Python program sometimes exit with "IndexError: string index out of range" and sometimes runs fine

Question

I'm learning Python and decided to upgrade a version of the hangman game that was used as an example in one of the courses I watched, the code turned out like this:

import random
word_dictionary = {
    1:  "mouse",
    2:  "house",
    3:  "show",
    4:  "see",
    5:  "leave",
    6:  "shower",
    7:  "showcase",
    8: "coding",
    9:  "elephant",
    10:  "apartment",
}

random_number = random.randint(1, 10)
random_word = word_dictionary[random_number]

secret_word = random_word
word_len = len(secret_word)
guess = None
tip = 0
tip_num = None
tip_previous = None
tip_position = None
tip_model_formula = "_"*word_len
tip_model_list = list(tip_model_formula)
send_tip = None

print("Word has", word_len, " letters")
while guess != secret_word:
    guess = input("Enter your guess: ")
    if guess != secret_word:
        print("Wrong answer! Try again!")
        tip += 1
        if tip == 5:
            tip_num = random.randint(0, word_len)
            tip_position = secret_word[tip_num]
            tip_model_list[tip_num] = tip_position
            send_tip = "".join(tip_model_list)
            tip_previous = tip_num
            print("here's a tip:\n" + send_tip)
        if tip == 10:
            tip_num = random.randint(0, word_len)
            while tip_num == tip_previous:
                tip_num = random.randint(0, word_len)
            tip_position = secret_word[tip_num]
            tip_model_list[tip_num] = tip_position
            send_tip = "".join(tip_model_list)
            tip_previous = tip_num
            print("here's a tip:\n" + send_tip)
        if tip == 15:
            tip_num = random.randint(0, word_len)
            while tip_num == tip_previous:
                tip_num = random.randint(0, word_len)
            tip_position = secret_word[tip_num]
            tip_model_list[tip_num] = tip_position
            send_tip = "".join(tip_model_list)
            tip_previous = tip_num
            print("here's a tip:\n" + send_tip)
        if tip == 16:
            print("You lost!")
            exit()

print("You win!")

(What this does is pick a random word from the dictionary to be the secret word and have the player take guesses on it, every 5 attempts, the player gets 1 tip, which is one letter of the word, after 16 attempts, you lose and have to restart).

When I run it, sometimes it runs as expected, but sometimes it exits with the following error:

Traceback (most recent call last):
  File "E:\Program Files\PyCharm Community Edition 2020.2\plugins\python-ce\helpers\pydev\pydevd.py", line 1448, in _exec
    pydev_imports.execfile(file, globals, locals)  # execute the script
  File "E:\Program Files\PyCharm Community Edition 2020.2\plugins\python-ce\helpers\pydev\_pydev_imps\_pydev_execfile.py", line 18, in execfile
    exec(compile(contents+"\n", file, 'exec'), glob, loc)
  File "C:/Users/rafae/PycharmProjects/Beginning/Beg.py", line 55, in <module>
    tip_position = secret_word[tip_num]
IndexError: string index out of range

I have no idea why this randomly happens and have been trying to figure out for an hour without success. Thanks for any help in advance.

Answer 1

Your bug is here:

            tip_num = random.randint(0, word_len)
            tip_position = secret_word[tip_num]

randint returns a number between its first and second arguments inclusive of both. That means that it can return a result of word_len, which is one past the end of secret_word.