[issue16272] C-API documentation clarification for tp_dictoffset

New submission from Chris Colbert:
The documentation of the tp_dictoffset is a bit unclear when describing the responsibilities of a base type with a nonzero tp_dictoffset. http://docs.python.org/c-api/typeobj.html
I feel there should some statement to the effect of:
"""
If a type defines a nonzero tp_dictoffset, that type is responsible for defining a `__dict__` slot as part of the tp_getset structures. Failure to do so will result in the dict being inaccesible from Python via `obj.__dict__` from instances of the type or subtypes.
"""
The reasoning is twofold:
1) `PyType_Ready` does not add the default getset members like `type_new` does. This prevents the instances of the type itself from retrieving `obj.__dict__`
2) `type_new` will provide the default `subtype_dict` getset member for subclasses, but this calls `get_builtin_base_with_dict` which will resolve to the most base type which is not heap allocated; in this case, the C type. Since this type has no `__dict__` getset member, the lookup fails.
Adding a bit of verbage about this "gotcha" would likely save some headaches in the future.
---------- assignee: docs@python components: Documentation messages: 173222 nosy: Chris.Colbert, docs@python priority: normal severity: normal status: open title: C-API documentation clarification for tp_dictoffset versions: Python 2.6, Python 2.7, Python 3.1, Python 3.2, Python 3.3, Python 3.4, Python 3.5
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Changes by Andrew Svetlov andrew.svetlov@gmail.com:
---------- nosy: +asvetlov
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Changes by Martin Kysel martin@martinkysel.com:
---------- nosy: +mkysel
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Mark Lawrence added the comment:
@Chris can you prepare a patch for this?
---------- nosy: +BreamoreBoy versions: -Python 2.6, Python 3.1, Python 3.2, Python 3.3
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eryksun added the comment:
It could also mention the generic getter and setter functions for the PyGetSetDef that were added in 3.3: PyType_GenericGetDict and PyType_GenericSetDict.
https://docs.python.org/3/c-api/object.html#c.PyType_GenericGetDict
---------- nosy: +eryksun
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Change by Mark Lawrence breamoreboy@gmail.com:
---------- nosy: -BreamoreBoy
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Change by Eryk Sun eryksun@gmail.com:
---------- Removed message: https://bugs.python.org/msg221020
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Change by Eryk Sun eryksun@gmail.com:
---------- type: -> enhancement versions: +Python 3.10, Python 3.8, Python 3.9 -Python 2.7, Python 3.4, Python 3.5
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participants (6)
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Andrew Svetlov
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Chris Colbert
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Eryk Sun
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eryksun
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Mark Lawrence
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Martin Kysel