test.py

#!/usr/bin/python
import cmd
import os

class MyShell(cmd.Cmd):
  def __init__(self):
    cmd.Cmd.__init__(self)
    self.prompt = Pwd()

  def do_cd(self,arg):
    if arg == '':
      arg = os.path.expanduser('~')
    os.chdir(arg)
  
  def emptyline(self):
    pass

class Pwd():
  def __repr__(self):
    return os.getcwd() + "% "

if __name__ == '__main__':
  MyShell().cmdloop()