为什么我的用于检查回文的python算法不起作用？

Question

So I was doing exercise 6 from chapter 7 from Think Python and it stroke me as odd that this piece of code I wrote as a solution didn't work:

def is_palindrome(word):
    if len(word) <= 1:
        return True
    elif first(word) == last(word):
        is_palindrome(middle(word))
    else:
        return False

It doesn't enter any recursion and I don't know why. It returns None for words longer than 1 character of length. Why is this? When the length of the word reaches 1 or less it should return True! Why doesn't it work?

PS: Here are first, last and middle deffinitions:

def first(word):
    return word[0]

def last(word):
    return word[-1]

def middle(word):
    return word[1:-1]

Answer 1

You're missing a return :

def is_palindrome(word):
    if len(word) <= 1:
        return True
    elif first(word) == last(word):
        return is_palindrome(middle(word))  # <--
    else:
        return False

So your current snippet is returning None once the elif block is entered, since you don't have an explicit return statement there. In other words, you do compute is_palindrome(middle(word)) , but you do nothing with the result.

Maybe working through a simple example would help. Consider calling the original function with an argument of 'aba' :

• function called
• word is now 'aba'
• len(word) <= 1 is False , if -body not entered.
• first(word) == last(word) is True , elif -body entered:
  • function called recursively:
  • word is now 'b'
  • len(word) <= 1 is True , if -body entered:
    • True returned
• Return value of recursive call discarded (since we have no return in elif )
• None returned

Answer 2

添加return

return is_palindrome(middle(word))

Answer 3

I think you are missing to return the method in 5th line of your function.

return is_palindrome(middle(word))

should be there instead of

 is_palindrome(middle(word))