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From: Peter Otten <__peter__@web.de>
Subject: Re: random.seed question (not reproducing same sequence)
Date: Tue, 15 Apr 2014 19:34:41 +0200
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Xref: csiph.com comp.lang.python:70276

Nick Mellor wrote:

> Hi guys,
> 
> (Python 2.7, Windows 7 64-bit)
> 
> Here's a bit of code stress-testing a method addUpdate_special_to_cart.
> The 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==4, once
> with special_qty==0, reseeding the Python random module number generator
> to a fixed seed (a string) between the iterations. special_qty==0 removes
> the special and restocks the products. The test relies on precisely 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
> passes.
> 
> 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=cart,
>                         stockbin=randrange(test_size),
>                                                     
special_id=randrange(test_size),
>                                                     special_qty=qty,
>                                                     
products=[(random.choice(PRODUCTS),
>                                                     
random.choice(range(10)))
>                                                         for r in
>                                                         
range(randrange(7))])

An exotic option: I notice that randrange() is the only random function not 
qualified with the module name. If you are using a fancy auto-reloading web 
framework things can get out of sync:

>>> import random
>>> from random import randrange
>>> random.seed(42)
>>> [randrange(10) for _ in range(5)]
[6, 0, 2, 2, 7]
>>> random.seed(42)
>>> [randrange(10) for _ in range(5)]
[6, 0, 2, 2, 7]
>>> reload(random)
<module 'random' from '/usr/lib/python2.7/random.pyc'>
>>> random.seed(42)
>>> [randrange(10) for _ in range(5)]
[6, 8, 0, 4, 0]