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Re: Is there a canonical way to check whether an iterable is ordered?

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On 9/18/2014 8:58 AM, Roy Smith wrote:

> I suspect what he meant was "How can I tell if I'm iterating over an
> ordered collection?", i.e. iterating over a list vs. iterating over a
> set.

One can check whether the iterable is a tuple, list, range, or tuple or 
list iterator (the latter not being re-iterable).

 >>> type(iter([]))
<class 'list_iterator'>
 >>> type(iter(()))
<class 'tuple_iterator'>

> Is there anything which requires an iterator to be deterministic?

No. An iterator can yields random number, input from a non-deterministic 
source -- human or mechanical, or items from a collection in shuffled 
order.  Generator that do such can easily be turned into the __iter__ 
method of a class.

 > For example, let's say I have an iterable, i, and I do:
>
> list1 = [item for item in i]
> list2 = [item for item in i]

If i is an iterator or other non-reiterable, list2 will be empty.
If i is an instance of a class with a non-deterministic __iter__ method, 
list2 will not necessarily be either empty or a copy of list1.

> am I guaranteed that list1 == list2?

Clearly not.

 > 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
> astonishment?

There should not be any astonishment.  'Iterable' is a much broader 
category than 'deterministically re-iterable iterable'.

-- 
Terry Jan Reedy

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Thread

Is there a canonical way to check whether an iterable is ordered? cool-RR <ram.rachum@gmail.com> - 2014-09-18 04:55 -0700
  Re: Is there a canonical way to check whether an iterable is ordered? Chris Angelico <rosuav@gmail.com> - 2014-09-18 22:10 +1000
    Re: Is there a canonical way to check whether an iterable is ordered? Roy Smith <roy@panix.com> - 2014-09-18 08:58 -0400
      Re: Is there a canonical way to check whether an iterable is ordered? Chris Angelico <rosuav@gmail.com> - 2014-09-18 23:33 +1000
        Re: Is there a canonical way to check whether an iterable is ordered? Roy Smith <roy@panix.com> - 2014-09-18 19:52 -0400
          Re: Is there a canonical way to check whether an iterable is ordered? Chris Angelico <rosuav@gmail.com> - 2014-09-19 12:45 +1000
          Re: Is there a canonical way to check whether an iterable is ordered? Terry Reedy <tjreedy@udel.edu> - 2014-09-19 18:02 -0400
          Re: Is there a canonical way to check whether an iterable is ordered? Chris Angelico <rosuav@gmail.com> - 2014-09-20 15:01 +1000
      Re: Is there a canonical way to check whether an iterable is ordered? Terry Reedy <tjreedy@udel.edu> - 2014-09-18 09:46 -0400
      Re: Is there a canonical way to check whether an iterable is ordered? Tim Chase <python.list@tim.thechases.com> - 2014-09-18 09:32 -0500
      Re: Is there a canonical way to check whether an iterable is ordered? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-09-19 15:15 +1000
        Re: Is there a canonical way to check whether an iterable is ordered? Chris Angelico <rosuav@gmail.com> - 2014-09-19 15:40 +1000
          Re: Is there a canonical way to check whether an iterable is ordered? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-09-19 20:59 +1000
            Re: Is there a canonical way to check whether an iterable is ordered? Chris Angelico <rosuav@gmail.com> - 2014-09-19 21:19 +1000
              Re: Is there a canonical way to check whether an iterable is ordered? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-09-19 21:58 +1000
                Re: Is there a canonical way to check whether an iterable is ordered? Chris Angelico <rosuav@gmail.com> - 2014-09-19 22:06 +1000
            Re: Is there a canonical way to check whether an iterable is ordered? Chris Angelico <rosuav@gmail.com> - 2014-09-19 21:25 +1000
              Re: Is there a canonical way to check whether an iterable is ordered? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-09-19 21:46 +1000
                Re: Is there a canonical way to check whether an iterable is ordered? Chris Angelico <rosuav@gmail.com> - 2014-09-19 21:56 +1000
                Re: Is there a canonical way to check whether an iterable is ordered? alister <alister.nospam.ware@ntlworld.com> - 2014-09-19 12:26 +0000
                Re: Is there a canonical way to check whether an iterable is ordered? Chris Angelico <rosuav@gmail.com> - 2014-09-19 22:36 +1000
  Re: Is there a canonical way to check whether an iterable is ordered? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-09-19 15:04 +1000

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