Pickling a class with a __getattr__

[Peter Bengtsson]

Hi, I'm trying to pickle an object instance of a class that is like a
dict but with a __getattr__ and I'm getting pickling errors.

This works but is not good enough.

$ python2.4

>>import cPickle as pickle
class Dict(dict):

.... pass
....

>>>

friend = Dict(name='Zahid', age=40)
friend

{'age': 40, 'name': 'Zahid'}

>>v=pickle.dumps(friend)
p=pickle.loads(v)
p

{'age': 40, 'name': 'Zahid'}

This is what happens when I'm trying to be clever:

>>import cPickle as pickle
class Dict(dict):

.... def __getattr__(self, key):
.... return self.__getitem__(key)
....

>>friend = Dict(name='Zahid', age=40)
friend

{'age': 40, 'name': 'Zahid'}

>>friend.name

'Zahid'

>>v=pickle.dumps(friend)

Traceback (most recent call last):
File "<stdin>", line 1, in ?
File "/usr/lib/python2.4/copy_reg.py", line 73, in _reduce_ex
getstate = self.__getstate__
File "<stdin>", line 3, in __getattr__
KeyError: '__getstate__'
Why can't I pickle the slightly more featurefull class there called
'Dict'? I've got my reasons for not going for a simple type dict but
feel that that is irrelevant right now.

[Peter Otten]

Peter Bengtsson wrote:

Hi, I'm trying to pickle an object instance of a class that is like a
dict but with a __getattr__ and I'm getting pickling errors.

This is what happens when I'm trying to be clever:

>>>import cPickle as pickle
class Dict(dict):

... def __getattr__(self, key):
... return self.__getitem__(key)
...

>>>friend = Dict(name='Zahid', age=40)
friend

{'age': 40, 'name': 'Zahid'}

>>>friend.name

'Zahid'

>>>v=pickle.dumps(friend)

Traceback (most recent call last):
File "<stdin>", line 1, in ?
File "/usr/lib/python2.4/copy_reg.py", line 73, in _reduce_ex
getstate = self.__getstate__
File "<stdin>", line 3, in __getattr__
KeyError: '__getstate__'
Why can't I pickle the slightly more featurefull class there called
'Dict'?

Because you allow your __getattr__() implementation to raise the wrong kind
of exception.

>>from cPickle import dumps, loads
class Dict(dict):

.... def __getattr__(self, key):
.... try:
.... return self[key]
.... except KeyError:
.... raise AttributeError
....

>>friend = Dict(name="Zaphod", age=42)
v = dumps(friend)
p = loads(v)
p

{'age': 42, 'name': 'Zaphod'}

Peter

[Peter Bengtsson]

On Apr 1, 5:48 pm, Peter Otten <[e-mail]:

Peter Bengtsson wrote:

Hi, I'm trying to pickle an object instance of a class that is like a
dict but with a __getattr__ and I'm getting pickling errors.
This is what happens when I'm trying to be clever:

>>import cPickle as pickle
class Dict(dict):

... def __getattr__(self, key):
... return self.__getitem__(key)
...

>>friend = Dict(name='Zahid', age=40)
friend

{'age': 40, 'name': 'Zahid'}

>>friend.name

'Zahid'

>>v=pickle.dumps(friend)

Traceback (most recent call last):
File "<stdin>", line 1, in ?
File "/usr/lib/python2.4/copy_reg.py", line 73, in _reduce_ex
getstate = self.__getstate__
File "<stdin>", line 3, in __getattr__
KeyError: '__getstate__'

Why can't I pickle the slightly more featurefull class there called
'Dict'?

Because you allow your __getattr__() implementation to raise the wrong kind
of exception.

>from cPickle import dumps, loads
class Dict(dict):

... def __getattr__(self, key):
... try:
... return self[key]
... except KeyError:
... raise AttributeError
...>>friend = Dict(name="Zaphod", age=42)

>v = dumps(friend)
p = loads(v)
p

{'age': 42, 'name': 'Zaphod'}

Peter

Thanks! That did the trick. I also noticed that I could define
__getstate__ and __setstate__ (needed for protocol 2) but your
solution works much better.