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Test failure in test_find_periodic_pattern in string_tests.py #105639

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sweeneyde opened this issue Jun 10, 2023 · 2 comments
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Test failure in test_find_periodic_pattern in string_tests.py #105639

sweeneyde opened this issue Jun 10, 2023 · 2 comments
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tests Tests in the Lib/test dir type-bug An unexpected behavior, bug, or error

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@sweeneyde
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sweeneyde commented Jun 10, 2023 •
edited by bedevere-bot
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A recent test failure:

FAIL: test_find_periodic_pattern (test.test_bytes.ByteArrayAsStringTest.test_find_periodic_pattern) (p='', text='')
Cover the special path for periodic patterns.
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/runner/work/cpython/cpython-ro-srcdir/Lib/test/string_tests.py", line 335, in test_find_periodic_pattern
    self.checkequal(reference_find(p, text),
  File "/home/runner/work/cpython/cpython-ro-srcdir/Lib/test/string_tests.py", line 60, in checkequal
    self.assertEqual(
AssertionError: -1 != 0

The issue is reference_find:

        def reference_find(p, s):
            for i in range(len(s)):
                if s.startswith(p, i):
                    return i
            return -1

When s == p == '', we get for i in range(0), and then -1 is returned instead of the actual result which should be 0.

Linked PRs
@sweeneyde sweeneyde added type-bug An unexpected behavior, bug, or error tests Tests in the Lib/test dir labels Jun 10, 2023
@bedevere-bot bedevere-bot mentioned this issue Jun 10, 2023
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@terryjreedy
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My immediate question was "What could have happened that this should suddenly fail?" The answer is "Nothing!" For each CI run, reference_find is called 1000 times with random input. There is obviously a non-zero probabilty of generating this corner case for which the test as a whole is wrong. Hence we get an occasional bogus failure. Some possible fixes:

  • Do not generate this corner case; I have no idea how right now.
  • Do not test this easy-to-detect case: if not p or not text: continue after text is generated, or perhaps if not p: continue after p is generated.
  • Change reference_find to return 0 for this case, making sure to not introduce another bug.

The test was added in #86138, with PR by Dennis Sweeny and Tim Peters.

@hugovk
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hugovk commented Mar 7, 2025

Thanks for the report, however it's been subsequently resolved by PR #110170 and issue #110160.

@hugovk hugovk closed this as completed Mar 7, 2025
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