I am writing a function intToRoman to convert numbers to roman numerals:

class Solution:
    def intToRoman(self, num: int) -> str:
        #Dictionary with keys for symbol and value for value
        symbols = { 
            'I' : 1, 
            'V' : 5,
            'X' : 10,
            'L' : 50,
            'C' : 100,
            'D' : 500,
            'M' : 1000
        } 
        str_form = str(num) #Get string form for extracting starting digit
        first_digit = str_form[0] # Get the first digit in string form for comparisons
        
        if first_digit != '4' and first_digit != '9': #If the value does not start with 4 or 9
            symbol_list = []
            
            for key,value in symbols.items(): # loop through to find the greatest key that does not return negative difference

                if (num >= value ):
                    best_key = key
                    
            
                else:
                    break;
            symbol_list += best_key # append this best key to the list
            number_to_subtract = symbols[best_key] 
            num = num - number_to_subtract # subtract the value from the number 
            symbol_list.append(self.intToRoman(num)) #recursion
            return ''.join(symbol_list)
        
        if (first_digit == '4' or first_digit == '9'): #If the value starts with 4 or 9
            symbol_list = []
            
            
            if (len(str_form) == 1 and first_digit == '4'):
                key_to_add = 'IV'
            elif ((len(str_form) == 1) and first_digit == '9'):
                key_to_add = 'IX'
            elif ((len(str_form) == 2) and first_digit == '4'):
                key_to_add = 'XL'
            elif ((len(str_form) == 2) and first_digit == '9'):
                key_to_add = 'XC'
            elif ((len(str_form) == 3) and first_digit == '4'):
                key_to_add = 'CD'
            elif ((len(str_form) == 3) and first_digit == '9'):
                key_to_add = 'CM'
            symbol_list += key_to_add #append to symbol_list
            if (len(str_form) > 1):
                truncated_str_num = str_form[1:] #get the truncated str num ; everything except first digit
                truncated_num = int(truncated_str_num)
                symbol_list.append(self.intToRoman(truncated_num)) #recursion
                return ''.join(symbol_list)
            else:
                return ''.join(symbol_list)
        
def main():
    solution = Solution()
    
    num = 3749
    expected = "MMMDCCXLIX"
    result = solution.intToRoman(num)
    print(f"Test case 3: num = {num}, expected = {expected}, result = {result}")


    num = 1994
    expected = "MCMXCIV"
    result = solution.intToRoman(num)
    print(f"Test case 3: num = {num}, expected = {expected}, result = {result}")

    num = 58
    expected = "LVIII"
    result = solution.intToRoman(num)
    print(f"Test case 3: num = {num}, expected = {expected}, result = {result}")

    

if __name__ == "__main__":
    main()'

For inputs that end with digits concerning the second if statement (starting with 4 or 9) it seems to work fine; see 3749 and 1994 above. But for inputs like 58, or even simpler just inputs like 8, or 7, I get the error:

File “intToRoman.py”, line 27, in intToRoman
symbol_list += best_key # append this best key to the list
^^^^^^^^^^^
UnboundLocalError: cannot access local variable ‘best_key’ where it is not associated with a value

I don’t understand how this is happening since best_key should be initialized no matter what in this clause:
if (num >= value ):
best_key = key
Since there is no number that doesn’t satisfy this condition. Is this my fault for not initializing best_key before entering the loop?

any help would be appreciated