writing pickle function

[Steve Holden]

Peter Otten wrote:
> perfreem at gmail.com wrote:
> 
>> i am using nested defaultdict from collections and i would like to
>> write it as a pickle object to a file. when i try:
>>
>> from collections import defaultdict
>> x = defaultdict(lambda: defaultdict(list))
>>
>> and then try to write to a pickle file, it says:
>>
>> TypeError: can't pickle function objects
>>
>> is there a way around this? it's simply a dictionary that i want to
>> write to file.. this works no problems with ordinary dicts.
> 
> The error message is misleading. You can pickle your defaultdict if you use
> a "normal" function instead of the lambda:
> 
> $ cat pickle_defaultdict.py
> import sys
> from collections import defaultdict
> from cPickle import dumps, loads
> 
> if "--lambda" in sys.argv:
>     make_inner = lambda: defaultdict(list)
> else:
>     def make_inner():
>         return defaultdict(list)
> 
> d = defaultdict(make_inner)
> d[1][2].append(42)
> e = loads(dumps(d))
> print e == d
> 
> $ python pickle_defaultdict.py
> True
> $ python pickle_defaultdict.py --lambda
> Traceback (most recent call last):
>   File "pickle_defaultdict.py", line 13, in <module>
>     e = loads(dumps(d))
>   File "/usr/lib/python2.5/copy_reg.py", line 69, in _reduce_ex
>     raise TypeError, "can't pickle %s objects" % base.__name__
> TypeError: can't pickle function objects
> 
But can you unpickle the objects you pickle? I believe that functions
are references relative to the modules they come from, and so won't be
found unless the same module can be imported at unpickle time. Could be
wrong, haven't read the source, but I understand that functions work the
same as classes.

regards
 Steve
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