Python代码不会在第一次运行时执行

Question

使用Spyder Python 3.6此代码不会执行，表示ispal方法未定义。但是，当我运行它并首先放入一个整数（比如我的字符串输入= 0）时，它会在运行之后识别该方法。似乎我必须先通过一个分支，而不是首先调用该方法。感谢批评。

s = input('enter a string: ') 
s1 = s 
s1 = s1.lower() 
s1 = s1.replace(',', '') 
s1 = s1.replace(' ', '') 

if s1.isalpha(): 
    if ispal(s1) == True: 
     print(s,' is a palindrome') 
    else: 
     print(s,' is not a palindrome') 
else: 
    print('you entered illegal chars') 


def ispal(s1): 
    if len(s1) <= 1: 
     return True 
    else: 
     #if the first and last char are the same 
     #and if all 
     return s1[0] == s1[-1] and ispal(s1[1:]) 

Answer 1

首先，如TGKL指出它的定义之前，你打电话ispal。因此调用之前定义，即：

def ispal(s1): 
    if len(s1) <= 1: 
     return True 
    else: 
     #if the first and last char are the same 
     #and if all 
     return s1[0] == s1[-1] and ispal(s1[1:]) 

if s1.isalpha(): 
    if ispal(s1) == True: 
     print(s,' is a palindrome') 
    else: 
     print(s,' is not a palindrome') 
else: 
    print('you entered illegal chars') 

其次你的回文递归函数是正确的，当你调用ispal里面本身除了。而不是ispal(s1[1:])你应该做ispal(s1[1:-1])这将删除刚刚测试的第一个和最后一个字符。

Answer 2

你必须首先定义你的方法，然后调用它：

s = raw_input('enter a string: ') #use raw_input so the text it takes will give to you directly a string without "" 
s1 = s 
s1 = s1.lower() 
s1 = s1.replace(',', '') 
s1 = s1.replace(' ', '') 

def ispal(s1): 
    if len(s1) <= 1: 
     return True 
    else: 
     #if the first and last char are the same 
     #and if all 
     return s1[0] == s1[-1] and ispal(s1[2:]) # here you put ispal(s1[1:]) it doesn't work properly :/ 

if s1.isalpha(): 
    if ispal(s1) == True: 
     print(s,' is a palindrome') 
    else: 
     print(s,' is not a palindrome') 
else: 
    print('you entered illegal chars')