RE: [Jython-users] creating custom arrays / extending java.util.A rrayList

Jeff, Justin,

>Here is __getitem__ from org/python/core/PyInstance.java:
>
>    public PyObject __getitem__(PyObject key) {
>        CollectionProxy proxy = getCollection();
>        if (proxy != CollectionProxy.NoProxy) {
>            PyObject ret = proxy.__finditem__(key);
>            if (ret == null) {
>                throw Py.KeyError(key.toString());
>            }
>            return ret;
>        }
>
>        PyObject ret = trySlice(key, "__getslice__", null);
>        if (ret != null)
>            return ret;
>
>        return invoke("__getitem__", key);
>    }
>
>
>You can see that the code checks whether your instance
>is a collection first and calls its __finditem__ before
>calling your __getitem__ method.
>
>You might try extending org.python.core.PyList.

You might also consider this to be a bug in Jython.
Ie. it might have checked for __getslice__ first,
then for __getitem__ and only when that fails, check
for a Collection proxy.
In case jython would use that order, it would have found
your __getitem__ before checking for being a Collection.

Having said that, I don't know precisily why the checks
are done in the order shown, ie. there might be another
reason for the current implementation.

One needs to inherit from a Collection (ArrayList in this case)
to get into this situation, and this is not done very frequently.

Kind regards,
Ype


Justin's original post (because I'm cross posting to jython-dev):

Hi,

Trying to create a custom array structure. Ideally I would like to subclass the 
built in list, but it seems this feature won't be available till version 2.2 
(as per CPython) so instead I am extending java.util.ArrayList as per below. 
This works fine, and I can access the object using standard array syntax (x[0] 
.. etc). However when I look under the hood / overload some of the built in 
methods, it appears they are different from CPython. In particular, __repr__ 
gets called as expected but __getitem__ does not (see below). (If I run the 
code in CPython 2,2, replacing ArrayList with list, it works as expected). 
Perhaps someone can shed some light on what is happening here.

thanks!

Justin

from java.util import *

class test(ArrayList):

        def __getitem__(self,i):
                print "__getitem__"+str(i)
                return ArrayList.get(self,i)

        def __repr__(self):
                print "__repr__"
                r=""
                for i in self:
                        r=r+str(i)+chr(10)
                return r

def main():
        r=test()
        r.add("one")
        r.add("two")
        print r # successfully calls __repr__
        print r[0] # returns the right result, but doens't call __getitem__

main()


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