[Python-checkins] bpo-27212: Modify islice recipe to consume initial values preceding start (GH-6195)
Raymond Hettinger
webhook-mailer at python.org
Mon Mar 26 21:29:36 EDT 2018
https://github.com/python/cpython/commit/da1734c58d2f97387ccc9676074717d38b044128
commit: da1734c58d2f97387ccc9676074717d38b044128
branch: master
author: Cheryl Sabella <cheryl.sabella at gmail.com>
committer: Raymond Hettinger <rhettinger at users.noreply.github.com>
date: 2018-03-26T18:29:33-07:00
summary:
bpo-27212: Modify islice recipe to consume initial values preceding start (GH-6195)
files:
A Misc/NEWS.d/next/Documentation/2018-03-22-19-23-04.bpo-27212.wrE5KR.rst
M Doc/library/itertools.rst
M Lib/test/test_itertools.py
diff --git a/Doc/library/itertools.rst b/Doc/library/itertools.rst
index 0b3829f19faf..a5a5356a9a1f 100644
--- a/Doc/library/itertools.rst
+++ b/Doc/library/itertools.rst
@@ -436,15 +436,24 @@ loops that truncate the stream.
# islice('ABCDEFG', 2, None) --> C D E F G
# islice('ABCDEFG', 0, None, 2) --> A C E G
s = slice(*args)
- it = iter(range(s.start or 0, s.stop or sys.maxsize, s.step or 1))
+ 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
- for i, element in enumerate(iterable):
- if i == nexti:
- yield element
- nexti = next(it)
+ 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
If *start* is ``None``, then iteration starts at zero. If *step* is ``None``,
then the step defaults to one.
@@ -688,8 +697,8 @@ which incur interpreter overhead.
# tail(3, 'ABCDEFG') --> E F G
return iter(collections.deque(iterable, maxlen=n))
- 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 4fcc02acbf44..effd7f0e21be 100644
--- a/Lib/test/test_itertools.py
+++ b/Lib/test/test_itertools.py
@@ -1192,6 +1192,7 @@ def test_islice(self):
(10, 20, 3),
(10, 3, 20),
(10, 20),
+ (10, 10),
(10, 3),
(20,)
]:
@@ -1218,6 +1219,10 @@ def test_islice(self):
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 @@ def test_takewhile(self):
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 @@ def test_permutations_sizeof(self):
... "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 @@ def test_permutations_sizeof(self):
>>> 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 @@ def test_main(verbose=None):
test_classes = (TestBasicOps, TestVariousIteratorArgs, TestGC,
RegressionTests, LengthTransparency,
SubclassWithKwargsTest, TestExamples,
+ TestPurePythonRoughEquivalents,
SizeofTest)
support.run_unittest(*test_classes)
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 000000000000..5910d2c17342
--- /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.
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