古塔格教科书上回文的代码

Question

我是编程新手，正在研究Guttag的书“使用Python和应用程序来理解数据的计算和编程简介”。在第4.3.2节的图4.9中，我无法理解他的回文代码。

这在书中给出

def isPalindrome(s): 
   """Assumes s is a str 
      Returns True if s is a palindrome; False otherwise. 
       Punctuation marks, blanks, and capitalization are ignored.""" 

   def toChars(s): 
      s = s.lower() 
      letters = '' 
      for c in s: 
        if c in 'abcdefghijklmnopqrstuvwxyz': 
            letters = letters + c 
      return letters 

   def isPal(s): 
      print('  isPal called with', s) 
      if len(s) <= 1: 
         print('  About to return True from base case') 
         return True 
      else: 
         answer = s[0] == s[-1] and isPal(s[1:-1]) 
         print('  About to return', answer, 'for', s) 
         return answer 

   return isPal(toChars(s)) 

def testIsPalindrome(): 
   print('Try dogGod') 
   print(isPalindrome('dogGod')) 
   print('Try doGood') 
   print(isPalindrome('doGood')) 

可打印

Try dogGod 
  isPal called with doggod 
  isPal called with oggo 
  isPal called with gg 
  isPal called with  
  About to return True from base case 
  About to return True for gg 
  About to return True for oggo 
  About to return True for doggod 
True 
Try doGood 
  isPal called with dogood 
  isPal called with ogoo 
  isPal called with go 
  About to return False for go 
  About to return False for ogoo 
  About to return False for dogood 
False 

我不明白为什么函数在生成后仍会继续

About to return True from base case

和

About to return False for go 

在此先感谢您的解释。