Question

When you call dict.values() the order of the returned items is dependent on the has value of the keys. This seems to be very consistent in all versions of cPython, however the python manual for dict simply states that the ordering is “arbitrary”.

I remember reading somewhere that there is actually a PEP which specifically states the expected ordering of the items() and values() methods.

FYI, if this behavior is indeed a guaranteed behavior of a class I am working on I could greatly simplify and speed up a class I am working on. On the other hand if this is merely an accidental and undocumented feature of cPython then it’s probably best not to trust it.

Answer 1

I suppose PEP-3106 is as close as it gets:

The specification implies that the order in which items are returned by
.keys(), .values() and .items() is the
same (just as it was in Python 2.x),
because the order is all derived from
the dict iterator (which is presumably
arbitrary but stable as long as a dict
isn’t modified). This can be expressed
by the following invariant:

list(d.items()) == list(zip(d.keys(),
d.values()))