On May 24, 2015 2:03 AM, "Ralf Gommers" <ralf.gommers@gmail.com> wrote:
>
> On Sun, May 24, 2015 at 10:22 AM, Antony Lee <antony.lee@berkeley.edu> wrote:
>>
>> Hi,
>>
>> As mentioned in
>>
>> #1450: Patch with Ziggurat method for Normal distribution
>> #5158: ENH: More efficient algorithm for unweighted random choice without replacement
>> #5299: using `random.choice` to sample integers in a large range
>> #5851: Bug in np.random.dirichlet for small alpha parameters
>>
>> some methods on np.random.RandomState are implemented either non-optimally (#1450, #5158, #5299) or have outright bugs (#5851), but cannot be easily changed due to backwards compatibility concerns.  While some have suggested new methods deprecating the old ones (see e.g. #5872), some consensus has formed around the following ideas (see #5299 for original discussion, followed by private discussions with @njsmith):
>>
>> - Backwards compatibility should only be provided to those who were explicitly instantiating a seeded RandomState object or reseeding a RandomState object to a given value, and drawing variates from it: using the global methods (or a None-seeded RandomState) was already non-reproducible anyways as e.g. other libraries could be drawing variates from the global RandomState (of which the free functions in np.random are actually methods).  Thus, the global RandomState object should use the latest implementation of the methods.
>
>
> The rest of the proposal looks good to me, but the reasoning on this point is shaky. np.random.seed() is *very* widely used, and works fine for a test suite where each test that needs random numbers calls seed(...) and is run with nose. Can you explain why you need to touch the behavior of the global methods in order to make RandomState(version=) work?

You're absolutely right about it being important to preserve the behavior of the global functions when seeded, but I think this is just a bug in the description of the proposal here, not in the proposal itself :-).

If you look at the PR, there's no change to how the global functions work -- they're still just a transparently thin wrapper around a hidden, global RandomState object, and thus IIUC changes to RandomState will automatically apply to the global functions as well.

So with this proposal, an unseeded RandomState uses the latest version -> therefore the global functions, which start out unseeded, start out using the latest version. If you call .seed() on an existing RandomState object and pass in a seed but no version= argument, the version gets reset to 0 -> therefore if you call the global seed() function and pass in a seed but no version= argument, the global RandomState gets reset to version 0 (at least until the next time seed() is called), and backcompat is preserved.

-n