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According to https://docs.python.org/2/c-api/buffer.html#the-new-style-py-buffer-struct if the suboffsets member of Py_buffer is non-NULL and all members of the array are negative, the buffer may be contiguous.
PyBuffer_IsContiguous() does not behave this way. It assumes that if the suboffsets member is non-NULL then at least one member of the array is non-negative, and thus assumes that the buffer is non-contiguous. This is not always correct.
One consequence of this bug is PyBuffer_ToContiguous() (and thus memoryview.tobytes()) falls back to slow (and currently buggy, see issue #23349) memory copying code.
Attached is a patch that fixes this bug. The patch is against 2.7 but can be trivially modified to apply to 3.x.