It is surprising though that the order was non-deterministic, not only undefined. Determinism is something I intuitively expect from read-only operations like iteration.
From a theoretical angle, the only way to build hash tables with some kind of provable runtime guarantees is to include randomness.
> Keys and values are iterated over in an arbitrary order which is non-random, varies across Python implementations, and depends on the dictionary’s history of insertions and deletions. If keys, values and items views are iterated over with no intervening modifications to the dictionary, the order of items will directly correspond.
That is, the order will be maintained between calls, so long as the dictionary had not been modified.
Going back to "str(some_dict) == str(some_dict)". I would not expect to always be the same, but for entirely different reasons. Consider:
class Strange():
def __repr__(self):
import random
return str(random.random())
some_dict = {1: Strange()}
>>> str(some_dict) == str(some_dict)
FalseI should have quoted the next section:
> If items(), keys(), values(), iteritems(), iterkeys(), and itervalues() are called with no intervening modifications to the dictionary, the lists will directly correspond.
Curiously, the "Dictionary view objects" section at https://docs.python.org/2.7/library/stdtypes.html#dictionary... has the same "Keys and values are iterated over in an arbitrary order which is non-random ..." text, but without being inside of a "CPython implementation detail" box.
I'd expect that any two ways of forming a dict with exactly same contents can result in different str(x) representation, and also that serializing and deserializing a dict can result in a different string representation.