isinstance(obj, object) returns True for _old style_ class instances

[thread13]

BPO	14671
Nosy	@birkenfeld, @benjaminp, @ezio-melotti

^{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 = None
closed_at = <Date 2012-04-26.06:00:10.340>
created_at = <Date 2012-04-26.02:53:35.538>
labels = ['interpreter-core', 'type-bug']
title = 'isinstance(obj, object) returns True for _old style_ class instances'
updated_at = <Date 2012-04-29.12:23:30.424>
user = 'https://bugs.python.org/thread13'

bugs.python.org fields:

activity = <Date 2012-04-29.12:23:30.424>
actor = 'ezio.melotti'
assignee = 'none'
closed = True
closed_date = <Date 2012-04-26.06:00:10.340>
closer = 'georg.brandl'
components = ['Interpreter Core']
creation = <Date 2012-04-26.02:53:35.538>
creator = 'thread13'
dependencies = []
files = []
hgrepos = []
issue_num = 14671
keywords = []
message_count = 8.0
messages = ['159351', '159352', '159353', '159354', '159358', '159438', '159439', '159599']
nosy_count = 4.0
nosy_names = ['georg.brandl', 'benjamin.peterson', 'ezio.melotti', 'thread13']
pr_nums = []
priority = 'normal'
resolution = 'wont fix'
stage = None
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue14671'
versions = ['Python 2.6']

[thread13]

$python
Python 2.6.5 (r265:79063, Apr 16 2010, 13:09:56) 
[GCC 4.4.3] on linux2
>>> class Old: pass
>>> class New(object): pass
>>> o = Old()
>>> n = New()
>>> isinstance(o, object)
True

This is it, basically. Is it a bug or a feature?

More tests :

>>> isinstance(o, Old)
True
>>> isinstance(o, New)
False
>>> isinstance(n, Old)
False
>>> isinstance(o, int)
False

Please note that some unimportant output was deleted from above.

PS. If this is a feature, how do I detect an old-style class then ?

[thread13]

In addition:

>>> issubclass(Old, object)
False

[benjaminp]

Yes, everything is a object. issubclass, though, works differently for old-style and new-style classes.

[thread13]

>> help(isinstance)

isinstance(...)
    isinstance(object, class-or-type-or-tuple) -> bool
    
    Return whether an object is an instance of a class or of a subclass thereof.
    (...)

So are the old-style class instances descendants of the object?

I feel like I am missing something (except for the fact that you have closed the bug).

[birkenfeld]

This is a result of how old-style classes are implemented.

If you look at type(Old()), you can see that it isn't Old, but "instance".

(And "instance" is a subclass of object again.)

"issubclass" for old-style classes doesn't check type(o) but o.__class__, which are different: the former is "instance" and the latter your class. That is one reason we removed old-style classes in Python 3...

[thread13]

I do not mean to reopen the bug (there are supposedly much more important things to work on in Python).

But just for the record, let me state that I feel like there is some misleading inconsistency here:

• by definition, a new style class is "Any class which inherits from object" ( see http://docs.python.org/glossary.html#term-new-style-class ) ;

• to support this statement, new classes are indeed explicitly defined in the form "NewClass(object)" ;

• now isinstance(), that is supposed to "return whether an object is an instance of a class or of a subclass thereof" (see help(isinstance)), returns True for old-style objects.

It also seems reasonable if the descendants of a class will inherit its powers, which -- in the case of the old-style classes -- they obviously don't.

Furthermore, I personally see no /point/ in returning True for isinstance(Old(), object): as it is quite misleading, one could easily have made it returning e.g. None as well.

As I completely accept the fact it's a feature -- ( may be slightly confusing, and probably also useless -- but ... hey, nobody's perfect ) -- should I take then calling

issubclass(obj.__class__, object) 

to be the official way to distinguish between the new-style and the old-style classes?

[benjaminp]

2012/4/26 Q <report@bugs.python.org>:

issubclass(obj.__class__, object)

to be the official way to distinguish between the new-style and the old-style classes?

Just do type(cls) is types.ClassType.