My functions have return statements, so why am I getting this error? TypeError: can only concatenate str (not "NoneType") to str

Question

This method is part of a class that converts inputs from infix to postfix. An example input would be: "11.897/3.4+9.2-0.4*6.9/12.6-16.7="

I have some helper methods below the main method (getPostFix).

I can't understand why the code result += ipop is causing the error. I use the same code snippets later in the code, so maybe I will have to fix those instances as well. I read some other answers in which the answers seemed to ask the OP - "well what if this function returns False?" But which functions and why would they do that?

    def getPostFix(self):

        result = ""

        # stack used to create postfix string
        self.stack = MyStack()

        input_string = self.getInput()
        input_string_split = [x for x in re.split("(\d*\.?\d*)", input_string) if x != '']

        for i in input_string_split:

            if isOperand(i):
                result += i

            elif isOperator(i):
                while True:
                    topItem = self.stack.getTop()
                    if self.stack.size == 0 or topItem == '(':
                        self.stack.push(i)
                        break
                    else:
                        precedence_i = getPrecedence(i)
                        precedence_topItem = getPrecedence(topItem)

                        if precedence_i > precedence_topItem:
                            self.stack.push(i)
                            break
                        else:
                            ipop = self.stack.pop()
                            result += ipop
                            #result += self.stack.pop()

            elif i == '(':
                self.stack.push(i)

            elif i == ')':
                ipop = self.stack.pop()

                while ipop != '(':
                    result += ipop
                    ipop = self.stack.pop()

            elif i == '=':
                ipop = self.stack.pop()
                result += ipop
                #result += self.stack.pop()

        while not self.stack.size == 0:
            ipop = self.stack.pop()
            result += ipop


def isOperand(c):
    return c >= '0' and c <= '9'

operators = "+-*/^"
        
def isOperator(c):
    return c in operators

def getPrecedence(c):
    result = 0

    for i in operators:
        result += 1

        # because + - and */ have the same value
        # these if statements will set their value equal to each other
        # for, example when I get to -, the result count is 2, but I subtract 1 to get 1
        # which is the same result count as +.
        if i == c:
            if c in '-/':
                result -= 1
            break

    return result

Here is the error code:

Traceback (most recent call last):
  File " ", line 9, in <module>
    print(calc)
  File " ", line 23, in __str__
    theCalc += 'Postfix input is: ' + self.getPostFix() + '\n'
  File " ", line 71, in getPostFix
    result += ipop
TypeError: can only concatenate str (not "NoneType") to str

Answer 1

Because sometimes you're popping at the end of the iterable and it returns None, try replacing all the lines of

result += ipop

with

result += ipop if ipop else ""  # Handles None cases and returns empty string.