Issue2514

Hi,

I'm using jython-standalone on maven, 2.7.0.

The output of class.__subclasses__() differs between Jython & Python.

In Python subclasses are in load order, in Jython the order is random.

I am attempting to run an application which loads files, then obtains the subclasses of the files by using __subclasses__()[-1]. Because it's not load order this causes the application to horrifically fail.

The application could be modified, but i'm in belief this should be fixed, I haven't looked into how complicated it is myself, so it might not be a walk in the park.

Somewhat related to http://bugs.jython.org/issue2487 in that we would expect this ordering is done in the same (currently buggy) code path.

Here's the relevant backing structure for subclasses in PyType:

    private Set<WeakReference<PyType>> subclasses = Generic.set();

Generic.set() in turn constructs the Set with a HashSet backing; synchronization is used on the subclasses for all usage of this set. So it would seem to be trivial to change this to using a LinkedHashSet instead to maintain original load order per Myles request to get the same behavior as CPython.

This code still will intersect with this other code I mentioned, but at least it's very much contained, with little to no risk I can see in making this change.

The solution that zyasoft (Jim Baker) suggested seems to work fine.

I tested this by changing it to a LinkedHashSet in Generic (May not be the ideal place but for the test I did this).

Then using the interpreter:

>>> Exception.__subclasses__()
[<type 'exceptions.StandardError'>, <type 'exceptions.StopIteration'>, <type 'exceptions.Warning'>, <class 'warnings._OptionError'>, <class 'sre_constants.error'>]

>>> class TestException(Exception):
...     test = 1
...
>>>

>>> Exception.__subclasses__()
[<type 'exceptions.StandardError'>, <type 'exceptions.StopIteration'>, <type 'exceptions.Warning'>, <class 'warnings._OptionError'>, <class 'sre_constants.error'>, <class '__main__.TestException'>]
>>> Exception.__subclasses__()
[<type 'exceptions.StandardError'>, <type 'exceptions.StopIteration'>, <type 'exceptions.Warning'>, <class 'warnings._OptionError'>, <class 'sre_constants.error'>, <class '__main__.TestException'>]


Indicating the order is now retained as previously the items would all be random.

So CPython 2.7 doesn't quite have the behavior of LinkedHashSet when the subclasses are GC'ed, as seen when the subclasses are deleted.

But it doesn't seem worth replicating that behavior - class deletion is relatively rare.

Fixed as of https://hg.python.org/jython/rev/ed451b497499