python - 如果没有选择有效，如何返回第一个 if 语句

Question

如果没有正确满足条件，我如何让 Python 移动到 if 语句的顶部。

我有一个基本的 if/else 语句，如下所示：

print "pick a number, 1 or 2"
a = int(raw_input("> ")

if a == 1:
    print "this"
if a == 2:
    print "that"
else:
   print "you have made an invalid choice, try again."

我想要的是提示用户为此 if 语句做出另一个选择，而不必重新启动整个程序，但是我对 Python 非常陌生，并且在任何地方都无法在线找到答案。

Answer 1

一个相当常见的方法是使用一个while True无限期运行break的循环，当输入有效时使用语句退出循环：

print "pick a number, 1 or 2"
while True:
    a = int(raw_input("> ")
    if a == 1:
        print "this"
        break
    if a == 2:
        print "that"
        break
    print "you have made an invalid choice, try again."

这里还有一个很好的方法来限制重试次数，例如：

print "pick a number, 1 or 2"
for retry in range(5):
    a = int(raw_input("> ")
    if a == 1:
        print "this"
        break
    if a == 2:
        print "that"
        break
    print "you have made an invalid choice, try again."
else:
    print "you keep making invalid choices, exiting."
    sys.exit(1)

Answer 2

使用 while 循环。

print "pick a number, 1 or 2"
a = None
while a not in (1, 2):

    a = int(raw_input("> "))

    if a == 1:
        print "this"
    if a == 2:
        print "that"
    else:
        print "you have made an invalid choice, try again."

Answer 3

您可以使用递归函数

def chk_number(retry)
    if retry==1
        print "you have made an invalid choice, try again."
    a=int(raw_input("> "))
    if a == 1:
        return "this"
    if a == 2:
        return "that"
    else:
        return chk_number(1)

print "Pick a number, 1 or 2"
print chk_number(0)