python的NameError: name 'xx‘ is not defined该怎么解决？

初学python 按着书上的代码打 想看看运行过程 代码如下

stack=[]
def pushit():
    stack:append(input(' Enter New String: ').strip())
def popit():
    if len(stack)==0:
        print('Cannot pop from an empty stack!')
    else:
        print ('Removes [','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:'''
while True:
    while True:
        try:
            choice=input(pr).strip()[0].lower()
        except (EOFError,KeyboardInterrupt,IndexError):
            choice='q'
        print('\nYou 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()

但是总提示NameError: name 'pr' is not defined ，该怎么解决这个问题？

def showmenu():
pr='''
p(U)sh
p(O)p
(V)iew
(Q)uit

Enter choice:'''
while True:
while True:
这里的while缩进貌似不对，否则它不在showmenu函数内了。
导致pr成为局部变量，而下面的代码成为全局的代码，认不到pr

应该这样写吧

stack=[]
def pushit():
    stack:append(input(' Enter New String: ').strip())
def popit():
    if len(stack)==0:
        print('Cannot pop from an empty stack!')
    else:
        print ('Removes [','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:'''
    while True:
        while True:
            try:
                choice=input(pr).strip()[0].lower()
            except (EOFError,KeyboardInterrupt,IndexError):
                choice='q'
            print('\nYou picked:[%s]'%choice)
            if choice not in 'uovq':
                print('Invalid option,try again')
            else:
                break
            if choice=='q':
                break
            CMDs[choice]()

showmenu()