mirror of https://github.com/python/cpython
259 lines
7.5 KiB
Python
259 lines
7.5 KiB
Python
""" Test Iterator Length Transparency
|
|
|
|
Some functions or methods which accept general iterable arguments have
|
|
optional, more efficient code paths if they know how many items to expect.
|
|
For instance, map(func, iterable), will pre-allocate the exact amount of
|
|
space required whenever the iterable can report its length.
|
|
|
|
The desired invariant is: len(it)==len(list(it)).
|
|
|
|
A complication is that an iterable and iterator can be the same object. To
|
|
maintain the invariant, an iterator needs to dynamically update its length.
|
|
For instance, an iterable such as range(10) always reports its length as ten,
|
|
but it=iter(range(10)) starts at ten, and then goes to nine after next(it).
|
|
Having this capability means that map() can ignore the distinction between
|
|
map(func, iterable) and map(func, iter(iterable)).
|
|
|
|
When the iterable is immutable, the implementation can straight-forwardly
|
|
report the original length minus the cumulative number of calls to next().
|
|
This is the case for tuples, range objects, and itertools.repeat().
|
|
|
|
Some containers become temporarily immutable during iteration. This includes
|
|
dicts, sets, and collections.deque. Their implementation is equally simple
|
|
though they need to permantently set their length to zero whenever there is
|
|
an attempt to iterate after a length mutation.
|
|
|
|
The situation slightly more involved whenever an object allows length mutation
|
|
during iteration. Lists and sequence iterators are dynanamically updatable.
|
|
So, if a list is extended during iteration, the iterator will continue through
|
|
the new items. If it shrinks to a point before the most recent iteration,
|
|
then no further items are available and the length is reported at zero.
|
|
|
|
Reversed objects can also be wrapped around mutable objects; however, any
|
|
appends after the current position are ignored. Any other approach leads
|
|
to confusion and possibly returning the same item more than once.
|
|
|
|
The iterators not listed above, such as enumerate and the other itertools,
|
|
are not length transparent because they have no way to distinguish between
|
|
iterables that report static length and iterators whose length changes with
|
|
each call (i.e. the difference between enumerate('abc') and
|
|
enumerate(iter('abc')).
|
|
|
|
"""
|
|
|
|
import unittest
|
|
from test import test_support
|
|
from itertools import repeat
|
|
from collections import deque
|
|
from UserList import UserList
|
|
from __builtin__ import len as _len
|
|
|
|
n = 10
|
|
|
|
def len(obj):
|
|
try:
|
|
return _len(obj)
|
|
except TypeError:
|
|
try:
|
|
# note: this is an internal undocumented API,
|
|
# don't rely on it in your own programs
|
|
return obj.__length_hint__()
|
|
except AttributeError:
|
|
raise TypeError
|
|
|
|
class TestInvariantWithoutMutations(unittest.TestCase):
|
|
|
|
def test_invariant(self):
|
|
it = self.it
|
|
for i in reversed(range(1, n+1)):
|
|
self.assertEqual(len(it), i)
|
|
next(it)
|
|
self.assertEqual(len(it), 0)
|
|
self.assertRaises(StopIteration, next, it)
|
|
self.assertEqual(len(it), 0)
|
|
|
|
class TestTemporarilyImmutable(TestInvariantWithoutMutations):
|
|
|
|
def test_immutable_during_iteration(self):
|
|
# objects such as deques, sets, and dictionaries enforce
|
|
# length immutability during iteration
|
|
|
|
it = self.it
|
|
self.assertEqual(len(it), n)
|
|
next(it)
|
|
self.assertEqual(len(it), n-1)
|
|
self.mutate()
|
|
self.assertRaises(RuntimeError, next, it)
|
|
self.assertEqual(len(it), 0)
|
|
|
|
## ------- Concrete Type Tests -------
|
|
|
|
class TestRepeat(TestInvariantWithoutMutations):
|
|
|
|
def setUp(self):
|
|
self.it = repeat(None, n)
|
|
|
|
def test_no_len_for_infinite_repeat(self):
