[2.7] bpo-27212: Modify islice recipe to consume initial values preceding start (GH-6195) (GH-6339)
(cherry picked from commit da1734c58d2f97387ccc9676074717d38b044128)
diff --git a/Doc/library/itertools.rst b/Doc/library/itertools.rst
index 57ba4d4..17303dd 100644
--- a/Doc/library/itertools.rst
+++ b/Doc/library/itertools.rst
@@ -411,12 +411,24 @@
# islice('ABCDEFG', 2, None) --> C D E F G
# islice('ABCDEFG', 0, None, 2) --> A C E G
s = slice(*args)
- it = iter(xrange(s.start or 0, s.stop or sys.maxint, s.step or 1))
- nexti = next(it)
- for i, element in enumerate(iterable):
- if i == nexti:
- yield element
- nexti = next(it)
+ start, stop, step = s.start or 0, s.stop or sys.maxint, s.step or 1
+ it = iter(xrange(start, stop, step)))
+ try:
+ nexti = next(it)
+ except StopIteration:
+ # Consume *iterable* up to the *start* position.
+ for i, element in izip(xrange(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 izip(xrange(i + 1, stop), iterable):
+ pass
If *start* is ``None``, then iteration starts at zero. If *step* is ``None``,
then the step defaults to one.
@@ -681,8 +693,8 @@
"Return function(0), function(1), ..."
return imap(function, count(start))
- def consume(iterator, n):
- "Advance the iterator n-steps ahead. If n is none, consume entirely."
+ 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
diff --git a/Lib/test/test_itertools.py b/Lib/test/test_itertools.py
index 1e6db34..0427979 100644
--- a/Lib/test/test_itertools.py
+++ b/Lib/test/test_itertools.py
@@ -802,6 +802,7 @@
(10, 20, 3),
(10, 3, 20),
(10, 20),
+ (10, 10),
(10, 3),
(20,)
]:
@@ -826,6 +827,10 @@
self.assertEqual(list(islice(it, 3)), range(3))
self.assertEqual(list(it), range(3, 10))
+ it = iter(range(10))
+ self.assertEqual(list(islice(it, 3, 3)), [])
+ self.assertEqual(list(it), range(3, 10))
+
# Test invalid arguments
self.assertRaises(TypeError, islice, xrange(10))
self.assertRaises(TypeError, islice, xrange(10), 1, 2, 3, 4)
@@ -1084,6 +1089,48 @@
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.maxint, s.step or 1
+ it = iter(xrange(start, stop, step))
+ try:
+ nexti = next(it)
+ except StopIteration:
+ # Consume *iterable* up to the *start* position.
+ for i, element in izip(xrange(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 izip(xrange(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(xrange(10))
+ self.assertEqual(list(self.islice(it, 3)), range(3))
+ self.assertEqual(list(it), range(3, 10))
+ it = iter(xrange(10))
+ self.assertEqual(list(self.islice(it, 3, 3)), [])
+ self.assertEqual(list(it), 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):
@@ -1577,6 +1624,17 @@
... "Return function(0), function(1), ..."
... return imap(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)
@@ -1678,6 +1736,14 @@
>>> list(islice(tabulate(lambda x: 2*x), 4))
[0, 2, 4, 6]
+>>> it = iter(xrange(10))
+>>> consume(it, 3)
+>>> next(it)
+3
+>>> consume(it)
+>>> next(it, 'Done')
+'Done'
+
>>> nth('abcde', 3)
'd'
@@ -1753,7 +1819,8 @@
def test_main(verbose=None):
test_classes = (TestBasicOps, TestVariousIteratorArgs, TestGC,
RegressionTests, LengthTransparency,
- SubclassWithKwargsTest, TestExamples)
+ SubclassWithKwargsTest, TestExamples,
+ TestPurePythonRoughEquivalents)
test_support.run_unittest(*test_classes)
# verify reference counting
diff --git a/Misc/NEWS.d/next/Documentation/2018-03-22-19-23-04.bpo-27212.wrE5KR.rst b/Misc/NEWS.d/next/Documentation/2018-03-22-19-23-04.bpo-27212.wrE5KR.rst
new file mode 100644
index 0000000..5910d2c
--- /dev/null
+++ b/Misc/NEWS.d/next/Documentation/2018-03-22-19-23-04.bpo-27212.wrE5KR.rst
@@ -0,0 +1,2 @@
+Modify documentation for the :func:`islice` recipe to consume initial values
+up to the start index.