ImmutableDict may fail to maintain iteration order on non-CPython implementations of Python 3.6

[gabbard]

51 made me notice our current code loses the ImmutableDict ordering guarantee on any 3.6 implementations which do not guarantee deterministic dict ordering in general because if we initialize from an Iterable of key-value pairs, we eventualy just pass the iterable directly to dict.

My inclination is to crash on module load in the same circumstances we currently crash when trying to initialize from a dict. That will prevent inadvertent bugs. If someone needs to run on a non-CPython 3.6 implementation, they can add the alternate implementation which maintains ordering. @ConstantineLignos ?

tag @berquist

[berquist]

Is it as simple as changing https://github.com/isi-vista/immutablecollections/blob/6985a25ffcd34bccc59ed423aceef257cdc681f5/immutablecollections/_immutabledict.py#L59 to

if not DICT_ITERATION_IS_DETERMINISTIC

? There should probably be a keyword argument to bypass this check (like for ImmutableSet).

[gabbard]

@berquist : Aha, yes, you are correct (on both counts)

[ConstantineLignos]

This all sounds good, just make sure that if that if is changed the error messages change accordingly.

[ConstantineLignos]

Wait, maybe I'm missing something, isn't this entire package supposed to be 3.6+? From setup.py:

      # 3.6 and up, but not Python 4
      python_requires='~=3.6',

[ConstantineLignos]

Sorry, I think was mixed up by the title. @rgabbard am I right that the issue is not about "Python < 3.6" as the title says, but only for non-CPython 3.6 where dicts are not guaranteed to have deterministic iteration order?

[gabbard]

@ConstantineLignos : Correct - apologies for the incorrect title