Need some help creating a word game

[perfringo]

Letters that are not present at all in the secret word show a "underscore _".

Letters that are present in the secret word, but in a different spot show in 'lowercase".

Letters that are present in the secret word at that exact spot, show in "uppercase".

It's always good to have a plan. So:

Letters that are present in the secret word at that exact spot, show in "uppercase".

There are many ways to achieve that but one of them is iterating over secret word and guess in parallel using built-in zip and check if characters are equal:

for guess, secret in zip(guess_word, secret_word):
    if guess == secret:
        # do something

Letters that are present in the secret word, but in a different spot show in 'lowercase".

If we already iterate over word we should also check whether character from guess_word is in secret_word. We can check it directly or use Python special data type for membership testing - set. In order to avoid overwriting result of if branch we should put this secondary check into elif branch ensuring that it will run only if characters are not equal:

elif guess in secret_word:
    # do something

# or

chars = set(secret_word)  # should be outside of for-loop

elif guess in chars:
    # do something

Letters that are not present at all in the secret word show a "underscore _".

If we have handled two cases in if and elif branch and anything that passes them is character what is not in same place nor in secret_word at all. So we can direct everything else to else branch:

else:
    # do something

Hint: using print function argument end we are able to print different loop iterations in one row:

for char in 'mosiah':
    print(char)
# will print:
m
o
s
i
a
h

for char in 'mosiah':
    print(char, end='')

# will print
mosiah

This of course assumes that validation of letters to be in same case and words are equal length has been made.