How Main is used...

main = Main(Scanner, Source)
main.run(sys.stdin, sys.stdout, sys.stderr, sys.argv)

Main will use docopts-ng to process command-line arguments and then build/configure the rest of the system (using the passed constructors for Scanner and Source) and then run the system.

The reason Scanner, Source, sys.stdin, etc are passed to Main is so that we can isolate Main's logic from the rest of the system during testing.