[Python-ideas] PEP 479: Change StopIteration handling inside generators

[random832 at fastmail.us]

On Tue, Nov 25, 2014, at 15:31, Chris Barker wrote:
> Once you start nesting these things, the distinction between
> "implementor"
> and "caller" gets mingled. And I think this is all about how nested
> generators behave, yes?

For something more concrete, we can consider a naive implementation of
iteration over adjacent pairs:

def pairs(x):
    i = iter(x)
    while True:
        yield next(i), next(i)

>>> list(pairs(range(1)))
[]
>>> list(pairs(range(2)))
[(0, 1)]
>>> list(pairs(range(3)))
[(0, 1)]
>>> list(pairs(range(4)))
[(0, 1), (2, 3)]
>>> list(pairs(range(5)))
[(0, 1), (2, 3)]
>>> list(pairs(range(6)))
[(0, 1), (2, 3), (4, 5)]

To work under the new paradigm, you need to catch StopIteration
explicitly:

def pairs(x):
    i = iter(x)
    while True:
        try:
            a = next(i)
            b = next(i)
        except StopIteration:
            return
        yield a, b