[Python-Dev] Python 3.6 dict becomes compact and gets a private version; and keywords become ordered (original) (raw)

Victor Stinner victor.stinner at gmail.com
Mon Sep 12 13:00:46 EDT 2016

2016-09-12 18:35 GMT+02:00 Guido van Rossum <guido at python.org>:

Couldn't we use the order in the actual hash table (which IIUC now contains just indexes into the ordered vector of key/value/hash structs)? That would probably simulate the pre-3.6 order quite effectively.

From what I understood, Python 3.6 dict got two different changes:

The second change depends on the first change.

When a key is deleted, the entry is marked as DUMMY. When we add a new item, DUMMY entries are skipped and we only append at the end of the key/value table. Sometimes, the key/value table is compacted to free memory: all DUMMY entries are removed.

It would be possible to add a flag to allow to reuse DUMMY entries, which means loosing the order. The order would only be lost when we add the first item after we removed at least one entry (when the first DUMMY entry is reused).

The OrderedDict would set the flag to preserve the order.

So technically, it is possible. The question is more what should be the "default" dict :-) Ordered or not? :-)

Victor

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