Re: [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
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Re: [Python-Dev] Syntax suggestion for imports

[John Barham]

Raymond Hettinger wrote:

> How about a new, simpler syntax:
>
> * import threading or dummy_threading as threading
> * import xml.etree.CElementTree or cElementTree or elementree.ElementTree as 
> ET
> * from cStringIO or StringIO import StringIO

These all look good to me.  The "short circuit" import syntax and
semantics are compact and intuitive.

> * import readline or emptymodule

This I find more problematic as "emptymodule" seems too magical.  Even
now any code that wants to use a module that might not have been
successfully imported needs to check if that's the case.  E.g., a
fuller current use-case would be:

try:
   readline = None
   import readline
except ImportError:
   pass

if readline is not None:
   readline.foo()
   ...

Conceivably emptymodule could act as a Null object but that could
create more problems than it solves.

  John
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