Candidate for a new itertool

[Aahz]

In article <6ca71455-2fb2-4dd0-a500-2a480d815f7a at v6g2000vbb.googlegroups.com>,
Raymond Hettinger  <python at rcn.com> wrote:
>
>For edge-triggered events, we need to convert a boundary-event
>predicate to groupby-style key function.  The code below encapsulates
>that process in a new itertool called split_on().
>
>Would love you guys to experiment with it for a bit and confirm that
>you find it useful.  Suggestions are welcome.

It seems a little esoteric for the standard library.  It was non-trivial
for me to make it work with what seems to me an obvious use-case
(although perhaps I'm missing something), and I'm not sure it has enough
general utility otherwise:

from math import log10

class C:
    def __init__(self, data):
        self.data = data
        self.index = 0
        self.sentinel = None

    def __iter__(self):
        return self

    def next(self):
        if self.index >= len(self.data):
            raise StopIteration
        value = self.data[self.index]
        self.index += 1
        return value

    def make_sentinel(self, value):
        return (int(value / 10) + 1) * 10

    def grouper(self, value):
        if self.sentinel is None:
            self.sentinel = self.make_sentinel(value)
            return False
        if value >= self.sentinel:
            self.sentinel = self.make_sentinel(value)
            return True
        return False

if __name__ == '__main__':
    for start in True, False:
        L_iter = C([11, 12, 13, 20, 32])
        for g in split_on(L_iter, L_iter.grouper, start):
            print list(g)
        print

[11, 12, 13]
[20]
[32]

[11, 12, 13, 20]
[32]
-- 
Aahz (aahz at pythoncraft.com)           <*>         http://www.pythoncraft.com/

"Programming language design is not a rational science. Most reasoning
about it is at best rationalization of gut feelings, and at worst plain
wrong."  --GvR, python-ideas, 2009-3-1