[Guido]
I would make one change: after looking at another use case, I'd like to change the outer iterator to produce (key, grouper) tuples. This way, you can write things like
totals = {} for key, group in sequence: totals[key] = sum(group)
Here is an implementation that translates readily into C. It uses Guido's syntax and meets my requirement that bad things don't happen when someone runs the outer iterator independently of the inner iterator. class groupby(object): __slots__ = ('keyfunc', 'it', 'tgtkey', 'currkey', 'currvalue') def __init__(self, key, iterable): NULL = 1+909.9j # In C, use the real NULL self.keyfunc = key self.it = iter(iterable) self.tgtkey = NULL self.currkey = NULL self.currvalue = NULL def __iter__(self): return self def next(self): while self.currkey == self.tgtkey: self.currvalue = self.it.next() # Exit on StopIteration self.currkey = self.keyfunc(self.currvalue) self.tgtkey = self.currkey return (self.currkey, self._grouper(self.currkey)) def _grouper(self, tgtkey): while self.currkey == tgtkey: yield self.currvalue self.currvalue = self.it.next() # Exit on StopIteration self.currkey = self.keyfunc(self.currvalue) import unittest class TestBasicOps(unittest.TestCase): def test_groupby(self): # Check zero length input self.assertEqual([], list(groupby(lambda r:r[0], []))) # Check normal input s = [(0, 10, 20), (0, 11,21), (0,12,21), (1,13,21), (1,14,22), (2,15,22), (3,16,23), (3,17,23)] dup = [] for k, g in groupby(lambda r:r[0], s): for elem in g: self.assertEqual(k, elem[0]) dup.append(elem) self.assertEqual(s, dup) # Check nested case dup = [] for k, g in groupby(lambda r:r[0], s): for ik, ig in groupby(lambda r:r[2], g): for elem in ig: self.assertEqual(k, elem[0]) self.assertEqual(ik, elem[2]) dup.append(elem) self.assertEqual(s, dup) # Check case where inner iterator is not used keys = [] for k, g in groupby(lambda r:r[0], s): keys.append(k) expectedkeys = set([r[0] for r in s]) self.assertEqual(set(keys), expectedkeys) self.assertEqual(len(keys), len(expectedkeys)) suite = unittest.TestSuite() suite.addTest(unittest.makeSuite(TestBasicOps)) unittest.TextTestRunner(verbosity=2).run(suite) Raymond