[Tutor] Pickling objects and then carrying on development

[Adam]

Tim Peters wrote:
> [Adam]
> 
>>OK - I have a kind of strange question. I'm developing an
>>application which acts as a catalogue for magazine articles,
>>storing each piece of information in an OO manner. I've
>>written those objects to disk through pickle, which is an
>>enormously cool feature of python.
>>
>>My question relates to the pickled object and also the code
>>that relates to those objects. If I enter data in and pickle
>>this data, and then later develop my code adding more
>>methods and functionality to each class, will the old data
>>automatically gain all this new functionality when it is
>>unpickled later?
> 
> 
> Yes, it will.  pickle stores only data, never code.  This isn't a
> necessary behavior, it's just how things are.  The code that makes up
> your class is not pickled.  Instead, when an instance of your class is
> pickled, the *path* to your class (its module and class name) is
> pickled, as a string.  Unpickling effectively imports the (current)
> class code via the path in that string.
> 
> Note that this may not always be what you want, though.  For example,
> if you rename your class, or move your class into a different module
> or package, then you'll no longer be able to load old pickles
> referencing that class (using its former name or location).
> 
> 
>>The sequence would be:
>>1) Data entry and pickle...
>>2) Develop and improve code
>>3) Load data
>>4) continue with data entry
>>5) pickle once more
>>
>>Does the new pickle pick up all the new code that I've been
>>working on,
> 
> 
> Yes, but because the code was never in the pickle, just the data, and
> a string recording from where to import the class.

Many thanks Peter - makes perfect sense.

adam