How to replace characters in a string using iteration and variables? (Python)

str.replace() returns the new string, store the new value:

word = word.replace(letter, guess)

Python strings are immutable and cannot be altered in-place.

However, you are replacing letter with the exact same value; letter == guess is only True if both are the same character.

I'd keep a separate set of correctly guessed letters instead, and rebuild the displayed underscores and correct guesses each time:

correct_guesses = set()
incorrect_guesses = set()

if guess in correct_guesses & incorrect_guesses:
    print('You already guessed that letter')

elif guess in word:
    # correct guess!
    correct_guesses.add(guess)
    display_word = ''.join(char if char in correct_guesses else '_' for char in word)

else:
    # incorrect guess!
    incorrect_guesses.add(guess)
    print('Oops, incorrect guess!')
    missNum += 1

I think I understand what you're getting at here.

I would probably rebuild the word-so-far on the spot instead of having a persistent string for it, keeping the tested letters separately. When the user tries a new character, make two checks:

1. See if the guess character has been guessed already: if guess in tried. If so, proceed however you like (penalize or ignore), but don't add the character to the tried-characters list.

2. If not, see if the character is in the target word: elif guess in word. If not, assess some penalty and add the guess to the tried-characters list.

3. For any other result: else. Add the guess to the tried-characters list.

To display the user's progress, make a blank string. Go through the target word character-at-a-time: for char in word, like you have been. But instead of trying to modify an extant string, just add the character to the end of the blank string if it's in the tried-characters string, or an underscore if not: show += char if char in tried else "_". Once that for loop is exhausted, display what you've got!

Alternatively, use .join with a slightly different iterator: show = "".join(char if char in tried else '_' for char in word). It'll iterate through word, keeping each letter if it's in your tried-characters string, or substituting an underscore if not, putting whatever is in "" between them (or nothing, if you leave it as ""). It looks like you already know that, though.

At the hazard of completely rewriting your code, this is what it might look like:

  ## init
word = "mauritius" # the word you're looking for. I chose this one.
tried = str()  # initialize a list of tested characters
tryNum = 3  # however many wrong guesses the user gets

  ...

  ## in your run loop...
if tryNum:  # evaluates 0 as Fasle, if you didn't already know
    guess = UserInput()  # some means to get the guess from the user; a single-character string.

    if   guess in tried:
        print "Tried that letter already!" 
    elif guess not in word:   # the letter hasn't been tested yet, but isn't in the word, either.
        print "Wrong! %d guesses left!" % tryNum
        tryNum -= 1
        tried += guess
    else:    # the guess is new (not tried) and it's valid (in the word)
        tried += guess

    show = str()  # this is the string you will display. make a blank one each time.
    for char in word:
        show += char if char in tried else "_"  # if the character has been tried, add it to the output. Otherwise, an underscore.
    print show  # display the word so far

    if show == word:
        print "You win!" # Congratulations! You hung a man.

else:  # Out of tries; the convict lives another day.
    print "Game Over!" # I am not sure how committed to this vocabulary-based execution you really are...

You can swap if tryNum: with while tryNum: and it should work all by itself, after initialization. If you do, there are fun things you can do with continues and breaks, but that goes a bit beyond the scope of your question.

You can swap show = str() and the for char in word: block out with the .join singleton in this next example, too. Change ''.join(..) to ' '.join(..) to add a space between characters/underscores!

This compressed version is probably a bit less Pythonic:

# post-init...
if tryNum:
    guess = UserInput()
    if guess in tried: pass
    elif guess not in word:
        print "Wrong! %d guesses left!" % tryNum
        tryNum -= 1
        tried  += guess
    else: tried += guess
    show = ''.join(char if char in tried else '_' for char in word)
    if show == word: print "You win!"
else: print "Game Over!"

This does not answer your first question of "What am I doing wrong?" but I think it might be a better way of going about what you intend? It might be a bit easier to maintain and expand for you, too.

Note: Go ahead and replace UserInput() with something like str(raw_input("Guess a letter!"))[0] if you want to try this thing out on its own.

Replacing a letter with an identical guess isn't going to do anything! I think you want to find the position in the word where the guessed letter appears, and replace the _ in that position with the letter. For that you will need to find every position where the letter occurs, e.g. using the index() method.

For instance, to replace the first occurrence of the guess:

# Illustration of the principle, not the complete answer.

word = 'faq'
display = '___'

# Put in a loop, one iteration for each guess input.
guess = 'a'
display = ''.join([letter if guess == letter else display[pos]
                   for pos, letter in enumerate(word)])
print display

Which will print _a_.