书籍

def collatz(num):
    num_pan = num % 2

    if num_pan == 0:
        num_finally = num // 2
        print (num_finally)
    else:
        num_finally = 3 * num + 1
        print (num_finally)

    return num_finally

def user():
    while True:
        num = input(">>:")
        try:
            num = int(num)
        except ValueError as e:
            print ("error {},please enter a number".format(e))
            continue
        break
    while True:
        num = collatz(num)
        if num == 1:
            break

user()

# >>:aa
# error invalid literal for int() with base 10: 'aa',please enter a number
# >>:10
# 5
# 16
# 8
# 4
# 2
# 1