diff options
| -rw-r--r-- | Doc/library/itertools.rst | 23 | ||||
| -rw-r--r-- | Lib/test/test_itertools.py | 67 | ||||
| -rw-r--r-- | Misc/NEWS.d/next/Documentation/2018-03-22-19-23-04.bpo-27212.wrE5KR.rst | 2 |
3 files changed, 85 insertions, 7 deletions
diff --git a/Doc/library/itertools.rst b/Doc/library/itertools.rst index 0b3829f..a5a5356 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 4fcc02a..effd7f0 100644 --- a/Lib/test/test_itertools.py +++ b/Lib/test/test_itertools.py @@ -1192,6 +1192,7 @@ class TestBasicOps(unittest.TestCase): (10, 20, 3), (10, 3, 20), (10, 20), + (10, 10), (10, 3), (20,) ]: @@ -1218,6 +1219,10 @@ class TestBasicOps(unittest.TestCase): 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 @@ class TestExamples(unittest.TestCase): 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 @@ Samuele ... "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 @@ perform as purported. >>> 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 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. |