[Python-Dev] Rationale behind lazy map/filter

[Chris Angelico]

On Wed, Oct 14, 2015 at 3:08 AM, Random832 <random832 at fastmail.com> wrote:
> If you are writing code that tries
> to resume iterating after the iterator has been exhausted, I have to
> ask: why?

A well-behaved iterator is supposed to continue raising StopIteration
forever once it's been exhausted. I don't know how much code actually
depends on this, but it wouldn't be hard to make a wrapper that raises
a different exception instead:

class iter:
    _orig_iter = iter
    def __init__(self, thing):
        self.iter = self._orig_iter(thing)
        self.exhausted = False
    def __iter__(self): return self
    def __next__(self):
        if self.exhausted: raise RuntimeError("Already exhausted")
        try: return next(self.iter)
        except StopIteration:
            self.exhausted = True
            raise

Play with that, and see where RuntimeErrors start coming up. I suspect
they'll be rare, but they will happen.

ChrisA