[Python-Dev] Repeatability of looping over dicts (original) (raw)

A.M. Kuchling amk at amk.ca
Fri Jan 4 20:50:23 CET 2008

This post describes work aimed at getting Django to run on Jython: http://zyasoft.com/pythoneering/2008/01/django-on-jython-minding-gap.html

One outstanding issue is whether to use Java's ConcurrentHashMap type to underly Jython's dict type. See <http://java.sun.com/j2se/1.5.0/docs/api/java/util/concurrent/ConcurrentHashMap.html>.

ConcurrentHashMap scales better in the face of threading because it doesn't lock the whole table when updating it, but iterating over the map can return elements in a different order each time. This would mean that list(dict_var) doesn't return values in the same order as a later call to list(dict_var), even if dict_var hasn't been modified.

Why? Under the hood, there are 32 different locks, each guarding a subset of the hash buckets, so if there are multiple threads iterating over the dictionary, they may not go through the buckets in order. <http://www.ibm.com/developerworks/java/library/j-jtp08223/> discusses the implementation, at least in 2003.

So, do Python implementations need to guarantee that list(dict_var) == a later result from list(dict_var)?

--amk

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