...

ConcurrentHashMap scales better in the face of threading .. . So, do Python implementations need to guarantee that list(dict_var) == a later result from list(dict_var)?

What code would break if we loosened this restriction?

I can imagine someone has code like this: for k in d: print '%5s' % k for k in d: print '-----' for v in d.values(): print '%5s' % v It seems likely to me that a lot of code using d.values() would care about the order of the values and the only order that matters is corresponding to some set of keys. There are probably a lot of functions that take keys and values separately, so it would not be uncommon to call those with a pattern like: save_config(configfile, d.keys(), d.values()). In the OP's context where multiple threads are running, it may be fair to say that all bets are off. Raymond