[python-uk] memoize & ordering of kwargs.items()

[Jonathan]

On 11/11/2011 09:34, René Dudfield wrote:
> On Fri, Nov 11, 2011 at 10:23 AM, Jonathan <tartley at tartley.com 
> <mailto:tartley at tartley.com>> wrote:
>
>     That's good to know René, but I *think* it's orthogonal to the
>     question. Please correct me if I'm wrong.
>
>     If PyPy returns items in a different order than CPython, that
>     doesn't matter to me, so long as every invocation of my function
>     in a particular process ends up receiving a particular  order,
>     that doesn't change until the process ends (or the cache is cleared.)
>
>     The dict subclass is a great idea - but if the change in order
>     isn't ever manifested by a regular dict, then it implies to me
>     that the 'sorted' call isn't actually required in real life, so
>     the need for this whole test disappears.
>
>     See also my imminent reply to Ross.
>
>         Jonathan
>
>
>
> Just because your code works Now, does not mean it will work in the 
> future.  Especially when the ordering is explicitly marked as a 
> non-deterministic implementation detail.  Relying on it to be 
> deterministic when it is stated to be non-deterministic will mean that 
> in the future you could get a failure.
>
> Relying on implementation quirks is fine, but not reliable.  In your 
> case though, the worst that can happen now is a cache miss.  In the 
> future though, your cache could be completely useless because every 
> time the data could be in a different order.
>
> Testing on multiple platforms allows you to sometimes see a failure 
> condition, which lets you test it.  Likewise, using a randomised dict 
> subclass will let you test your code as if there is an error condition.
>
> cya.
>
> ps.  my randomised items subclass is buggy... I forgot that 
> random.shuffle works in place, and doesn't return a shuffled sequence.
>
>
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Hey René,
Thanks for that! I guess I was getting distracted because I was 
interpreting the situation as the fact that .items() for an *arbitrary* 
dictionary may well be non-deterministic, but in this particular case, 
for dictionaries newly-created by the '**' mechanism, they appear to be 
non-deterministic. I was forgetting that this is (presumably) also an 
implementation detail. Regardless, it sounds like you are probably right 
on all counts. Thanks for your enlightening thoughts.
Cheers,
     Jonathan

-- 
Jonathan Hartley    tartley at tartley.com    http://tartley.com
Made of meat.       +44 7737 062 225       twitter/skype: tartley


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