I tested with defaultextension='' on Windows and it works well. If no extension is entered (no '.' in name), .py or .txt is added for the .py/w and .txt filetype choices. If there is a . in the name, nothing is added. If one wants x.y.py, one must type the whole name. So typing x.pyw does not get .py added on top of it. This needs to be documented in the Idle help file.

Should the change only be made to 3.3, or also to 3.2/2.7? The old behavior of typing .py still works, so that does not break. Typing no . to get no extension (rare) will break unless one selects '*.* all files' (which one should do anyway as it is standard Windows behavior). I think uniform across versions is better so people who learn leaving out .py on 3.3 can also leave it out in future 2.7 and 3.2, which should come out about the same time as 3.3.0.

Whether just one or two or three versions, how do people used to the old (buggy in my opinion) behavior find out about the change. A splash line on startup "See help for new 'save as' behavior" ?

Anything I should know about *nix or mac behavior, at least for the doc? Ned, does this change work on macs?