8000 bpo-27212: Modify islice recipe to consume initial values preceding start by csabella · Pull Request #6195 · python/cpython · GitHub
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bpo-27212: Modify islice recipe to consume initial values preceding start #6195

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Merged
merged 4 commits into from
Mar 27, 2018
Merged

bpo-27212: Modify islice recipe to consume initial values preceding start #6195

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0f51d3d
bpo-27212: Modify islice recipe to consume initial values
csabella Mar 22, 2018
93504c6
Requested changes and tests.
csabella Mar 24, 2018
e73983d
Move try block and update blurb
csabella Mar 25, 2018
6a064a3
Remove unneeded return
csabella Mar 26, 2018
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24 changes: 17 additions & 7 deletions Doc/library/itertools.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -436,15 +436,25 @@ 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
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
return
for i, element in enumerate(iterable):
if i == nexti:
yield element
nexti = next(it)

If *start* is ``None``, then iteration starts at zero. If *step* is ``None``,
then the step defaults to one.
Expand Down Expand Up @@ -688,8 +698,8 @@ 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
Expand Down
68 changes: 68 additions & 0 deletions Lib/test/test_itertools.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1192,6 +1192,7 @@ def test_islice(self):
(10, 20, 3),
(10, 3, 20),
(10, 20),
(10, 10),
(10, 3),
(20,)
]:
Expand All @@ -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)
Expand Down Expand Up @@ -1604,6 +1609,49 @@ 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
return
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@rhettinger rhettinger Mar 26, 2018 •
edited
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Let's drop the "return" on the final line so that it doesn't look like an early-out as opposed to a normal end.

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Done.


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.
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This is the part that I'm still thinking about.

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 @@ -2158,6 +2206,17 @@ 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)
Expand Down Expand Up @@ -2298,6 +2357,14 @@ 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'

Expand Down Expand Up @@ -2386,6 +2453,7 @@ def test_main(verbose=None):
test_classes = (TestBasicOps, TestVariousIteratorArgs, TestGC,
RegressionTests, LengthTransparency,
SubclassWithKwargsTest, TestExamples,
TestPurePythonRoughEquivalents,
SizeofTest)
support.run_unittest(*test_classes)

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.
0