.getrandbits
            k = n.bit_length()  # don't use (n-1) here because n can be 1
            r = getrandbits(k)          # 0 <= r < 2**k
            while r >= n:
                r = getrandbits(k)
            return r

        for i in xrange(len(x) - 1, 0, -1):
            # pick an element in x[:i+1] with which to exchange x[i]
            j = _randbelow(i+1)
            x[i], x[j] = x[j], x[i]

which gives you the same output on 2.7 as 3.5:

>>> random.seed(42)
>>> print(random.random())
0.639426798458
>>> l = list(range(20))
>>> newshuffle(l)
>>> print(l)
[3, 5, 2, 15, 9, 12, 16, 19, 6, 13, 18, 14, 10, 1, 11, 4, 17, 7, 8, 0]