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

[Terry Reedy]

On 11/15/2012 9:58 AM, Stefan Behnel wrote:
> Greg Ewing, 15.11.2012 11:48:
>> martin at v.loewis.de wrote:
>>> It's faster than calling dict() because the dict code will
>>> create a second dictionary, and discard the keywords dictionary.
>>
>> Perhaps in the case where dict() is called with keyword
>> args only, it could just return the passed-in keyword
>> dictionary instead of creating another one?
>
> This should work as long as this still creates a copy of d at some point:
>
>      d = {...}
>      dict(**d)

I was thinking that CPython could check the ref count of the input 
keyword dict to determine whether it is newly created and can be 
returned or is pre-existing and must be copied. But it seems not so.

 >>> def d(**x): return sys.getrefcount(x)

 >>> import sys
 >>> d(a = 3)
2
 >>> d(**{'a': 3})
2
 >>> b = {'a': 3}
 >>> d(**b)
2

I was expecting 3 for the last one.

-- 
Terry Jan Reedy