Python dicts are now ordered

Re:Python dicts are now ordered by klyrs on Friday February 07, 2020 @11:10AM (#22267340)

> Code that ordered the dictionary keys before iterating are now slightly innefficient due to extra work of sorting a sorted list.

Dicts are ordered, specifically insertion-ordered, not sorted.

Sometimes they'll be sorted:

  D = {i:i for i in range(10)}

... specifically, when you insert them in sorted order. But then you can break the sortedness on your next insert:

  D[-1] = -1

What it allows is parallel iteration:

  zip(D.keys(), D.values())

now being synonymous with

  D.items()

This enables, nay, encourages people to write code that is very subtly broken in 3.5 and below.

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Re:Python dicts are now ordered by throwaway287391 on Friday February 07, 2020 @12:27PM (#22268376)

The property you mention at the end has been true of dicts since long before 3.5, at least since 2.7 if not forever. See https://docs.python.org/2/library/stdtypes.html#dict.items :

> If items(), keys(), values(), iteritems(), iterkeys(), and itervalues() are called with no intervening modifications to the dictionary, the lists will directly correspond. This allows the creation of (value, key) pairs using zip(): pairs = zip(d.values(), d.keys()).

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Re:Python dicts are now ordered by klyrs on Friday February 07, 2020 @01:10PM (#22268866)

Sorry, yes, I was hasty. Ordered dicts supports such a thing with intervening modifications:

  oldkeys = list(D.keys())
  D[z] = foo(z)

now if you take

  zip(oldkeys, D.values())

then you're guaranteed to iterate over oldkeys, with the proper values associated with those keys -- if z was an oldkey, its value got updated; otherwise, it comes after oldkeys and gets dropped out of zip.

The subtlety of this is what I, and perhaps others, find the most jarring.

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