Thanks, Maciej. 

I tweaked the patch a bit, you might want to take a look just for your own information.  Mostly I fixed the warning stuff, which I didn't explain very well.  The idea is that if the default is used (no value is specified), we want there to be a warning.  But if a value *is* specified, there should be no warning (the user knows what they want).  To accomplish that we make the actual default value None, and check for that.  I also had to modify the tests so that warnings aren't issued, as well as test that they actually get issued when the default is used.

I also added versionchanged directives and a whatsnew entry, and expanded the decode_data docs a bit.