> That's because dictionaries are not stored sequentially and the
> order of retrieval is not guaranteed - it can even change
> during the execution of a program so you should never
> depend on it. That's because dictionaries are optimised
> for random access via the keys not to be iterated over.
Moreover, most implementations *deliberately* randomize their iteration
order to avoid a particular kind of hash collision attack out there in
the wild. See:
https://en.wikipedia.org/wiki/Collision_attack
https://arstechnica.com/business/2011/12/huge-portions-of-web-vulnerable-to-hashing-denial-of-service-attack/
https://mail.python.org/pipermail/python-dev/2011-December/115116.html
for some details.
> I've a vague memory that recent versions of Python may have
> a special dictionary type that does return items in the order
> they were inserted, but I may be mixing that up with
> another language... Hopefully someone else can provide
> a steer there.
In Python, it's collections.OrderedDict:
https://docs.python.org/3.6/library/collections.html#collections.OrderedDict
Other languages have similar libraries. Java has LinkedHashMap, for example:
https://docs.oracle.com/javase/8/docs/api/java/util/LinkedHashMap.html
Best of wishes!
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