|
|
# The repeat() object can also be infinite
|
|
self.assertRaises(TypeError, len, repeat(None))
|
|
|
|
class TestXrange(TestInvariantWithoutMutations):
|
|
|
|
def setUp(self):
|
|
self.it = iter(range(n))
|
|
|
|
class TestXrangeCustomReversed(TestInvariantWithoutMutations):
|
|
|
|
def setUp(self):
|
|
self.it = reversed(range(n))
|
|
|
|
class TestTuple(TestInvariantWithoutMutations):
|
|
|
|
def setUp(self):
|
|
self.it = iter(tuple(range(n)))
|
|
|
|
## ------- Types that should not be mutated during iteration -------
|
|
|
|
class TestDeque(TestTemporarilyImmutable):
|
|
|
|
def setUp(self):
|
|
d = deque(range(n))
|
|
self.it = iter(d)
|
|
self.mutate = d.pop
|
|
|
|
class TestDequeReversed(TestTemporarilyImmutable):
|
|
|
|
def setUp(self):
|
|
d = deque(range(n))
|
|
self.it = reversed(d)
|
|
self.mutate = d.pop
|
|
|
|
class TestDictKeys(TestTemporarilyImmutable):
|
|
|
|
def setUp(self):
|
|
d = dict.fromkeys(range(n))
|
|
self.it = iter(d)
|
|
self.mutate = d.popitem
|
|
|
|
class TestDictItems(TestTemporarilyImmutable):
|
|
|
|
def setUp(self):
|
|
d = dict.fromkeys(range(n))
|
|
self.it = iter(d.items())
|
|
self.mutate = d.popitem
|
|
|
|
class TestDictValues(TestTemporarilyImmutable):
|
|
|
|
def setUp(self):
|
|
d = dict.fromkeys(range(n))
|
|
self.it = iter(d.values())
|
|
self.mutate = d.popitem
|
|
|
|
class TestSet(TestTemporarilyImmutable):
|
|
|
|
def setUp(self):
|
|
d = set(range(n))
|
|
self.it = iter(d)
|
|
self.mutate = d.pop
|
|
|
|
## ------- Types that can mutate during iteration -------
|
|
|
|
class TestList(TestInvariantWithoutMutations):
|
|
|
|
def setUp(self):
|
|
self.it = iter(range(n))
|
|
|
|
def test_mutation(self):
|
|
d = list(range(n))
|
|
it = iter(d)
|
|
next(it)
|
|
next(it)
|
|
self.assertEqual(len(it), n-2)
|
|
d.append(n)
|
|
self.assertEqual(len(it), n-1) # grow with append
|
|
d[1:] = []
|
|
self.assertEqual(len(it), 0)
|
|
self.assertEqual(list(it), [])
|
|
d.extend(range(20))
|
|
self.assertEqual(len(it), 0)
|
|
|
|
class TestListReversed(TestInvariantWithoutMutations):
|
|
|
|
def setUp(self):
|
|
self.it = reversed(range(n))
|
|
|
|
def test_mutation(self):
|
|
d = list(range(n))
|
|
it = reversed(d)
|
|
next(it)
|
|
next(it)
|
|
self.assertEqual(len(it), n-2)
|
|
d.append(n)
|
|
self.assertEqual(len(it), n-2) # ignore append
|
|
d[1:] = []
|
|
self.assertEqual(len(it), 0)
|
|
self.assertEqual(list(it), []) # confirm invariant
|
|
d.extend(range(20))
|
|
self.assertEqual(len(it), 0)
|
|
|
|
class TestSeqIter(TestInvariantWithoutMutations):
|
|
|
|
def setUp(self):
|
|
self.it = iter(UserList(range(n)))
|
|
|
|
def test_mutation(self):
|
|
d = UserList(range(n))
|
|
it = iter(d)
|
|
next(it)
|
|
next(it)
|
|
self.assertEqual(len(it), n-2)
|
|
d.append(n)
|
|
self.assertEqual(len(it), n-1) # grow with append
|
|
d[1:] = []
|
|
self.assertEqual(len(it), 0)
|
|
self.assertEqual(list(it), [])
|
|
d.extend(range(20))
|
|
self.assertEqual(len(it), 0)
|
|
|
|
class TestSeqIterReversed(TestInvariantWithoutMutations):
|
|
|
|
def setUp(self):
|
|
self.it = reversed(UserList(range(n)))
|
|
|
|
def test_mutation(self):
|
|
d = UserList(range(n))
|
|
it = reversed(d)
|
|
next(it)
|
|
next(it)
|
|
self.assertEqual(len(it), n-2)
|
|
d.append(n)
|
|
self.assertEqual(len(it), n-2) # ignore append
|
|
d[1:] = []
|
|
self.assertEqual(len(it), 0)
|
|
self.assertEqual(list(it), []) # confirm invariant
|
|
d.extend(range(20))
|
|
self.assertEqual(len(it), 0)
|
|
|
|
|
|
def test_main():
|
|
unittests = [
|
|
TestRepeat,
|
|
TestXrange,
|
|
TestXrangeCustomReversed,
|
|
TestTuple,
|
|
TestDeque,
|
|
TestDequeReversed,
|
|
TestDictKeys,
|
|
TestDictItems,
|
|
TestDictValues,
|
|
TestSet,
|
|
TestList,
|
|
TestListReversed,
|
|
TestSeqIter,
|
|
TestSeqIterReversed,
|
|
]
|
|
test_support.run_unittest(*unittests)
|
|
|
|
if __name__ == "__main__":
|
|
test_main()
|