stack.py

#!/usr/bin/env python
stack = []

def pushit():
    stack.append(raw_input(' Enter New String: ').strip())

def popit():
    if len(stack) == 0:
        print 'Cannot pop from an empty stack!'
    else:
        print 'Removed [', `stack.pop()`, ']'

def viewstack():
    print stack

CMDs = {'u': pushit, 'o':popit, 'v':viewstack}
def showmenu():

    pr="""
    P(U)sh
    p(O)p
    (V)iew
    (Q)uit
    
Enter choice:"""
#注意下面第一个while和上面的pr对齐
    while True:

        while True:

            try:

                choice = raw_input(pr).strip()[0].lower()
            except (EOFError, KeyboardInterrupt, IndexError):
                choice = 'q'

            print '
 You picked: [%s]' % choice
            if choice not in 'uovq':

                print 'Invalid option, try again'
            else:
                break

        if choice == 'q':

            break
        CMDs[choice]()
if __name__ == '__main__':
    showmenu()

NameError: name 'pr' is not defined

当出现此错误时，需注意的是将第一个while与上面的pr对齐。