Recursive palindrome check - having issues when recalling the function

Question

The problem is simple, check if palindrome or not using recursion. They also provided a template so I can't change that. The template:

def isPalindrome(s): # Wrapper function
   def isPalindromeRec(s,low,high):
      """ Recursive function which checks if substring s[low ... high]     is palindrome
      returns a True/False value"""

   n = len(s)
   return isPalindromeRec(s,0,n-1)

I am nearly there but I think I am having trouble understanding how recursion exactly works. (especially how the values changes in the recursion)

My code:

def isPalindrome(s): # Wrapper function
    def isPalindromeRec(s,low,high):
        if len(s)<=1:
            return True
        else:
            if s[0]==s[len(s)-1]:
                return isPalindromeRec(s[low+1:high],low+1,high-1)
            else:
                return False

    n = len(s)
    return isPalindromeRec(s,0,n-1)
print(isPalindrome("anna"))
print(isPalindrome("civic"))
print(isPalindrome("a"))
print(isPalindrome("tx1aa1xt"))
print(isPalindrome(""))
print(isPalindrome("Civic"))
print(isPalindrome("ab"))

this is the output:

runfile('/Users/Rayan/Desktop/AUB Spring 2019/EECE 230 /HW/Homework 7/Problem2.py', wdir='/Users/Rayan/Desktop/AUB Spring 2019/EECE 230 /HW/Homework 7')
True
True
True
False
True
False
False

the first false should be True. Thank you for your help!

Answer 1

Rewrote it:

def isPalindrome(s):
    def isPalindromeRec(s,low,high):

        if (low == high): 
            return True

        if (s[low] != s[high]) : 
            return False

        if (low < high + 1) : 
            return isPalindromeRec(s, low + 1, high - 1); 

        return True

    n = len(s) 
    if (n == 0) : 
        return True

    return isPalindromeRec(s, 0, n - 1); 

print(isPalindrome("anna"))
print(isPalindrome("civic"))
print(isPalindrome("a"))
print(isPalindrome("tx1aa1xt"))
print(isPalindrome(""))
print(isPalindrome("Civic"))
print(isPalindrome("ab"))

output:
True
True
True
True
True
False
False