我寫了下面的python程序#! /usr/bin/pythondef checkIndex(key):    if not isinstance(key, (int, long)): raise TypeError    if key<0: raise IndexErrorclass ArithmeticSequence:    def __init__(self, start=0, step=1):        self.start = start      # Store the start value        self.step = step        # Store the step value        self.changed = {}       # No items have been modified    def __getitem__(self, key):        checkIndex(key)        try: return self.changed[key]        except KeyError:            return self.start + key*self.step    def __setitem__(self, key, value):        checkIndex(key)        self.changed[key] = value我做的時(shí)候程序是my.pychmod +x my.pypython my.py在執(zhí)行完此步驟后，我將回到bash shelluser@ubuntu:~/python/$ pythonPython 2.7.3 (default, Aug  1 2012, 05:14:39) [GCC 4.6.3] on linux2Type "help", "copyright", "credits" or "license" for more information.>>> s=ArithmeticSequence(1,2)Traceback (most recent call last):  File "<stdin>", line 1, in <module>NameError: name 'ArithmeticSequence' is not defined我如何給程序輸入并運(yùn)行它，因?yàn)樗驯４嬖趘i中