Why isn't my cipher working

[AutoPython]

Hey, it's me again. I have a Ceaser Cypher here:

letters=list("abcdefghijklmnopqrstuvwxyz")

def Encrypt():
    for i in range(int((len(word)-1))):
        curletter="".join(letters).find(word[i])
        if (curletter+key)>(25):
            if (curletter+key)<0:
                i=(curletter+key)
                i=[(curletter+key)-(curletter+key)]
            else:
                i=(curletter+key)-(25)
        print (curletter+key)
        word[i]=letters[curletter+key]
    return

def Decrypt():
    return
    
word=list(input("Type in some letters:"))


key=int(input("Type in a key:"))

choice=int(input("1) Encrypt\n2) Decrypt\n:"))
if choice==1:
    Encrypt()
else:
    Decrypt()


print("".join(word))

If you run it and type in something that is 26 or more letters (no spaces) and you use a key of 1, it runs, and you get an index error. However, when I used print to see why letters[currentletter+key] was out of range, the number preceding it was 26, when the list only goes to 25. I see no reason why this code:

if (curletter+key)>(25):
    if (curletter+key)<0:
        i=(curletter+key)
        i=[(curletter+key)-(curletter+key)]
     else:
        i=(curletter+key)-(25)

Doesn't filter out numbers larger then 25. i is the "for" variable, currentletter is the list number of the letter you're on (a=0, b=1, c=2 etc). Any help from some python coding gurus would be much appreciated.

[woooee]

This will never be true

if (curletter+key)>(25):
            if (curletter+key)<0:

[TrustyTony]

Additionally what happens when user inputs key 123456? Why you use global variables, not parameters. Also would be natural to return values.

[AutoPython]

if (curletter+key)>(25):
            if (curletter+key)<0:

I don't know why I didn't see that.
Even after removing it however, 26 still slips through.

Additionally what happens when user inputs key 123456?

I never realized that, but all you have to do is divide it, because any key past 26 can be reduced.

Why you use global variables, not parameters. Also would be natural to return values

As far as global variables, are you implying I just need to take out the return? Or should I put all the variables in the methods parameters and skip the return command.

Either way, the program still isn't working and 26 still isn't getting filtered out.

[TrustyTony]

check % operator and ord and chr functions.

[AutoPython]

I can use % to check for remainders and stuff, thanks.However, I get how ord and chr work, but wouldn't it simply be a more less, built in list? If it's a substitute for the out of range problem, it still wouldn't work because I would have some random character in the cipher.

[TrustyTony]

If you use modulo you do not go out of range.

[woooee]

And this will always be zero.

i=[(curletter+key)-(curletter+key)]

You can just add or subtract the shift value, depending on whether it is encrypt or decrypt, and test for < 0 or > length and add or subtract length. You can in fact use the same function for encryption and decryption by sending a switch to the function telling it which you want to do.

def do_some_testing(letter_in):
    ## decrypt = shift each letter backward by 3 letters
    letters=list("abcdefghijklmnopqrstuvwxyz")
    shift = 3 

    if letter_in in letters:
        current_location = letters.index(letter_in)
        new_location = current_location - shift
        if new_location < 0:
            new_location += len(letters)
        return(letters[new_location])
    return("*")                         ## error

##------------------------------------------------
print "z -->",
print do_some_testing("z")

print "a -->",
print do_some_testing("a")

print "-"*30
orig_string = "d txlfn eurzq ira"
decrypt_list = []
for letter in orig_string:
    decrypt_list.append(do_some_testing(letter))  
print orig_string
print "".join(decrypt_list), "\n"

[AutoPython]

Thanks woooe. I'll implement that code into my version.