[Python-Dev] backwards and forwards compatibility, the exact contents of pickle files, and IntEnum

[Glenn Linderman]

On 3/14/2015 10:52 PM, Ethan Furman wrote:
> I'm not sure exactly how to phrase this inquiry, so please bear with me.
>
> What exactly does backwards compatibility mean as far as pickle goes?  We have the various protocols, we have the
> contents of pickle files created at those protocols, and we have different versions of Python.
>
> Should a pickle file created in Python 3.4 with protocol 3 and the contents of socket.AF_INET be unpicklable with a
> Python 3.3 system?  Because currently it cannot be as socket.AF_INET became an IntEnum in 3.4.
>
> I'm thinking the answer is yes, it should be.
>
> So how do we fix it?  There are a couple different options:
>
>    - modify IntEnum pickle methods to return the name only
>
>    - modify IntEnum pickle methods to pickle just like the int they represent
>
> The first option has the advantage that in 3.4 and above, you'll get back the IntEnum version.
>
> The second option has the advantage that the actual pickle contents are the same [1] as in previous versions.
>
> So, the final question:  Do the contents of a pickle file at a certain protocol have to be the some between versions, or
> is it enough if unpickling them returns the correct object?
In general, it would seem to me that each version of Python could add 
object types that are foreign to prior versions, and so as long as 
version N can unpickle anything produced by versions <=N, all would be well.

While I am _not_ an expert in pickle, this would just be my expectation 
based on years of experience with compatibility issues in non-Python 
non-pickle systems.  Pickle may have stronger compatibility goals than I 
would expect, and so must you, if you are thinking the answer to the 
above is "yes, it should be". :)
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