Issue1605019

Assigning a method bound to an instance of a class C to a class D rebinds the method to instances of D instead of using the already bound instance C.  This causes methods in test_descr to fail.

Im trying to understand the bug here.  Ive got a test case together:
class x:
    def zoom1(self):
        print self

class z:
    pass

a = x()
a.zoom1()

t = z()
z.zoom1 = a.zoom1
t.zoom1()

and they do the same thing for me in Python as in Jython.

leouser

Leo, if you haven't already, try running the test 'methods()' from test_descr.py (which currently fails for me) and see if you can find the cause of the failure.

- kz

Ahh, I see the bug.  This is odd, since I would expect a PyMethod to be there, which would be bound to C.  Ill have to see what's up.

leouser

Well, the method is created in PyMethod _doget.

From altering this line in _doget:
if (__builtin__.issubclass(container.fastGetClass(), im_class))

To:
if (__builtin__.issubclass(container.fastGetClass(), im_class) && im_self == null){

the code moves further along in the test, but bombs here:
verify(repr(C.foo.__get__(C(1))).startswith("<bound method "))

at the last line in methods.  Ill see what the deal is here.


Im trying to think if the == null makes sense as a test.  It guards against the problem, but what else can go wrong. 

leouser

Ahh, I see the bug.  This is odd, since I would expect a PyMethod to be there, which would be bound to C.  Ill have to see what's up.

leouser

The final bomb can be navigated through by giving the PyMethod a dictionary with a __get__ function.  This in turn requires _doget to be further enhanced to create a new PyMethod if the container is null but wherefrom is not null.  wherefrom is used as the "self" argument.  Also the toString needs to return a "bound method" instead of a "method" string.

Im trying to locate the spec for a unbound methods + __get__ invocations to see if there is clear behavior as what to do with it.

leouser

oh yeah,

there are worlds of differences between CPython and Jythons bound and unbound methods:
Jython:
>>> class x:
...    def zoo(self):
...       print "Zoo"
>>> a = x.zoo
>>> dir(a)
['__dict__', '__doc__', '__name__', 'im_class', 'im_func', 'im_self']

Python:
>>> class z:
...    def mork(self):
...      print "mork"
...
>>> x = z.mork
['__call__', '__class__', '__cmp__', '__delattr__', '__doc__', '__get__', '__get attribute__', '__hash__', '__init__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__str__', 'im_class', 'im_func', 'im_self']

leouser

This article is instructive about descriptors:
http://users.rcn.com/python/download/Descriptor.htm

It appears that unbound methods are descriptors and if passed in an Object it they need to create a bound method to the Object.

leouser

the problem is very simple

a) further calls on the __get__ behavior of a bound method should simply return the bound method untouched
   (see 
    http://svn.python.org/view/python/trunk/Objects/classobject.c  instancemethod_descr_get
    or https://codespeak.net/viewvc/pypy/dist/pypy/interpreter/function.py  def descr_method_get)

b) PyMethod is still 2.1 code, it needs to be converted to new-style conventions

Fixed in r3067.