Small improvements to the itertools docs (pythonGH-123885)

Doc/library/itertools.rst

@@ -474,7 +474,7 @@ loops that truncate the stream.
  If *start* is zero or ``None``, iteration starts at zero. Otherwise,
  elements from the iterable are skipped until *start* is reached.
 
- If *stop* is ``None``, iteration continues until the iterator is
+ If *stop* is ``None``, iteration continues until the iterable is
  exhausted, if at all. Otherwise, it stops at the specified position.
 
  If *step* is ``None``, the step defaults to one. Elements are returned
@@ -503,6 +503,10 @@ loops that truncate the stream.
  yield element
  next_i += step
 
+ If the input is an iterator, then fully consuming the *islice*
+ advances the input iterator by ``max(start, stop)`` steps regardless
+ of the *step* value.
+
 
 .. function:: pairwise(iterable)
 
@@ -601,6 +605,8 @@ loops that truncate the stream.
  # product('ABCD', 'xy') → Ax Ay Bx By Cx Cy Dx Dy
  # product(range(2), repeat=3) → 000 001 010 011 100 101 110 111
 
+ if repeat < 0:
+ raise ValueError('repeat argument cannot be negative')
  pools = [tuple(pool) for pool in iterables] * repeat
 
  result = [[]]
@@ -684,6 +690,8 @@ loops that truncate the stream.
  Roughly equivalent to::
 
  def tee(iterable, n=2):
+ if n < 0:
+ raise ValueError('n must be >= 0')
  iterator = iter(iterable)
  shared_link = [None, None]
  return tuple(_tee(iterator, shared_link) for _ in range(n))
@@ -703,6 +711,12 @@ loops that truncate the stream.
  used anywhere else; otherwise, the *iterable* could get advanced without
  the tee objects being informed.
 
+ When the input *iterable* is already a tee iterator object, all
+ members of the return tuple are constructed as if they had been
+ produced by the upstream :func:`tee` call. This "flattening step"
+ allows nested :func:`tee` calls to share the same underlying data
+ chain and to have a single update step rather than a chain of calls.
+
  ``tee`` iterators are not threadsafe. A :exc:`RuntimeError` may be
  raised when simultaneously using iterators returned by the same :func:`tee`
  call, even if the original *iterable* is threadsafe.

Lib/test/test_itertools.py

@@ -1288,12 +1288,16 @@ def product1(*args, **kwds):
  else:
  return
 
- def product2(*args, **kwds):
+ def product2(*iterables, repeat=1):
  'Pure python version used in docs'
- pools = list(map(tuple, args)) * kwds.get('repeat', 1)
+ if repeat < 0:
+ raise ValueError('repeat argument cannot be negative')
+ pools = [tuple(pool) for pool in iterables] * repeat
+
  result = [[]]
  for pool in pools:
  result = [x+[y] for x in result for y in pool]
+
  for prod in result:
  yield tuple(prod)
 
@@ -2062,6 +2066,161 @@ def test_islice_recipe(self):
  self.assertEqual(next(c), 3)
 
