test_slice.py

# tests for slice objects; in particular the indices method.

import itertools
import operator
import sys
import unittest
import weakref

from pickle import loads, dumps
from test import support


def evaluate_slice_index(arg):
    """
    Helper function to convert a slice argument to an integer, and raise
    TypeError with a suitable message on failure.

    """
    if hasattr(arg, '__index__'):
        return operator.index(arg)
    else:
        raise TypeError(
            "slice indices must be integers or "
            "None or have an __index__ method")

def slice_indices(slice, length):
    """
    Reference implementation for the slice.indices method.

    """
    # Compute step and length as integers.
    length = operator.index(length)
    step = 1 if slice.step is None else evaluate_slice_index(slice.step)

    # Raise ValueError for negative length or zero step.
    if length < 0:
        raise ValueError("length should not be negative")
    if step == 0:
        raise ValueError("slice step cannot be zero")

    # Find lower and upper bounds for start and stop.
    lower = -1 if step < 0 else 0
    upper = length - 1 if step < 0 else length

    # Compute start.
    if slice.start is None:
        start = upper if step < 0 else lower
    else:
        start = evaluate_slice_index(slice.start)
        start = max(start + length, lower) if start < 0 else min(start, upper)

    # Compute stop.
    if slice.stop is None:
        stop = lower if step < 0 else upper
    else:
        stop = evaluate_slice_index(slice.stop)
        stop = max(stop + length, lower) if stop < 0 else min(stop, upper)

    return start, stop, step


# Class providing an __index__ method.  Used for testing slice.indices.

class MyIndexable(object):
    def __init__(self, value):
        self.value = value

    def __index__(self):
        return self.value


class SliceTest(unittest.TestCase):

    def test_constructor(self):
        self.assertRaises(TypeError, slice)
        self.assertRaises(TypeError, slice, 1, 2, 3, 4)

    def test_repr(self):
        self.assertEqual(repr(slice(1, 2, 3)), "slice(1, 2, 3)")

    def test_hash(self):
        # Verify clearing of SF bug #800796
        self.assertRaises(TypeError, hash, slice(5))
        with self.assertRaises(TypeError):
            slice(5).__hash__()

    def test_cmp(self):
        s1 = slice(1, 2, 3)
        s2 = slice(1, 2, 3)
        s3 = slice(1, 2, 4)
        self.assertEqual(s1, s2)
        self.assertNotEqual(s1, s3)
        self.assertNotEqual(s1, None)
        self.assertNotEqual(s1, (1, 2, 3))
        self.assertNotEqual(s1, "")

        class Exc(Exception):
            pass

        class BadCmp(object):
            def __eq__(self, other):
                raise Exc

        s1 = slice(BadCmp())
        s2 = slice(BadCmp())
        self.assertEqual(s1, s1)
        self.assertRaises(Exc, lambda: s1 == s2)

        s1 = slice(1, BadCmp())
        s2 = slice(1, BadCmp())
        self.assertEqual(s1, s1)
        self.assertRaises(Exc, lambda: s1 == s2)

        s1 = slice(1, 2, BadCmp())
        s2 = slice(1, 2, BadCmp())
        self.assertEqual(s1, s1)
        self.assertRaises(Exc, lambda: s1 == s2)

    def test_members(self):
        s = slice(1)
        self.assertEqual(s.start, None)
        self.assertEqual(s.stop, 1)
        self.assertEqual(s.step, None)

        s = slice(1, 2)
        self.assertEqual(s.start, 1)
        self.assertEqual(s.stop, 2)
        self.assertEqual(s.step, None)

        s = slice(1, 2, 3)
        self.assertEqual(s.start, 1)
        self.assertEqual(s.stop, 2)
        self.assertEqual(s.step, 3)

        class AnyClass:
            pass

        obj = AnyClass()
        s = slice(obj)
        self.assertTrue(s.stop is obj)

    def check_indices(self, slice, length):
        try:
            actual = slice.indices(length)
        except ValueError:
            actual = "valueerror"
        try:
            expected = slice_indices(slice, length)
        except ValueError:
            expected = "valueerror"
        self.assertEqual(actual, expected)

        if length >= 0 and slice.step != 0:
            actual = range(*slice.indices(length))
            expected = range(length)[slice]
            self.assertEqual(actual, expected)

