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[Python-Dev] PEP 0424: A method for exposing a length hint

Antoine Pitrou solipsis at pitrou.net
Sun Jul 15 17:06:28 CEST 2012 
On Sun, 15 Jul 2012 16:33:23 +0200
Christian Heimes <lists at cheimes.de> wrote:
> Am 15.07.2012 16:22, schrieb Antoine Pitrou:
> > On Mon, 16 Jul 2012 00:08:41 +1000
> > Nick Coghlan <ncoghlan at gmail.com> wrote:
> >> Right, I agree on the value in being able to return something to say "this
> >> cannot be converted to a concrete container".
> > 
> > Who would be able to return that, apart from trivial cases like
> > itertools.cycle()?
> 
> For example most numerical sequence iterators like Fibonacci generator,
> prime number sequence generator and even trivial cases like even natural
> number generator.

First, you can't implement __length_hint__ for a generator, which is the
preferred (the most practical) way of writing iterators in pure Python.

Second, not all iterators will implement __length_hint__ (because it's
optional and, really, of rather little use). So, as a user, you cannot
hope that `list(some_iterator)` will always raise instead of filling
your memory with an infinite stream of values: you have to be careful
anyway.

Even if __length_hint__ is implemented, its result may be wrong.
That's the whole point: it's a *hint*; an iterator might tell you it's
finite while it's infinite, or the reverse.


My conclusion is that an infinite iterator is a documentation issue.
Just tell the user that it doesn't stop, and let them shoot themselves
in the foot in they want to.

Regards

Antoine.


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