[Python-Dev] Iterating over objects of unknown length

[Oleg Broytmann]

On Thu, Sep 27, 2007 at 01:33:47PM +1200, Greg Ewing wrote:
> Oleg Broytmann wrote:
> >if sqlQuery:
> >   for row in sqlQuery: ...
> >else:
> >   # no rows
> >
> >To prevent users from writing such code the class implements __nonzero__()
> >that always raises an exception.
> 
> I'm not sure I like that idea. It's common practice to write
> 'if x:' as a shorthand for 'if x is not None:' when it's known
> that x is an object that doesn't have a notion of emptiness.
> Another thing is that any code doing "if x" to test for
> emptiness is clearly expecting x to be a sequence, *not*
> an iterator, and you've violated the contract by passing
> it one. This is what you may be running into with the libraries
> you mention.

   In most cases the code in those libraries is, using the word of Mr. van
Rossum, "archaic". It was developed for old versions of Python (long before
Python has got the iterator protocol). I will file bug reports and patches
(I have filed one about logginig/__init__.py) to allow developers to either
fix the code or document the fact the code really requires a finite
sequence.
   Unfortunately now when my code no longer raises an exception it would be
harder to spot the buggy libraries.

> Generally I think it's a bad idea to try to protect people
> from themselves when doing so can interfere with legitimate
> usage.

   I agree. I admitted in mailing list it was my design mistake. The
offending __nonzero__ was removed from SVN today.

Oleg.
-- 
     Oleg Broytmann            http://phd.pp.ru/            phd at phd.pp.ru
           Programmers don't die, they just GOSUB without RETURN.