Hi Victor, I’m not sure about changing the data type. As Python 3 grows older, there is potentially more code being written that you break by fixing a bug like this. It is incompatible if you used to write

>>> print(struct.Struct('hi').format.decode())
hi

I have used this decode() trick in the past to build composite format strings; e.g.: <https://bugs.python.org/issue16349#msg174083>. If you change the data type this code will raise AttributeError. At a minimum you should acknowledge it in the “porting” section of What’s New.

Also, if you make this change, maybe update the module doc string. See the end of format-str.patch.