Adafruit intensive working on port micropython to SamD21.
I have tested it and it is amazing ! Everything seems working
REPL, Mass storage ....unfortunately I have no idea about Pyhon, but It is time to start learning.

https://github.com/adafruit/micropython

And what is best????

Cool you can leave zero bootloader, just upload binary and winxi is micropython board, If you want go

back to arduino ...just press twice reset button and use arduino IDE .... great ...