bug in copy.deepcopy or in getattr or in my understanding?

[Emin]

Dear Gabriel,

Thank you for your reply. As you guessed, I want to be able to select
the method at runtime as in your final example, but when I tried your
suggestion I got the same error (see below). I think the problem is
that getattr is donig something different than in my example where I
explicitly get it from the dict (see the very end of the transcript
below):


--------------------------- Transcript Follows
----------------------------------------------------------

Python 2.5 (r25:51908, Sep 19 2006, 09:52:17) [MSC v.1310 32 bit
(Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> # The following shows that using getattr to grab a method is
>>> # incompatible with copy.deepcopy
>>> class a:
... 	def foo(self): pass
...
>>> class b(a):
... 	def __init__(self):
... 		self.x = getattr(a,'foo')
...
>>> import copy
>>> c=b()
>>> copy.deepcopy(c)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "c:\Python25\lib\copy.py", line 162, in deepcopy
    y = copier(x, memo)
  File "c:\Python25\lib\copy.py", line 291, in _deepcopy_inst
    state = deepcopy(state, memo)
  File "c:\Python25\lib\copy.py", line 162, in deepcopy
    y = copier(x, memo)
  File "c:\Python25\lib\copy.py", line 254, in _deepcopy_dict
    y[deepcopy(key, memo)] = deepcopy(value, memo)
  File "c:\Python25\lib\copy.py", line 189, in deepcopy
    y = _reconstruct(x, rv, 1, memo)
  File "c:\Python25\lib\copy.py", line 322, in _reconstruct
    y = callable(*args)
  File "c:\Python25\lib\copy_reg.py", line 92, in __newobj__
    return cls.__new__(cls, *args)
TypeError: instancemethod expected at least 2 arguments, got 0
>>>
>>> # The following shows that getattr is doing something different
>>> # than looking in the __dict__ of base classes
>>> b.__bases__[0].__dict__['foo']
<function foo at 0x009F0CF0>
>>> getattr(a,'foo')
<unbound method a.foo>
>>>


On Jan 4, 10:08 pm, Gabriel Genellina <gagsl... at yahoo.com.ar> wrote:
> At Thursday 4/1/2007 17:26, Emin wrote:
>
> >I got some unexpected behavior in getattr and copy.deepcopy (see
> >transcript below). I'm not sure if this is actually a bug in
> >copy.deepcopy or if I'm doing something too magical with getattr.
> >Comments would be appreciated.Both examples are different. #1 stores a *bound* method into an
> instance attribute. Bound methods carry a reference to "self", this
> creates a cyclic reference that may cause problems to the garbage
> collector (and confuses deepcopy, apparently).
> #2 uses and *unbound* method and it's the usual way.
>
> > >>> class a:
> >...     def foo(self):
> >...             print 'hi'
> >...
> > >>> class b(a): #1
> >...     def __init__(self):
> >...             self.y = getattr(self,'foo')
>
> > >>> class b(a): #2
> >...     def __init__(self):
> >...             self.x = self.__class__.__bases__[0].__dict__['foo']For #2 you can simply say:
>
> class b(a):
>      x = a.foo
>
> If you have to choose at runtime (a simplified version of your own code):
>
> class b(a):
>      def __init__(self):
>          name = select_method_to_use(..., default="foo")
>          self.x = getattr(a, name)
>
> You *know* your bases because you wrote them in the class statement
> (or use super() instead of playing with __bases__); and getattr works
> fine here so you don't need to mess with the __dict__ details.
>
> (Note that #1 was *almost* right, you had to replace self by the base class)
>
> --
> Gabriel Genellina
> Softlab SRL
>
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