[Python-Dev] performance of {} versus dict(), de fmd(**kw): return kw trumps all ; -)

[Chris Withers]

On 14/11/2012 10:11, martin at v.loewis.de wrote:
>
> Zitat von Chris Withers <chris at simplistix.co.uk>:
>
>> a_dict = dict(
>>     x = 1,
>>     y = 2,
>>     z = 3,
>>     ...
>>     )
>
>> What can we do to speed up the former case?
>
> It should be possible to special-case it. Rather than creating
> a new dictionary from scratch, one could try to have the new dictionary
> the same size as the original one, and copy all entries.

Indeed, Doug, what are your views on this? Also, did you have a 
real-world example where this speed difference was causing you a problem?

> I don't know how much this would gain, though. You still have to
> create two dictionary objects. For a better speedup, try
>
> def xdict(**kwds):
>    return kwds

Hah, good call, this trumps both of the other options:

$ python2.7 -m timeit -n 1000000 -r 5 -v 
"{'a':1,'b':2,'c':3,'d':4,'e':5,'f':6,'g':7}"
raw times: 1.45 1.45 1.44 1.45 1.45
1000000 loops, best of 5: 1.44 usec per loop
$ python2.6 -m timeit -n 1000000 -r 5 -v 'dict(a=1,b=2,c=3,d=4,e=5,f=6,g=7)'
raw times: 2.37 2.36 2.36 2.37 2.37
1000000 loops, best of 5: 2.36 usec per loop$ python2.6 -m timeit -n 
1000000 -r 5 -v 'def md(**kw): return kw; md(a=1,b=2,c=3,d=4,e=5,f=6,g=7)'
raw times: 0.548 0.533 0.55 0.577 0.539
1000000 loops, best of 5: 0.533 usec per loop

For the naive observer (ie: me!), why is that?

Chris

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