Using Recursion in Python for Assignment Stumped on TypeError

Question

I'm working on a set of code where I have to turn a loop into a recursive function and the professor has not been able to help nor my friends I've reached out to. The loop function is this:

 def __str__(self):
     """ Returns a string representation of the list with a space between each item.
         Precondition:  none
         Postcondition:  Returns a string representation of the list with a space between each item.
     """
     resultStr = "(head)"
     current = self._head
     while current != None:
         resultStr += " " + str(current.getData())
         current = current.getNext()
     return resultStr + " (tail)"

The recursive function I have typed so far is this:

def __str__(self):
        """ Returns a string representation of the list with a space between each item.
            Precondition:  none
            Postcondition:  Returns a string representation of the list with a space between each item.
        """
        
        def strHelper(current):
            if current != None:
                str(current.getData()) + " " + strHelper(current.getNext())
            else:
                return ""
        
        # Start of __str__ method execution by passing pointer to first node in list
        return "(head) " + strHelper(self._head) + "(tail)"

The professor basically said, "This should work!" but I still get a TypeError reading "can only concatenate str (not "NoneType") to str" on the final return line. What do I do?

Dont do "something" + None + "some other thing" ... check if the things in your stringconcattenation are None and avoid concatting them in that case... — Patrick Artner
– Patrick Artner, Commented Jun 25, 2020 at 13:27
Paste your full code in pythontutor.com/visualize.html#mode=edit and step through the code to see where you have a None value where you expect a string. — BioGeek
– BioGeek, Commented Jun 25, 2020 at 13:36
Your if case in strHelper doesn't do anything, since you forgot the return keyword. strHelper returns either None (after computing and discarding a str value) or the empty string. — chepner
– chepner, Commented Jun 25, 2020 at 13:42

chepner · Accepted Answer · 2020-06-25 13:48:13Z

You forgot the return keyword in your helper function.

def __str__(self):
        
    def strHelper(current):
        if current is not None:
            return str(current.getData()) + " " + strHelper(current.getNext())
        else:
            return ""
        
    return "(head) " + strHelper(self._head) + "(tail)"

I would consider replacing recursion with iteration, though.

def __str__(self):

    def strHelper(current):
        while current is not None:
            yield str(current.getData())
            current = current.getNext()

    return f'(head) {" ".join(strHelper(self._head))} (tail)'

Your string helper also forms the basis for a more general iterator:

def __iter__(self):
    current = self._head
    while current is not None:
        yield current
        current = current.getNext()

def __str__(self):
    return f'(head) {" ".join(str(x.getData()) for x in self)} (tail)'

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