 
+ def test_tee_recipe(self):
+
+ # Begin tee() recipe ###########################################
+
+ def tee(iterable, n=2):
+ if n < 0:
+ raise ValueError('n must be >= 0')
+ iterator = iter(iterable)
+ shared_link = [None, None]
+ return tuple(_tee(iterator, shared_link) for _ in range(n))
+
+ def _tee(iterator, link):
+ try:
+ while True:
+ if link[1] is None:
+ link[0] = next(iterator)
+ link[1] = [None, None]
+ value, link = link
+ yield value
+ except StopIteration:
+ return
+
+ # End tee() recipe #############################################
+
+ n = 200
+
+ a, b = tee([]) # test empty iterator
+ self.assertEqual(list(a), [])
+ self.assertEqual(list(b), [])
+
+ a, b = tee(irange(n)) # test 100% interleaved
+ self.assertEqual(lzip(a,b), lzip(range(n), range(n)))
+
+ a, b = tee(irange(n)) # test 0% interleaved
+ self.assertEqual(list(a), list(range(n)))
+ self.assertEqual(list(b), list(range(n)))
+
+ a, b = tee(irange(n)) # test dealloc of leading iterator
+ for i in range(100):
+ self.assertEqual(next(a), i)
+ del a
+ self.assertEqual(list(b), list(range(n)))
+
+ a, b = tee(irange(n)) # test dealloc of trailing iterator
+ for i in range(100):
+ self.assertEqual(next(a), i)
+ del b
+ self.assertEqual(list(a), list(range(100, n)))
+
+ for j in range(5): # test randomly interleaved
+ order = [0]*n + [1]*n
+ random.shuffle(order)
+ lists = ([], [])
+ its = tee(irange(n))
+ for i in order:
+ value = next(its[i])
+ lists[i].append(value)
+ self.assertEqual(lists[0], list(range(n)))
+ self.assertEqual(lists[1], list(range(n)))
+
+ # test argument format checking
+ self.assertRaises(TypeError, tee)
+ self.assertRaises(TypeError, tee, 3)
+ self.assertRaises(TypeError, tee, [1,2], 'x')
+ self.assertRaises(TypeError, tee, [1,2], 3, 'x')
+
+ # Tests not applicable to the tee() recipe
+ if False:
+ # tee object should be instantiable
+ a, b = tee('abc')
+ c = type(a)('def')
+ self.assertEqual(list(c), list('def'))
+
+ # test long-lagged and multi-way split
+ a, b, c = tee(range(2000), 3)
+ for i in range(100):
+ self.assertEqual(next(a), i)
+ self.assertEqual(list(b), list(range(2000)))
+ self.assertEqual([next(c), next(c)], list(range(2)))
+ self.assertEqual(list(a), list(range(100,2000)))
+ self.assertEqual(list(c), list(range(2,2000)))
+
+ # test invalid values of n
+ self.assertRaises(TypeError, tee, 'abc', 'invalid')
+ self.assertRaises(ValueError, tee, [], -1)
+
+ for n in range(5):
+ result = tee('abc', n)
+ self.assertEqual(type(result), tuple)
+ self.assertEqual(len(result), n)
+ self.assertEqual([list(x) for x in result], [list('abc')]*n)
+
+
+ # Tests not applicable to the tee() recipe
+ if False:
+ # tee pass-through to copyable iterator
+ a, b = tee('abc')
+ c, d = tee(a)
+ self.assertTrue(a is c)
+
+ # test tee_new
+ t1, t2 = tee('abc')
+ tnew = type(t1)
+ self.assertRaises(TypeError, tnew)
+ self.assertRaises(TypeError, tnew, 10)
+ t3 = tnew(t1)
+ self.assertTrue(list(t1) == list(t2) == list(t3) == list('abc'))
+
+ # test that tee objects are weak referencable
+ a, b = tee(range(10))
+ p = weakref.proxy(a)
+ self.assertEqual(getattr(p, '__class__'), type(b))
+ del a
+ gc.collect() # For PyPy or other GCs.
+ self.assertRaises(ReferenceError, getattr, p, '__class__')
+
+ ans = list('abc')
+ long_ans = list(range(10000))
+
+ # Tests not applicable to the tee() recipe
+ if False:
+ # check copy
+ a, b = tee('abc')
+ self.assertEqual(list(copy.copy(a)), ans)
+ self.assertEqual(list(copy.copy(b)), ans)
+ a, b = tee(list(range(10000)))
+ self.assertEqual(list(copy.copy(a)), long_ans)
+ self.assertEqual(list(copy.copy(b)), long_ans)
+
+ # check partially consumed copy
+ a, b = tee('abc')
+ take(2, a)
+ take(1, b)
+ self.assertEqual(list(copy.copy(a)), ans[2:])
+ self.assertEqual(list(copy.copy(b)), ans[1:])
+ self.assertEqual(list(a), ans[2:])
+ self.assertEqual(list(b), ans[1:])
+ a, b = tee(range(10000))
+ take(100, a)
+ take(60, b)
+ self.assertEqual(list(copy.copy(a)), long_ans[100:])
+ self.assertEqual(list(copy.copy(b)), long_ans[60:])
+ self.assertEqual(list(a), long_ans[100:])
+ self.assertEqual(list(b), long_ans[60:])
+
+ # Issue 13454: Crash when deleting backward iterator from tee()
+ forward, backward = tee(repeat(None, 2000)) # 20000000
+ try:
+ any(forward) # exhaust the iterator
+ del backward
+ except:
+ del forward, backward
+ raise
+
+
 class TestGC(unittest.TestCase):
 
  def makecycle(self, iterator, container):