[Python-ideas] Nudging beginners towards a more accurate mental model for loop else clauses

[Nick Coghlan]

(context for python-ideas: my recently checked in changes to the
tutorial, that added the final paragraph to
http://docs.python.org/tutorial/controlflow.html#break-and-continue-statements-and-else-clauses-on-loops)

On Fri, Jun 8, 2012 at 5:29 PM, Stephen J. Turnbull <stephen at xemacs.org> wrote:
> Note: reply-to set to python-ideas.
>
> Nick Coghlan writes:
>
>  > The inaccuracies in the analogy are why this is in the tutorial, not the
>  > language reference. All 3 else clauses are really their own thing.
>
> Nick, for the purpose of the tutorial, actually there are 4 else
> clauses: you need to distinguish *while* from *for*.  It was much
> easier for me to get confused about *for*.

The only thing I'm trying to do with the tutorial update is to
encourage beginners to be start thinking in terms of try/except/else
when they first encounter for/break/else and while/break/else. That's
it.

Yes, ultimately once people fully understand how it works under the
hood (including the loop-and-a-half construct for infinite while
loops), they'll release it's actually closely related to conditionals
as well, but anyone that places too much weight on the following
obvious parallel is going to be confused for a long time. After all:

  if iterable:
    ...
  else:
    ...

is *very* similar in appearance to:

  for x in iterable:
    ...
  else:
    ...

I believe that parallel is 99% of the reason why people get confused
about the meaning of the latter.

The point of the tutorial update is to give readers a slight nudge
towards thinking of the latter as:

  for x in iterable:
    ...
  except break:  # Implicit in the semantics of loops
    pass
  else:
    ...

Would it be worth adding the "except break:" clause to the language
just to make it crystal clear what is actually going on? I don't think
so, but it's still a handy way to explain the semantics while gently
steering people away from linking for/else and if/else too closely. I
actually agree all of the else clauses really *are* quite closely
related (hence the consistent use of the same keyword), but the
relationship is *not* the intuitively obvious one that comes to mind
when you just look at the similarity in the concrete syntax
specifically of for/else and if/else.

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia