I'm implementing the buffer API and some of memoryview for Jython. I
have read with interest, and mostly understood, the discussion in Issue
#10181 that led to the v3.3 re-implementation of memoryview and
much-improved documentation of the buffer API. Although Jython is
targeting v2.7 at the moment, and 1-D bytes (there's no Jython NumPy),
I'd like to lay a solid foundation that benefits from the recent CPython
work. I hope that some of the complexity in memoryview stems from legacy
considerations I don't have to deal with in Jython.
I am puzzled that PEP 3118 makes some specifications that seem
unnecessary and complicate the implementation. Would those who know the
API inside out answer a few questions?
My understanding is this: When a consumer requests a buffer from the
exporter it specifies using flags how it intends to navigate it. If the
buffer actually needs more apparatus than the consumer proposes, this
raises an exception. If the buffer needs less apparatus than the
consumer proposes, the exporter has to supply what was asked for. For
example, if the consumer sets PyBUF_STRIDES, and the buffer can only be
navigated by using suboffsets (PIL-style) this raises an exception.
Alternatively, if the consumer sets PyBUF_STRIDES, and the buffer is
just a simple byte array, the exporter has to supply shape and strides
arrays (with trivial values), since the consumer is going to use those
Is there any harm is supplying shape and strides when they were not
requested? The PEP says: "PyBUF_ND ... If this is not given then shape
will be NULL". It doesn't stipulate that strides will be null if
PyBUF_STRIDES is not given, but the library documentation says so.
suboffsets is different since even when requested, it will be null if
Similar, but simpler, the PEP says "PyBUF_FORMAT ... If format is not
explicitly requested then the format must be returned as NULL (which
means "B", or unsigned bytes)". What would be the harm in returning "B"?
One place where this really matters is in the implementation of
memoryview. PyMemoryView requests a buffer with the flags PyBUF_FULL_RO,
so even a simple byte buffer export will come with shape, strides and
format. A consumer (of the memoryview's buffer API) might specify
PyBUF_SIMPLE: according to the PEP I can't simply give it the original
buffer since required fields (that the consumer will presumably not
access) are not NULL. In practice, I'd like to: what could possibly go