Python3, __slots__ and serialization

Ned Batchelder ned at
Sat Feb 8 19:39:43 CET 2014

On 2/8/14 1:29 PM, Ned Batchelder wrote:
> On 2/8/14 1:06 PM, Eric Jacoboni wrote:
>> Hi,
>> Say i want create a class with a __slots__ tuple in order to prevent
>> creation of new attributes from outside the class.
>> Say i want to serialize instances of this class... With pickle, all is
>> ok : i can dump an object to a file, then reload it.
>> With PyYAML, i can dump an object to a file, but trying to reload it
>> fails.
>> If i comment out the __slots__ tuple, all is fine but, then, the class
>> is "open"... Is there some magic to have both a __slots__ tuple and a
>> working yaml.load() ?
> Preventing attribute creation runs counter to Python's philosophy, so it
> doesn't surprise me that it causes problems.  There might be a way to
> make it all work together, but I don't know.
> __slots__ wasn't meant to be a way to prevent attribute creation, it was
> meant as a way to save space by avoiding the need for an attributes
> dictionary.
> Why do you need to prevent attribute creation?  If someone uses your
> class and wants to use it in ways you didn't anticipate, why should you
> try to stop them?

I also looked at why it fails.  I get this error:

   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
line 71, in load
       return loader.get_single_data()
line 39, in get_single_data
       return self.construct_document(node)
line 43, in construct_document
       data = self.construct_object(node)
line 90, in construct_object
       data = constructor(self, tag_suffix, node)
line 610, in construct_python_object_new
       return self.construct_python_object_apply(suffix, node, newobj=True)
line 601, in construct_python_object_apply
       self.set_python_instance_state(instance, state)
line 563, in set_python_instance_state
       setattr(object, key, value)
TypeError: can't set attributes of built-in/extension type 'object'

This is set_python_instance_state, the failing code:

     def set_python_instance_state(self, instance, state):
         if hasattr(instance, '__setstate__'):
             slotstate = {}
             if isinstance(state, tuple) and len(state) == 2:
                 state, slotstate = state
             if hasattr(instance, '__dict__'):
             elif state:
             for key, value in slotstate.items():
                 setattr(object, key, value)

Two things seem clear to me:  1) you can define __setstate__ to do what 
you need to do, and 2) this code is simply wrong.  The last line should 
use "instance", not "object", and it might be that the last two lines 
should really be indented withing the elif.

All that said, skip __slots__: it isn't helping you with any actual 
problem, and it's forcing you to jump through hoops, or run code that 
most Python programs don't run.

Ned Batchelder,

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