[issue23756] Tighten definition of bytes-like objects

[Martin Panter]

Martin Panter added the comment:

After doing a bit of reading and experimenting, I think we should at least restrict bytes-like objects to “C-contiguous”. Any looser definition risks memoryview(byteslike).tobytes() returning the bytes in a different order than their true in-memory order. Fortran-style contiguous arrays aren’t enough:

>>> import _testbuffer, sys
>>> fortran = memoryview(_testbuffer.ndarray([11, 12, 21, 22], format="B", flags=0, shape=[2, 2], strides=[1, 2], offset=0))
>>> fortran.f_contiguous
True
>>> fortran.c_contiguous
False
>>> fortran.tolist()
[[11, 21], [12, 22]]
>>> tuple(bytes(fortran))
(11, 21, 12, 22)
>>> sys.stdout.buffer.write(fortran)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
BufferError: memoryview: underlying buffer is not C-contiguous

So I am proposing a patch which:

* Restricts the bytes-like object definition to C-contiguous buffers
* Explains what I think is actually meant by “contiguous” in the C API buffer protocol page. Turns out it is generally a more strict definition than I originally assumed.
* Explains why memoryview.tobytes() is out of order for non C-contiguous buffers
* Has a couple other fixes taking into acount memoryview.tolist() doesn’t work for zero dimensions, and is nested for more than one dimension

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keywords: +patch
Added file: http://bugs.python.org/file38780/c-contig.patch

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<http://bugs.python.org/issue23756>
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