>
> def read2():
> expr = ""
> while expr != "quit":
> expr = raw_input("Lisp> ")
> print parse(expr)
> read2()
^^^^^^^^^^^^^^^^^
> print "Good session!"
You shouldn't call read2() inside read2()...
just remove that line and retry...
Each time you call read2() recursively, a
new expr is initialized to "", so the condition
never becomes true