1 """ Test Iterator Length Transparency 2 3 Some functions or methods which accept general iterable arguments have 4 optional, more efficient code paths if they know how many items to expect. 5 For instance, map(func, iterable), will pre-allocate the exact amount of 6 space required whenever the iterable can report its length. 7 8 The desired invariant is: len(it)==len(list(it)). 9 10 A complication is that an iterable and iterator can be the same object. To 11 maintain the invariant, an iterator needs to dynamically update its length. 12 For instance, an iterable such as range(10) always reports its length as ten, 13 but it=iter(range(10)) starts at ten, and then goes to nine after next(it). 14 Having this capability means that map() can ignore the distinction between 15 map(func, iterable) and map(func, iter(iterable)). 16 17 When the iterable is immutable, the implementation can straight-forwardly 18 report the original length minus the cumulative number of calls to next(). 19 This is the case for tuples, range objects, and itertools.repeat(). 20 21 Some containers become temporarily immutable during iteration. This includes 22 dicts, sets, and collections.deque. Their implementation is equally simple 23 though they need to permanently set their length to zero whenever there is 24 an attempt to iterate after a length mutation. 25 26 The situation slightly more involved whenever an object allows length mutation 27 during iteration. Lists and sequence iterators are dynamically updatable. 28 So, if a list is extended during iteration, the iterator will continue through 29 the new items. If it shrinks to a point before the most recent iteration, 30 then no further items are available and the length is reported at zero. 31 32 Reversed objects can also be wrapped around mutable objects; however, any 33 appends after the current position are ignored. Any other approach leads 34 to confusion and possibly returning the same item more than once. 35 36 The iterators not listed above, such as enumerate and the other itertools, 37 are not length transparent because they have no way to distinguish between 38 iterables that report static length and iterators whose length changes with 39 each call (i.e. the difference between enumerate('abc') and 40 enumerate(iter('abc')). 41 42 """ 43 44 import unittest 45 from itertools import repeat 46 from collections import deque 47 from operator import length_hint 48 49 n = 10 50 51 52 class TestInvariantWithoutMutations: 53 54 def test_invariant(self): 55 it = self.it 56 for i in reversed(range(1, n+1)): 57 self.assertEqual(length_hint(it), i) 58 next(it) 59 self.assertEqual(length_hint(it), 0) 60 self.assertRaises(StopIteration, next, it) 61 self.assertEqual(length_hint(it), 0) 62 63 class TestTemporarilyImmutable(TestInvariantWithoutMutations): 64 65 def test_immutable_during_iteration(self): 66 # objects such as deques, sets, and dictionaries enforce 67 # length immutability during iteration 68 69 it = self.it 70 self.assertEqual(length_hint(it), n) 71 next(it) 72 self.assertEqual(length_hint(it), n-1) 73 self.mutate() 74 self.assertRaises(RuntimeError, next, it) 75 self.assertEqual(length_hint(it), 0) 76 77 ## ------- Concrete Type Tests ------- 78 79 class TestRepeat(TestInvariantWithoutMutations, unittest.TestCase): 80 81 def setUp(self): 82 self.it = repeat(None, n) 83 84 class TestXrange(TestInvariantWithoutMutations, unittest.TestCase): 85 86 def setUp(self): 87 self.it = iter(range(n)) 88 89 class TestXrangeCustomReversed(TestInvariantWithoutMutations, unittest.TestCase): 90 91 def setUp(self): 92 self.it = reversed(range(n)) 