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...@yahoo.com.arwrote:
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
__________________________________________________
Preguntá. Respondé. Descubrí.
Todo lo que querías saber, y lo que ni imaginabas,
está en Yahoo! Respuestas (Beta).
¡Probalo ya!http://www.yahoo.com.ar/respuestas
