gh-154566: Fix array.byteswap() corrupting 'Zd' arrays with more than one element#154567
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gh-154566: Fix array.byteswap() corrupting 'Zd' arrays with more than one element#154567PhysicistJohn wants to merge 1 commit into
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…e than one element The 16-byte-item loop in array_array_byteswap_impl() advanced the buffer pointer by only 8 bytes per iteration, even though each iteration swaps a full 16-byte item (two independent 8-byte halves for the real/imaginary double components). This caused every item after the first to overlap the previous iteration's byte window and come out scrambled. A single byteswap() call gave wrong bytes for the second item onward. Calling byteswap() twice happened to round-trip back to the original value (the corruption is self-canceling under double application), which is why the existing test suite -- which only checked a double-call round-trip -- didn't catch it. Added a regression test that checks the actual byte-level result of a single call.
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skirpichev
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Jul 24, 2026
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| Fix :meth:`array.array.byteswap` corrupting data for ``'Zd'`` (complex | ||
| double) arrays with more than one element: the 16-byte item loop advanced | ||
| the buffer pointer by only 8 bytes per iteration, causing items after the | ||
| first to be scrambled. A single :meth:`!byteswap` call produced incorrect | ||
| results while a double call happened to round-trip back to the original | ||
| value, which is why the existing test suite did not catch it. |
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Suggested change
| Fix :meth:`array.array.byteswap` corrupting data for ``'Zd'`` (complex | |
| double) arrays with more than one element: the 16-byte item loop advanced | |
| the buffer pointer by only 8 bytes per iteration, causing items after the | |
| first to be scrambled. A single :meth:`!byteswap` call produced incorrect | |
| results while a double call happened to round-trip back to the original | |
| value, which is why the existing test suite did not catch it. | |
| Fix :meth:`array.array.byteswap` corrupting data for ``'Zd'`` (complex | |
| double) arrays with more than one element: the 16-byte item loop advanced | |
| the buffer pointer by only 8 bytes per iteration, causing items after the | |
| first to be scrambled. |
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| # halves (real, imag) independently. The pre-existing test only | ||
| # checked that byteswap() twice round-trips to the original, | ||
| # which passes even if a single call scrambles multi-item arrays. |
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| # halves (real, imag) independently. The pre-existing test only | |
| # checked that byteswap() twice round-trips to the original, | |
| # which passes even if a single call scrambles multi-item arrays. | |
| # halves (real, imag) independently. |
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| a = array.array(self.typecode, [1+2j, 3+4j, 5+6j]) | ||
| original = a.tobytes() | ||
| a.byteswap() | ||
| expected = bytearray() | ||
| for i in range(0, len(original), 16): | ||
| item = original[i:i + 16] | ||
| expected += item[7::-1] + item[15:7:-1] | ||
| self.assertEqual(a.tobytes(), bytes(expected)) |
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I suggest to do this in a more generic way:
- Move to CFPTest
- use self.examples
- support all possible itemsize's
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array.array('Zd', ...).byteswap()scrambled every item after the firstbecause the 16-byte-item loop advanced the pointer by 8 bytes instead of
16. Fixes gh-154566.
Adds a regression test that checks the actual byte-level result of a
single
byteswap()call (the existing test only checked a double-callround-trip, which passes even under the bug since the corruption happens
to be self-canceling on the second call).