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257 lines
7.5 KiB
Python
257 lines
7.5 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 xrange(10) always reports its length as ten,
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but it=iter(xrange(10)) starts at ten, and then goes to nine after it.next().
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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, xrange 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 permantently 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 dynanamically 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 test import test_support
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from itertools import repeat
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from collections import deque
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from UserList import UserList
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from __builtin__ import len as _len
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n = 10
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def len(obj):
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try:
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return _len(obj)
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except TypeError:
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try:
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# note: this is an internal undocumented API,
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# don't rely on it in your own programs
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return obj.__length_hint__()
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except AttributeError:
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raise TypeError
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class TestInvariantWithoutMutations(unittest.TestCase):
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def test_invariant(self):
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it = self.it
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for i in reversed(xrange(1, n+1)):
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self.assertEqual(len(it), i)
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it.next()
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self.assertEqual(len(it), 0)
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self.assertRaises(StopIteration, it.next)
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self.assertEqual(len(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(len(it), n)
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it.next()
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self.assertEqual(len(it), n-1)
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self.mutate()
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self.assertRaises(RuntimeError, it.next)
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self.assertEqual(len(it), 0)
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## ------- Concrete Type Tests -------
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class TestRepeat(TestInvariantWithoutMutations):
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def setUp(self):
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self.it = repeat(None, n)
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def test_no_len_for_infinite_repeat(self):
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# The repeat() object can also be infinite
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self.assertRaises(TypeError, len, repeat(None))
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class TestXrange(TestInvariantWithoutMutations):
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def setUp(self):
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self.it = iter(xrange(n))
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class TestXrangeCustomReversed(TestInvariantWithoutMutations):
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def setUp(self):
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self.it = reversed(xrange(n))
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class TestTuple(TestInvariantWithoutMutations):
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def setUp(self):
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self.it = iter(tuple(xrange(n)))
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## ------- Types that should not be mutated during iteration -------
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class TestDeque(TestTemporarilyImmutable):
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def setUp(self):
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d = deque(xrange(n))
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self.it = iter(d)
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self.mutate = d.pop
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class TestDequeReversed(TestTemporarilyImmutable):
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def setUp(self):
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d = deque(xrange(n))
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self.it = reversed(d)
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self.mutate = d.pop
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class TestDictKeys(TestTemporarilyImmutable):
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def setUp(self):
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d = dict.fromkeys(xrange(n))
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self.it = iter(d)
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self.mutate = d.popitem
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class TestDictItems(TestTemporarilyImmutable):
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def setUp(self):
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d = dict.fromkeys(xrange(n))
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self.it = d.iteritems()
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self.mutate = d.popitem
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class TestDictValues(TestTemporarilyImmutable):
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def setUp(self):
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d = dict.fromkeys(xrange(n))
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self.it = d.itervalues()
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self.mutate = d.popitem
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class TestSet(TestTemporarilyImmutable):
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def setUp(self):
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d = set(xrange(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):
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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 = range(n)
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it = iter(d)
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it.next()
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it.next()
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self.assertEqual(len(it), n-2)
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d.append(n)
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self.assertEqual(len(it), n-1) # grow with append
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d[1:] = []
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self.assertEqual(len(it), 0)
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self.assertEqual(list(it), [])
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d.extend(xrange(20))
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self.assertEqual(len(it), 0)
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class TestListReversed(TestInvariantWithoutMutations):
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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 = range(n)
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it = reversed(d)
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it.next()
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it.next()
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self.assertEqual(len(it), n-2)
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d.append(n)
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self.assertEqual(len(it), n-2) # ignore append
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d[1:] = []
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self.assertEqual(len(it), 0)
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self.assertEqual(list(it), []) # confirm invariant
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d.extend(xrange(20))
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self.assertEqual(len(it), 0)
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class TestSeqIter(TestInvariantWithoutMutations):
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def setUp(self):
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self.it = iter(UserList(range(n)))
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def test_mutation(self):
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d = UserList(range(n))
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it = iter(d)
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it.next()
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it.next()
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self.assertEqual(len(it), n-2)
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d.append(n)
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self.assertEqual(len(it), n-1) # grow with append
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d[1:] = []
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self.assertEqual(len(it), 0)
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self.assertEqual(list(it), [])
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d.extend(xrange(20))
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self.assertEqual(len(it), 0)
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class TestSeqIterReversed(TestInvariantWithoutMutations):
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def setUp(self):
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self.it = reversed(UserList(range(n)))
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def test_mutation(self):
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d = UserList(range(n))
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it = reversed(d)
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it.next()
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it.next()
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self.assertEqual(len(it), n-2)
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d.append(n)
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self.assertEqual(len(it), n-2) # ignore append
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d[1:] = []
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self.assertEqual(len(it), 0)
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self.assertEqual(list(it), []) # confirm invariant
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d.extend(xrange(20))
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self.assertEqual(len(it), 0)
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if __name__ == "__main__":
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unittests = [
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TestRepeat,
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TestXrange,
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TestXrangeCustomReversed,
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TestTuple,
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TestDeque,
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TestDequeReversed,
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TestDictKeys,
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TestDictItems,
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TestDictValues,
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TestSet,
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TestList,
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TestListReversed,
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TestSeqIter,
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TestSeqIterReversed,
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]
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test_support.run_unittest(*unittests)
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