Where to set default data - where received, or where used

[Dennis Carachiola]

I'm programming a project which will use a file to save parameters
needed by the program.  There are already two previous file formats,
each of which can only be run by the version of the program which
created them.  I'm trying to avoid that problem in the future.  To do
that, I intend to use a dictionary, and default data.  I believe that
most new versions will add parameters.

Each version of the program will have reasonable default values for
each key in the dictionary handled by that version.  If an older file
is used, the data values in that file replace the keys that exist.
The program then can operate on the values supplied by the older data
file and the default values.  Conversely, if a newer file is used, the
data in the file replaces the keys in the dictionary.  The program
then simply doesn't access the newer data values.  I'm hoping that
this will make the file backward and forward compatible.

Here's my question.  I could do this by creating the dictionary with
the default values, then read the file into it.  Or I could use a
'get' with default values at the location in the program where those
values are used.

>From what I can see, creating the dictionary with default values puts
everything in one place.  While, supplying the default values at the
place where they're used places the default values nearest the place
where actually used.

I can't decide on one way over the other.  Can anyone give me some
ideas if one is a preferred method, or other criteria I've overlooked?

Thanks,

Den