I suggest to take a look at a D.learn thread:
http://forum.dlang.org/thread/morxorwjkjejowynjqkn@forum.dlang.org

There is a common need to have a (lazy) range of keys and their corresponding values of an associative array. Sometimes iterating them in parallel with a foreach is not enough, and performing a lookup for each key generated by byKeys is not efficient.

In Python2 the two lazy iterables of an associartive array given by iterkeys() and itervalues() are guaranteed to have the same order (Python doesn't allow you to modify the associative array while iterating on it).

In D the current implementation gives them in the same order (again if you don't modify the associative array in the meantime), because this is the most natural way to implement the algorithm. But as far as I know there is no guarantee this is always true.

To solve this problem I have suggested to introduce another AA pseudomethod, named byPair, that returns a lazy range of key-value tuples, similar to the dict.iteritems() method of Python2:

http://d.puremagic.com/issues/show_bug.cgi?id=5466

But Andrei has not introduced this enhancement probably because it requires to import the std.typecons module from the object.d, tying it even more to Phobos.

I think the D docs should be updated, to state that the byKey and byValue yield corresponding keys-values:
http://d.puremagic.com/issues/show_bug.cgi?id=8473

Bye,
bearophile