[New-bugs-announce] [issue13797] Add a __buffer__ special method to retrieve a PEP 3118 object

[Nick Coghlan]

New submission from Nick Coghlan <ncoghlan at gmail.com>:

Currently, there's no straightforward way to create new classes in Python that delegate the PEP 3118 buffer APIs to another object that supports them (e.g. bytes, bytearray, memoryview, array.array, a NumPy array).

I see a few possible ways to go about this:

1. Add a single new special method, __buffer__. This creates an entry in the underlying getbuffer slot, but leaves releasebuffer empty.

The getbuffer wrapper then calls __buffer__ and delegates the getbuffer call to the returned object.

Any created Py_buffer instances refer directly to the underlying PEP 3118 supporting object, not the original object that defines __buffer__

The downside of this approach is that you can't implement a completely new PEP 3118 supporting object in pure Python (since you don't get access to the flags)

2. As above, but any Py_buffer structs returned are updated to refer to the original object, not the underlying one. A releasebuffer wrapper is also created, that calls back into __buffer__ to retrieve the object again. This approach has problems if __buffer__ can change to refer to different objects over time.

3. Define separate __getbuffer__ and __releasebuffer__ special methods. All C types implementing the PEP 3118 slots get these special methods automatically (as with any other slot). The flag variables are passed into __getbuffer__, and it is expected to return a memoryview object defining the view. The underlying getbuffer slot wrapper stores the current object in the Py_buffer "obj" slot, and the returned memoryview in "internal". The rest of the Py_buffer fields are populated based on the memoryview contents.

The releasebuffer wrapper then reverses this process, by passing the memoryview instance from the internal slot into the __releasebuffer__ call.

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components: Interpreter Core
messages: 151347
nosy: ncoghlan, pitrou, skrah
priority: normal
severity: normal
status: open
title: Add a __buffer__ special method to retrieve a PEP 3118 object
versions: Python 3.3

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<http://bugs.python.org/issue13797>
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