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Fri Feb 17 18:52:23 CET 2012 · https://mail.python.org/pipermail/python-dev/2012-February/116610.html

 On 17 February 2012 at 17:42 Jim Jewett <jimjjewett at gmail.com> wrote:

> On Fri, Feb 17, 2012 at 1:50 AM, "Martin v. Löwis" <martin at v.loewis.de>
wrote:
> >>> Good idea. However, how do you track per-dict how large the
> >>> table is?
>
> [Or, rather, what is the highest index needed to store any values
> that are actually set for this instance.]
>
> > To determine whether it needs to grow the array, it needs to find out
> > how large the array is, no? So: how do you do that?
>
> Ah, now I understand; you do need a single ssize_t either on the dict
> or at the head of the values array to indicate how many slots it has
> actually allocated.  It *may* also be worthwhile to add a second
> ssize_t to indicate how many are currently in use, for faster results
> in case of len.  But the dict is guaranteed to have at least one free
> slot, so that extra index will never make the allocation larger than
> the current code.

The dict already has a field indicating how many items are in use,
the ma_used field.

Cheers,
Mark.

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[Python-Dev] PEP for new dictionary implementation