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

Guido van Rossum guido at python.org
Fri Jan 4 23:54:49 CET 2008

On Jan 4, 2008 11:50 AM, A.M. Kuchling <amk at amk.ca> wrote:

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(dictvar) doesn't return values in the same order as a later call to list(dictvar), even if dictvar 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(dictvar) == a later result from list(dictvar)?

What code would break if we loosened this restriction? I guess defining d.items() as zip(d.keys(), d.values()) would no longer fly, but does anyone actually depend on this? Just like we changed how we think about auto-closing files once Jython came along, I think this is at least worth considering.

-- --Guido van Rossum (home page: http://www.python.org/~guido/)

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