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

Terry Reedy tjreedy at
Thu Sep 18 15:46:29 CEST 2014

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

More information about the Python-list mailing list