[Python-ideas] Control Flow - Never Executed Loop Body

[Michael Selik]

On Thu, Mar 24, 2016 at 5:34 PM Chris Barker <chris.barker at noaa.gov> wrote:

> From the perspective of many, many (most?) people.  I use for loops and
>> while loops *alot*, and I can count the times I have wanted to use the
>> `else` clause in the meaning of "all my tests failed, so do this block" on
>> the fingers of one hand.
>
>
> I"ve only used for .. else a handful of times, and honestly, probably
> forgot to use it when it would have been the right way to do something far
> more times :-)
>
> but I can't think of a SINGLE time when I wanted to know that the loop
> didn't run because it's iterable was empty.
>

More specifically, a time when the loop didn't run because the iterable was
empty AND the iterable didn't have a len to make that a simple check.

Which suddenly gives me an idea...

    class Peeking:

        def __init__(self, iterator):
            self.it = iter(iterator)
            self.sentinel = object()
            self.buf = next(self.it, self.sentinel)

        def __iter__(self):
            return self

        def __next__(self):
            if self.buf is self.sentinel:
                raise StopIteration
            value = self.buf
            self.buf = next(self.it, self.sentinel)
            return value

        def __bool__(self):
            return self.buf is not self.sentinel


If you find yourself in a situation where you want to check empty as if
your iterator was a sequence (ie. len(seq) == 0 means False), give this
wrapper a shot just before you do the loop.

    >>> bool(Peeking(range(0)))
    False
    >>> bool(Peeking(range(1)))
    True

We could even make it a context manager to monkey-patch a buffering inside
the context for the bool check, then undo the patch on leaving the context
to stop buffering.
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