How to handle TypeError exception raised by python dict?

Question

I cannot seem to find the error to this code. I keep getting this as my output log.

 STDERR
Traceback (most recent call last):
  File "main.py", line 18, in <module>
    static ("Avoid success at all costs!")
  File "main.py", line 10, in static
    test.assert_equals(rot13(d),sol(d))
  File "/home/codewarrior/solution.py", line 16, in rot13
    newkey = (get_keyA(i)+13)
TypeError: unsupported operand type(s) for +: 'NoneType' and 'int'

The purpose of this code is to active Rot13, if there is a numerical in there or a space, it'll just leave it as original.

ABC = {1: "A", 2: "B", 3:"C", 4: "D", 5: "E", 6: "F", 7: "G", 8: "H", 9: "I", 10: "J", 11: "K",
       12: "L", 13: "M", 14: "N", 15: "O", 16: "P", 17: "Q", 18: "R", 19: "S",
       20: "T", 21: "U", 22: "V", 23: "W", 24: "X", 25: "Y", 26: "Z"}
abc = {1: "a", 2: "b", 3:"c", 4: "d", 5: "e", 6: "f", 7: "g", 8: "h", 9: "i", 10: "j", 11: "k",
       12: "l", 13: "m", 14: "n", 15: "o", 16: "p", 17: "q", 18: "r", 19: "s",
       20: "t", 21: "u", 22: "v", 23: "w", 24: "x", 25: "y", 26: "z"} 

def rot13(message):
    string = list(message)
    new = ""
    for i in string:
        if (i.isnumeric() == False):
            if (i != " "):
                if (i.islower() == False):
                    newkey = (get_keyA(i)+13)
                    if (newkey > 26):
                        newkey -= 26
                    new += ABC[newkey]
                if (i.islower() == True):
                    newkey = (get_keya(i)+13)
                    if (newkey > 26):
                        newkey -= 26
                    new += abc[newkey] 
            else:
                new += i
        else :
            new += i
    return new
    
    
def get_keyA(val): 
    for key, value in ABC.items(): 
        if val == value: 
            return key 

def get_keya(val): 
    for key, value in abc.items(): 
        if val == value: 
            return key 

Yes, I'm not using encode. I want to try this out without using it. Thank you for reading and thanks for the help!

Answer 1

your functions are written wrongly and non pythonic. we will fix later. the bug lays in this line:

newkey = (get_keyA(i)+13)

you have called the method with i that doesnot exist in the ABC dict . and you have get None return value from this method. hence, None + 13 is something that python cant handle (None + int) . hence the error!

Any way this methods are unusable. python have built in methods for searching a dict and get the return value:

dict.get(key[, value]) 

get(key[, default]) Return the value for key if key is in the dictionary, else default. If default is not given, it defaults to None, so that this method never raises a KeyError.

Just use it. i recommend you to add a try except on this code block(above) Suppose that, if the key i is not exist in the dict , then you can return a default value that suits your program, so th e usage become:

ABC.get(i, <some_default_value>) 

for example: suppose i want to return 0 value if the key i isnot exist in the dict, and then i want to add it to the dict with default value 'foo', so the code becomes:

if not ABC.get(i,0):
    ABC[i] = 'foo'  #add the key i to this dict with the value 'foo'

hence the methods you have wrote ( get_keyA(val) and get_keya(val) ), can be deleted!