    def test_indices(self):
        self.assertEqual(slice(None           ).indices(10), (0, 10,  1))
        self.assertEqual(slice(None,  None,  2).indices(10), (0, 10,  2))
        self.assertEqual(slice(1,     None,  2).indices(10), (1, 10,  2))
        self.assertEqual(slice(None,  None, -1).indices(10), (9, -1, -1))
        self.assertEqual(slice(None,  None, -2).indices(10), (9, -1, -2))
        self.assertEqual(slice(3,     None, -2).indices(10), (3, -1, -2))
        # issue 3004 tests
        self.assertEqual(slice(None, -9).indices(10), (0, 1, 1))
        self.assertEqual(slice(None, -10).indices(10), (0, 0, 1))
        self.assertEqual(slice(None, -11).indices(10), (0, 0, 1))
        self.assertEqual(slice(None, -10, -1).indices(10), (9, 0, -1))
        self.assertEqual(slice(None, -11, -1).indices(10), (9, -1, -1))
        self.assertEqual(slice(None, -12, -1).indices(10), (9, -1, -1))
        self.assertEqual(slice(None, 9).indices(10), (0, 9, 1))
        self.assertEqual(slice(None, 10).indices(10), (0, 10, 1))
        self.assertEqual(slice(None, 11).indices(10), (0, 10, 1))
        self.assertEqual(slice(None, 8, -1).indices(10), (9, 8, -1))
        self.assertEqual(slice(None, 9, -1).indices(10), (9, 9, -1))
        self.assertEqual(slice(None, 10, -1).indices(10), (9, 9, -1))

        self.assertEqual(
            slice(-100,  100     ).indices(10),
            slice(None).indices(10)
        )
        self.assertEqual(
            slice(100,  -100,  -1).indices(10),
            slice(None, None, -1).indices(10)
        )
        self.assertEqual(slice(-100, 100, 2).indices(10), (0, 10,  2))

        self.assertEqual(list(range(10))[::sys.maxsize - 1], [0])

        # Check a variety of start, stop, step and length values, including
        # values exceeding sys.maxsize (see issue #14794).
        vals = [None, -2**100, -2**30, -53, -7, -1, 0, 1, 7, 53, 2**30, 2**100]
        lengths = [0, 1, 7, 53, 2**30, 2**100]
        for slice_args in itertools.product(vals, repeat=3):
            s = slice(*slice_args)
            for length in lengths:
                self.check_indices(s, length)
        self.check_indices(slice(0, 10, 1), -3)

        # Negative length should raise ValueError
        with self.assertRaises(ValueError):
            slice(None).indices(-1)

        # Zero step should raise ValueError
        with self.assertRaises(ValueError):
            slice(0, 10, 0).indices(5)

        # Using a start, stop or step or length that can't be interpreted as an
        # integer should give a TypeError ...
        with self.assertRaises(TypeError):
            slice(0.0, 10, 1).indices(5)
        with self.assertRaises(TypeError):
            slice(0, 10.0, 1).indices(5)
        with self.assertRaises(TypeError):
            slice(0, 10, 1.0).indices(5)
        with self.assertRaises(TypeError):
            slice(0, 10, 1).indices(5.0)

        # ... but it should be fine to use a custom class that provides index.
        self.assertEqual(slice(0, 10, 1).indices(5), (0, 5, 1))
        self.assertEqual(slice(MyIndexable(0), 10, 1).indices(5), (0, 5, 1))
        self.assertEqual(slice(0, MyIndexable(10), 1).indices(5), (0, 5, 1))
        self.assertEqual(slice(0, 10, MyIndexable(1)).indices(5), (0, 5, 1))
        self.assertEqual(slice(0, 10, 1).indices(MyIndexable(5)), (0, 5, 1))

    def test_setslice_without_getslice(self):
        tmp = []
        class X(object):
            def __setitem__(self, i, k):
                tmp.append((i, k))

        x = X()
        x[1:2] = 42
        self.assertEqual(tmp, [(slice(1, 2), 42)])

    def test_pickle(self):
        s = slice(10, 20, 3)
        for protocol in (0,1,2):
            t = loads(dumps(s, protocol))
            self.assertEqual(s, t)
            self.assertEqual(s.indices(15), t.indices(15))
            self.assertNotEqual(id(s), id(t))

    def test_cycle(self):
        class myobj(): pass
        o = myobj()
        o.s = slice(o)
        w = weakref.ref(o)
        o = None
        support.gc_collect()
        self.assertIsNone(w())

if __name__ == "__main__":
    unittest.main()