[docs] [issue16272] C-API documentation clarification for tp_dictoffset

Chris Colbert report at bugs.python.org
Thu Oct 18 02:27:43 CEST 2012

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.

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 at python
components: Documentation
messages: 173222
nosy: Chris.Colbert, docs at 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 at bugs.python.org>

More information about the docs mailing list