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Apr 2, 2018
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[2.7] bpo-27212: Modify islice recipe to consume initial values prece… #6339

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28 changes: 20 additions & 8 deletions Doc/library/itertools.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -411,12 +411,24 @@ 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(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.
Expand Down Expand Up @@ -681,8 +693,8 @@ which incur interpreter overhead.
"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
Expand Down
69 changes: 68 additions & 1 deletion Lib/test/test_itertools.py
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Original file line number Diff line number Diff line change
Expand Up @@ -802,6 +802,7 @@ def test_islice(self):
(10, 20, 3),
(10, 3, 20),
(10, 20),
(10, 10),
(10, 3),
(20,)
]:
Expand All @@ -826,6 +827,10 @@ def test_islice(self):
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)
Expand Down Expand Up @@ -1084,6 +1089,48 @@ 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.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):
Expand Down Expand Up @@ -1577,6 +1624,17 @@ def __init__(self, newarg=None, *args):
... "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)
Expand Down Expand Up @@ -1678,6 +1736,14 @@ def __init__(self, newarg=None, *args):
>>> 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'

Expand Down Expand Up @@ -1753,7 +1819,8 @@ def __init__(self, newarg=None, *args):
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
Expand Down
2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Documentation/2018-03-22-19-23-04.bpo-27212.wrE5KR.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,2 @@
Modify documentation for the :func:`islice` recipe to consume initial values
up to the start index.