[Python-Dev] slightly inconsistent set/list pop behaviour

[John Barham]

Tennessee Leeuwenburg wrote:
> Now, I know that sets aren't ordered, but...
>
> foo = set([1,2,3,4,5])
> bar = [1,2,3,4,5]
>
> foo.pop() will reliably return 1
> while bar.pop() will return 5
>
> discuss :)

As designed.

If you play around a bit it becomes clear that what set.pop() returns
is independent of the insertion order:

PythonWin 2.5.2 (r252:60911, Mar 27 2008, 17:57:18) [MSC v.1310 32 bit
(Intel)] on win32.
>>> foo = set([5,4,3,2,1]) # Order reversed from above
>>> foo.pop()
1
>>> foo = set([-1,0,1,2,3,4,5])
>>> foo.pop()
0
>>> foo = set([-1,1,2,3,4,5])
>>> foo.pop()
1

As the documentation says
(http://docs.python.org/library/stdtypes.html#set.pop) set.pop() is
free to return an arbitrary element.

list.pop() however always returns the last element of the list, unless
of course you specify some other index:
http://docs.python.org/library/stdtypes.html#mutable-sequence-types,
point 6.

 John