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

_______________________________________ Python tracker report@bugs.python.org http://bugs.python.org/issue16272 _______________________________________