[Python-Dev] Repeatability of looping over dicts

[A.M. Kuchling]

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