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Date: Tue, 15 Apr 2014 17:07:50 -0700
Subject: Re: random.seed question (not reproducing same sequence)
From: Dan Stromberg <drsalists@gmail.com>
To: Nick Mellor <thebalancepro@gmail.com>
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Xref: csiph.com comp.lang.python:70305

You could easily provide your own random number generator, if you
don't need cryptographic-strength random numbers.

EG:
http://stromberg.dnsalias.org/svn/lcgrng/trunk/lcgrng.py

That way you can be certain nothing else is using it.


On Tue, Apr 15, 2014 at 8:54 AM, Nick Mellor <thebalancepro@gmail.com> wrot=
e:
> Hi guys,
>
> (Python 2.7, Windows 7 64-bit)
>
> Here's a bit of code stress-testing a method addUpdate_special_to_cart. T=
he test adds and updates random "specials" (multiple products bundled at an=
 advantageous price) of various sizes to thousands of shopping carts, then =
restocks the whole darn lot. The test passes if the stock level afterwards =
is the same as it was before executing the code for all products.
>
> addUpdate_special_to_cart is working perfectly. But the test isn't.
>
> The test iterates over the same code twice, once with special_qty=3D=3D4,=
 once with special_qty=3D=3D0, reseeding the Python random module number ge=
nerator to a fixed seed (a string) between the iterations. special_qty=3D=
=3D0 removes the special and restocks the products. The test relies on prec=
isely the same random number sequence on both runs.
>
> Can you think of a reason why the random number generator should fall out=
 of sync between the two iterations? Because that's what's happening by the=
 look of it: occasionally products are returned to the wrong stockbin. No "=
random" module method is used anywhere else while this code is executing.
>
> When I assign something non-random to the stockbin parameter, the test pa=
sses.
>
> Best wishes,
>
>
>
> Nick
>
> for qty in [4, 0]:
>                 random.seed(seed)
>                 for cart in range(test_size):
>                     for special in range(randrange(3)):
>                         s.addUpdate_special_to_cart(cart=3Dcart, stockbin=
=3Drandrange(test_size),
>                                                     special_id=3Drandrang=
e(test_size), special_qty=3Dqty,
>                                                     products=3D[(random.c=
hoice(PRODUCTS), random.choice(range(10)))
>                                                         for r in range(ra=
ndrange(7))])
> --
> https://mail.python.org/mailman/listinfo/python-list