Is there a canonical way to check whether an iterable is ordered?
steve+comp.lang.python at pearwood.info
Fri Sep 19 07:15:16 CEST 2014
Roy Smith wrote:
> Is there anything which requires an iterator to be deterministic?
py> def spam():
... while True:
... n = random.randint(0, 10)
... s = ' '.join(['spam']*n)
... if not s:
... yield s + '!'
py> for s in spam():
spam spam spam spam spam spam!
spam spam spam spam spam spam spam spam spam!
spam spam spam spam spam spam spam!
> example, let's say I have an iterable, i, and I do:
> list1 = [item for item in i]
> list2 = [item for item in i]
Don't do that. Just write:
list1 = list(i)
list2 = list(i)
> am I guaranteed that list1 == list2?
However, as far as I am aware, there are no built-ins that will fail that
test, yet. Although the iteration order of dicts and sets is arbitrary, I
think that (at least to date) it will be the same order every time you
iterate over the dict or set within a single run of the Python interpreter.
(Provided the dict or set hasn't changed.)
That's not a language guarantee though. It's an implementation detail. In
principle, it could be different each time:
s = set("abcd")
=> returns ['d', 'a', 'b', 'c']
=> returns ['c', 'a', 'd', 'b']
> It will be for all the collections
> I can think of in the standard library, but if I wrote my own class with
> an __iter__() which yielded the items in a non-deterministic order,
> would I be violating something other than the principle of least
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