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super behaviour and abstract base classes (either implementation or documentation/error message is wrong) #64702
Description
| BPO | 20503 |
|---|---|
| Nosy | @rhettinger, @gerritholl, @vadmium, @eryksun, @miss-islington |
| PRs | |
| Dependencies |
Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.
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GitHub fields:
assignee = 'https://github.com/rhettinger'
closed_at = <Date 2021-04-05.20:12:39.078>
created_at = <Date 2014-02-03.15:36:42.585>
labels = ['interpreter-core', 'type-bug', '3.8', '3.9', '3.10']
title = 'super behaviour and abstract base classes (either implementation or documentation/error message is wrong)'
updated_at = <Date 2021-04-05.20:12:39.077>
user = 'https://github.com/gerritholl'bugs.python.org fields:
activity = <Date 2021-04-05.20:12:39.077>
actor = 'rhettinger'
assignee = 'rhettinger'
closed = True
closed_date = <Date 2021-04-05.20:12:39.078>
closer = 'rhettinger'
components = ['Interpreter Core']
creation = <Date 2014-02-03.15:36:42.585>
creator = 'Gerrit.Holl'
dependencies = ['23674']
files = []
hgrepos = []
issue_num = 20503
keywords = ['patch']
message_count = 7.0
messages = ['210141', '235852', '255686', '387731', '387797', '390254', '390258']
nosy_count = 6.0
nosy_names = ['rhettinger', 'docs@python', 'Gerrit.Holl', 'martin.panter', 'eryksun', 'miss-islington']
pr_nums = ['25175', '25202']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue20503'
versions = ['Python 3.8', 'Python 3.9', 'Python 3.10']
Activity
gerritholl commentedon Feb 3, 2014
When using an abstract base class, super(type, obj) throws a TypeError stating "obj must be an instance (...) of type", even though isinstance(obj, type) returns True. I'm not sure what is supposed to happen here, but either the error message and the documentation for super would need to be reformulated, or there is an issue with the implementation, or I am misunderstanding something.
[-]super behavioru and abstract base classes (either implementation or documentation/error message is wrong)[/-][+]super behaviour and abstract base classes (either implementation or documentation/error message is wrong)[/+]eryksun commentedon Feb 12, 2015
Given super(cls, obj), cls needs to be somewhere in type(obj).__mro__. Thus the implementation checks PyType_IsSubtype instead of the more generic PyObject_IsSubclass.
In this case int's MRO is unrelated to numbers.Number:
>>> print(*int.__mro__, sep='\n') <class 'int'> <class 'object'>It gets registered as a subclass via numbers.Integral.register(int).
>>> print(*numbers.Integral._abc_registry) <class 'int'>issubclass calls PyObject_IsSubclass, which uses the __subclasscheck__ API. In this case ABCMeta.__subclasscheck__ recursively checks the registry and caches the result to speed up future checks.
>>> numbers.Number.__subclasscheck__(int) True >>> print(*numbers.Number._abc_cache) <class 'int'>vadmium commentedon Dec 2, 2015
I am proposing some documentation changes in bpo-23674 which would address this.
eryksun commentedon Feb 26, 2021
The docs still need to clarified that isinstance(obj, type) is a necessary but not sufficient condition for success. It would also be helpful if the error message were less confusing in the case of registered subclasses such as numbers.Number.
rhettinger commentedon Feb 28, 2021
This is a bit out of date. The isinstance() docs now specify that the predicate tests for direct, indirect, and virtual inheritance. Likewise, the super() docs already specify that only the __mro__ is searched. So those docs are already precise.
Am thinking of adding a FAQ entry about when direct inheritance is required and include the OP's example. FWIW, this isn't really about super(). It affects any attribute lookup or any other property of inheritance. A true result from instance() doesn't imply that actual inheritance has occurred.
rhettinger commentedon Apr 5, 2021
New changeset 7bc25ec by Raymond Hettinger in branch 'master':
bpo-20503: Show how isinstance() works with ABC registered classes. (GH-25175)
7bc25ec
rhettinger commentedon Apr 5, 2021
New changeset 028d528 by Miss Islington (bot) in branch '3.9':
bpo-20503: Show how isinstance() works with ABC registered classes. (GH-25175) (GH-25202)
028d528