bpo-27212: Modify islice recipe to consume initial values preceding start (GH-6195) (#GH-6266)
(cherry picked from commit da1734c58d2f97387ccc9676074717d38b044128)
Co-authored-by: Cheryl Sabella <cheryl.sabella@gmail.com>
diff --git a/Lib/test/test_itertools.py b/Lib/test/test_itertools.py
index 4fcc02a..effd7f0 100644
--- a/Lib/test/test_itertools.py
+++ b/Lib/test/test_itertools.py
@@ -1192,6 +1192,7 @@
(10, 20, 3),
(10, 3, 20),
(10, 20),
+ (10, 10),
(10, 3),
(20,)
]:
@@ -1218,6 +1219,10 @@
self.assertEqual(list(islice(it, 3)), list(range(3)))
self.assertEqual(list(it), list(range(3, 10)))
+ it = iter(range(10))
+ self.assertEqual(list(islice(it, 3, 3)), [])
+ self.assertEqual(list(it), list(range(3, 10)))
+
# Test invalid arguments
ra = range(10)
self.assertRaises(TypeError, islice, ra)
@@ -1604,6 +1609,48 @@
self.assertEqual(list(takewhile(lambda x: x<5, [1,4,6,4,1])), [1,4])
+class TestPurePythonRoughEquivalents(unittest.TestCase):
+
+ @staticmethod
+ def islice(iterable, *args):
+ s = slice(*args)
+ start, stop, step = s.start or 0, s.stop or sys.maxsize, s.step or 1
+ it = iter(range(start, stop, step))
+ try:
+ nexti = next(it)
+ except StopIteration:
+ # Consume *iterable* up to the *start* position.
+ for i, element in zip(range(start), iterable):
+ pass
+ return
+ try:
+ for i, element in enumerate(iterable):
+ if i == nexti:
+ yield element
+ nexti = next(it)
+ except StopIteration:
+ # Consume to *stop*.
+ for i, element in zip(range(i + 1, stop), iterable):
+ pass
+
+ def test_islice_recipe(self):
+ self.assertEqual(list(self.islice('ABCDEFG', 2)), list('AB'))
+ self.assertEqual(list(self.islice('ABCDEFG', 2, 4)), list('CD'))
+ self.assertEqual(list(self.islice('ABCDEFG', 2, None)), list('CDEFG'))
+ self.assertEqual(list(self.islice('ABCDEFG', 0, None, 2)), list('ACEG'))
+ # Test items consumed.
+ it = iter(range(10))
+ self.assertEqual(list(self.islice(it, 3)), list(range(3)))
+ self.assertEqual(list(it), list(range(3, 10)))
+ it = iter(range(10))
+ self.assertEqual(list(self.islice(it, 3, 3)), [])
+ self.assertEqual(list(it), list(range(3, 10)))
+ # Test that slice finishes in predictable state.
+ c = count()
+ self.assertEqual(list(self.islice(c, 1, 3, 50)), [1])
+ self.assertEqual(next(c), 3)
+
+
class TestGC(unittest.TestCase):
def makecycle(self, iterator, container):
@@ -2158,6 +2205,17 @@
... "Return function(0), function(1), ..."
... return map(function, count(start))
+>>> import collections
+>>> def consume(iterator, n=None):
+... "Advance the iterator n-steps ahead. If n is None, consume entirely."
+... # Use functions that consume iterators at C speed.
+... if n is None:
+... # feed the entire iterator into a zero-length deque
+... collections.deque(iterator, maxlen=0)
+... else:
+... # advance to the empty slice starting at position n
+... next(islice(iterator, n, n), None)
+
>>> def nth(iterable, n, default=None):
... "Returns the nth item or a default value"
... return next(islice(iterable, n, None), default)
@@ -2298,6 +2356,14 @@
>>> list(islice(tabulate(lambda x: 2*x), 4))
[0, 2, 4, 6]
+>>> it = iter(range(10))
+>>> consume(it, 3)
+>>> next(it)
+3
+>>> consume(it)
+>>> next(it, 'Done')
+'Done'
+
>>> nth('abcde', 3)
'd'
@@ -2386,6 +2452,7 @@
test_classes = (TestBasicOps, TestVariousIteratorArgs, TestGC,
RegressionTests, LengthTransparency,
SubclassWithKwargsTest, TestExamples,
+ TestPurePythonRoughEquivalents,
SizeofTest)
support.run_unittest(*test_classes)