non-Pythonic fate of __abstractmethods__

[YaroslavHalchenko]

BPO	10006
Nosy	@benjaminp, @Trundle, @durban

^{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/benjaminp'
closed_at = <Date 2010-10-02.00:05:23.953>
created_at = <Date 2010-10-01.13:47:31.944>
labels = []
title = 'non-Pythonic fate of __abstractmethods__'
updated_at = <Date 2010-10-24.17:34:12.899>
user = 'https://bugs.python.org/YaroslavHalchenko'

bugs.python.org fields:

activity = <Date 2010-10-24.17:34:12.899>
actor = 'eric.araujo'
assignee = 'benjamin.peterson'
closed = True
closed_date = <Date 2010-10-02.00:05:23.953>
closer = 'benjamin.peterson'
components = []
creation = <Date 2010-10-01.13:47:31.944>
creator = 'Yaroslav.Halchenko'
dependencies = []
files = []
hgrepos = []
issue_num = 10006
keywords = []
message_count = 5.0
messages = ['117798', '117814', '117853', '117861', '117885']
nosy_count = 4.0
nosy_names = ['benjamin.peterson', 'Trundle', 'daniel.urban', 'Yaroslav.Halchenko']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = None
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue10006'
versions = ['Python 3.2']

[YaroslavHalchenko]

We ran into this while generating documentation for our project (PyMVPA) with recent sphinx and python2.6 (fine with 2.5, failed for 2.6, 2.7, 3.1), which relies on traversing all attributes given by "dir(obj)", BUT apparently __abstractmethods__ becomes very special -- it is given by "dir(obj)" since it is present in obj.__class__, but getattr(obj, "__abstractmethods__") fails for classes derived from type. E.g. following sample demonstrates it:

print("in type's dir" , '__abstractmethods__' in dir(type))
print(type.__abstractmethods__)

class type3(type):
    pass

print("in type3's dir" , '__abstractmethods__' in dir(type3))
print(type3.__abstractmethods__)

results in output:

$> python2.6 trash/type_subclass.py
("in type's dir", True)
<attribute '__abstractmethods__' of 'type' objects>
("in type3's dir", True)
Traceback (most recent call last):
  File "trash/type_subclass.py", line 9, in <module>
    print(type3.__abstractmethods__)
AttributeError: __abstractmethods__


$> python3.1 trash/type_subclass.py 
in type's dir True
<attribute '__abstractmethods__' of 'type' objects>
in type3's dir True
Traceback (most recent call last):
  File "trash/type_subclass.py", line 9, in <module>
    print(type3.__abstractmethods__)
AttributeError: __abstractmethods__

And that seems to be the only attribute behaving like that (others are fine and accessible). Some people even seems to provide workarounds already, e.g.:
http://bitbucket.org/DasIch/bpython-colorful/src/19bb4cb0a65d/bpython/repl.py
when __abstractmethods__ is accessed only for the subclasses of ABCmeta ...

so, is it a bug or a feature (so we have to take care about it in all traversals of attributes given by dir())? ;)

[benjaminp]

I see the problem; will consider/fix later today hopefully.

[benjaminp]

As of r85154, type.__abstractmethods__ now raises an AttributeError, too.

[YaroslavHalchenko]

yikes... surprising resolution -- I expected that fix would either makes __abstractmethods__ accessible in derived "type"s or becomes absent from output of dir() -- but none of those has happened. Now we ended up with a consistent non-Pythonic fate of __abstractmethods__ listed in output of dir() but not accessible. is that a feature?

[benjaminp]

2010/10/1 Yaroslav Halchenko <report@bugs.python.org>:

Yaroslav Halchenko <yarikoptic@gmail.com> added the comment:

yikes... surprising resolution -- I expected that fix would either makes __abstractmethods__ accessible in derived "type"s or becomes absent from output of dir() -- but none of those has happened.  Now we ended up with a consistent non-Pythonic fate of __abstractmethods__ listed in output of dir() but not accessible.  is that a feature?

type has no __abstractmethods__, so it should raise an AttributeError.
Note descriptors are allowed to raise AttributeError even if they're
in dir().