Has anyone ever heard of a UNIX shell written in a reasonable language, like Python?
From all appearances, Python IS a shell.  It runs with #! and it can run interactively.  Between the os and shutil packages you have all of the features of standard Unix shells.
Since you can do anything in Python with simple, powerful scripts, you don't really need to spend any time messing with the other shells.
UNIX shell written in a reasonable language?