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228 lines
7.1 KiB
Python
228 lines
7.1 KiB
Python
""" Test Iterator Length Transparency
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Some functions or methods which accept general iterable arguments have
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optional, more efficient code paths if they know how many items to expect.
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For instance, map(func, iterable), will pre-allocate the exact amount of
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space required whenever the iterable can report its length.
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The desired invariant is: len(it)==len(list(it)).
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A complication is that an iterable and iterator can be the same object. To
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maintain the invariant, an iterator needs to dynamically update its length.
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For instance, an iterable such as range(10) always reports its length as ten,
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but it=iter(range(10)) starts at ten, and then goes to nine after next(it).
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Having this capability means that map() can ignore the distinction between
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map(func, iterable) and map(func, iter(iterable)).
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When the iterable is immutable, the implementation can straight-forwardly
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report the original length minus the cumulative number of calls to next().
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This is the case for tuples, range objects, and itertools.repeat().
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Some containers become temporarily immutable during iteration. This includes
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dicts, sets, and collections.deque. Their implementation is equally simple
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though they need to permanently set their length to zero whenever there is
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an attempt to iterate after a length mutation.
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The situation slightly more involved whenever an object allows length mutation
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during iteration. Lists and sequence iterators are dynamically updatable.
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So, if a list is extended during iteration, the iterator will continue through
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the new items. If it shrinks to a point before the most recent iteration,
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then no further items are available and the length is reported at zero.
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Reversed objects can also be wrapped around mutable objects; however, any
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appends after the current position are ignored. Any other approach leads
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to confusion and possibly returning the same item more than once.
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The iterators not listed above, such as enumerate and the other itertools,
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are not length transparent because they have no way to distinguish between
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iterables that report static length and iterators whose length changes with
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each call (i.e. the difference between enumerate('abc') and
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enumerate(iter('abc')).
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"""
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import unittest
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from itertools import repeat
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from collections import deque
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from operator import length_hint
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n = 10
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class TestInvariantWithoutMutations:
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def test_invariant(self):
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it = self.it
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for i in reversed(range(1, n+1)):
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self.assertEqual(length_hint(it), i)
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next(it)
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self.assertEqual(length_hint(it), 0)
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self.assertRaises(StopIteration, next, it)
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self.assertEqual(length_hint(it), 0)
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class TestTemporarilyImmutable(TestInvariantWithoutMutations):
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def test_immutable_during_iteration(self):
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# objects such as deques, sets, and dictionaries enforce
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# length immutability during iteration
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it = self.it
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self.assertEqual(length_hint(it), n)
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next(it)
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self.assertEqual(length_hint(it), n-1)
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self.mutate()
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self.assertRaises(RuntimeError, next, it)
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self.assertEqual(length_hint(it), 0)
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## ------- Concrete Type Tests -------
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class TestRepeat(TestInvariantWithoutMutations, unittest.TestCase):
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def setUp(self):
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self.it = repeat(None, n)
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class TestXrange(TestInvariantWithoutMutations, unittest.TestCase):
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def setUp(self):
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self.it = iter(range(n))
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class TestXrangeCustomReversed(TestInvariantWithoutMutations, unittest.TestCase):
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def setUp(self):
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self.it = reversed(range(n))
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class TestTuple(TestInvariantWithoutMutations, unittest.TestCase):
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def setUp(self):
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self.it = iter(tuple(range(n)))
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## ------- Types that should not be mutated during iteration -------
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class TestDeque(TestTemporarilyImmutable, unittest.TestCase):
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def setUp(self):
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d = deque(range(n))
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self.it = iter(d)
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self.mutate = d.pop
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class TestDequeReversed(TestTemporarilyImmutable, unittest.TestCase):
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def setUp(self):
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d = deque(range(n))
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self.it = reversed(d)
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self.mutate = d.pop
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class TestDictKeys(TestTemporarilyImmutable, unittest.TestCase):
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def setUp(self):
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d = dict.fromkeys(range(n))
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self.it = iter(d)
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self.mutate = d.popitem
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class TestDictItems(TestTemporarilyImmutable, unittest.TestCase):
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def setUp(self):
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d = dict.fromkeys(range(n))
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self.it = iter(d.items())
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self.mutate = d.popitem
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class TestDictValues(TestTemporarilyImmutable, unittest.TestCase):
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def setUp(self):
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d = dict.fromkeys(range(n))
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self.it = iter(d.values())
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self.mutate = d.popitem
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class TestSet(TestTemporarilyImmutable, unittest.TestCase):
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def setUp(self):
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d = set(range(n))
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self.it = iter(d)
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self.mutate = d.pop
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## ------- Types that can mutate during iteration -------
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class TestList(TestInvariantWithoutMutations, unittest.TestCase):
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def setUp(self):
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self.it = iter(range(n))
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def test_mutation(self):
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d = list(range(n))
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it = iter(d)
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next(it)
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next(it)
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self.assertEqual(length_hint(it), n - 2)
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d.append(n)
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self.assertEqual(length_hint(it), n - 1) # grow with append
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d[1:] = []
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self.assertEqual(length_hint(it), 0)
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self.assertEqual(list(it), [])
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d.extend(range(20))
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self.assertEqual(length_hint(it), 0)
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class TestListReversed(TestInvariantWithoutMutations, unittest.TestCase):
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def setUp(self):
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self.it = reversed(range(n))
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def test_mutation(self):
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d = list(range(n))
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it = reversed(d)
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next(it)
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next(it)
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self.assertEqual(length_hint(it), n - 2)
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d.append(n)
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self.assertEqual(length_hint(it), n - 2) # ignore append
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d[1:] = []
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self.assertEqual(length_hint(it), 0)
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self.assertEqual(list(it), []) # confirm invariant
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d.extend(range(20))
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self.assertEqual(length_hint(it), 0)
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## -- Check to make sure exceptions are not suppressed by __length_hint__()
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class BadLen(object):
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def __iter__(self):
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return iter(range(10))
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def __len__(self):
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raise RuntimeError('hello')
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class BadLengthHint(object):
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def __iter__(self):
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return iter(range(10))
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def __length_hint__(self):
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raise RuntimeError('hello')
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class NoneLengthHint(object):
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def __iter__(self):
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return iter(range(10))
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def __length_hint__(self):
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return NotImplemented
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class TestLengthHintExceptions(unittest.TestCase):
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def test_issue1242657(self):
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self.assertRaises(RuntimeError, list, BadLen())
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self.assertRaises(RuntimeError, list, BadLengthHint())
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self.assertRaises(RuntimeError, [].extend, BadLen())
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self.assertRaises(RuntimeError, [].extend, BadLengthHint())
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b = bytearray(range(10))
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self.assertRaises(RuntimeError, b.extend, BadLen())
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self.assertRaises(RuntimeError, b.extend, BadLengthHint())
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def test_invalid_hint(self):
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# Make sure an invalid result doesn't muck-up the works
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self.assertEqual(list(NoneLengthHint()), list(range(10)))
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if __name__ == "__main__":
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unittest.main()
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