93 94 class TestTuple(TestInvariantWithoutMutations, unittest.TestCase): 95 96 def setUp(self): 97 self.it = iter(tuple(range(n))) 98 99 ## ------- Types that should not be mutated during iteration ------- 100 101 class TestDeque(TestTemporarilyImmutable, unittest.TestCase): 102 103 def setUp(self): 104 d = deque(range(n)) 105 self.it = iter(d) 106 self.mutate = d.pop 107 108 class TestDequeReversed(TestTemporarilyImmutable, unittest.TestCase): 109 110 def setUp(self): 111 d = deque(range(n)) 112 self.it = reversed(d) 113 self.mutate = d.pop 114 115 class TestDictKeys(TestTemporarilyImmutable, unittest.TestCase): 116 117 def setUp(self): 118 d = dict.fromkeys(range(n)) 119 self.it = iter(d) 120 self.mutate = d.popitem 121 122 class TestDictItems(TestTemporarilyImmutable, unittest.TestCase): 123 124 def setUp(self): 125 d = dict.fromkeys(range(n)) 126 self.it = iter(d.items()) 127 self.mutate = d.popitem 128 129 class TestDictValues(TestTemporarilyImmutable, unittest.TestCase): 130 131 def setUp(self): 132 d = dict.fromkeys(range(n)) 133 self.it = iter(d.values()) 134 self.mutate = d.popitem 135 136 class TestSet(TestTemporarilyImmutable, unittest.TestCase): 137 138 def setUp(self): 139 d = set(range(n)) 140 self.it = iter(d) 141 self.mutate = d.pop 142 143 ## ------- Types that can mutate during iteration ------- 144 145 class TestList(TestInvariantWithoutMutations, unittest.TestCase): 146 147 def setUp(self): 148 self.it = iter(range(n)) 149 150 def test_mutation(self): 151 d = list(range(n)) 152 it = iter(d) 153 next(it) 154 next(it) 155 self.assertEqual(length_hint(it), n - 2) 156 d.append(n) 157 self.assertEqual(length_hint(it), n - 1) # grow with append 158 d[1:] = [] 159 self.assertEqual(length_hint(it), 0) 160 self.assertEqual(list(it), []) 161 d.extend(range(20)) 162 self.assertEqual(length_hint(it), 0) 163 164 165 class TestListReversed(TestInvariantWithoutMutations, unittest.TestCase): 166 167 def setUp(self): 168 self.it = reversed(range(n)) 169 170 def test_mutation(self): 171 d = list(range(n)) 172 it = reversed(d) 173 next(it) 174 next(it) 175 self.assertEqual(length_hint(it), n - 2) 176 d.append(n) 177 self.assertEqual(length_hint(it), n - 2) # ignore append 178 d[1:] = [] 179 self.assertEqual(length_hint(it), 0) 180 self.assertEqual(list(it), []) # confirm invariant 181 d.extend(range(20)) 182 self.assertEqual(length_hint(it), 0) 183 184 ## -- Check to make sure exceptions are not suppressed by __length_hint__() 185 186 187 class BadLen(object): 188 def __iter__(self): 189 return iter(range(10)) 190 191 def __len__(self): 192 raise RuntimeError('hello') 193 194 195 class BadLengthHint(object): 196 def __iter__(self): 197 return iter(range(10)) 198 199 def __length_hint__(self): 200 raise RuntimeError('hello') 201 202 203 class NoneLengthHint(object): 204 def __iter__(self): 205 return iter(range(10)) 206 207 def __length_hint__(self): 208 return NotImplemented 209 210 211 class TestLengthHintExceptions(unittest.TestCase): 212 213 def test_issue1242657(self): 214 self.assertRaises(RuntimeError, list, BadLen()) 215 self.assertRaises(RuntimeError, list, BadLengthHint()) 216 self.assertRaises(RuntimeError, [].extend, BadLen()) 217 self.assertRaises(RuntimeError, [].extend, BadLengthHint()) 218 b = bytearray(range(10)) 219 self.assertRaises(RuntimeError, b.extend, BadLen()) 220 self.assertRaises(RuntimeError, b.extend, BadLengthHint()) 221 222 def test_invalid_hint(self): 223 # Make sure an invalid result doesn't muck-up the works 224 self.assertEqual(list(NoneLengthHint()), list(range(10))) 225 226 227 if __name__ == "__main__": 228 unittest.main() 229
