Text Shift function in Python

You are looping over the list of characters, and i is thus a character. You then try to store that back into data using the i character as an index. That won't work.

Use enumerate() to get indexes and the values:

def shifttext(shift):
    input=raw_input('Input text here: ')
    data = list(input)
    for i, char in enumerate(data):
        data[i] = chr((ord(char) + shift) % 26)
    output = ''.join(data)
    return output

You can simplify this with a generator expression:

def shifttext(shift):
    input=raw_input('Input text here: ')
    return ''.join(chr((ord(char) + shift) % 26) for char in input)

But now you'll note that your % 26 won't work; the ASCII codepoints start after 26:

>>> ord('a')
97

You'll need to use the ord('a') value to be able to use a modulus instead; subtracting puts your values in the range 0-25, and you add it again afterwards:

    a = ord('a')
    return ''.join(chr((ord(char) - a + shift) % 26) + a) for char in input)

but that will only work for lower-case letters; which might be fine, but you can force that by lowercasing the input:

    a = ord('a')
    return ''.join(chr((ord(char) - a + shift) % 26 + a) for char in input.lower())

If we then move asking for the input out of the function to focus it on doing one job well, this becomes:

def shifttext(text, shift):
    a = ord('a')
    return ''.join(chr((ord(char) - a + shift) % 26 + a) for char in text.lower())

print shifttext(raw_input('Input text here: '), 3)

and using this on the interactive prompt I see:

>>> print shifttext(raw_input('Input text here: '), 3)
Input text here: Cesarsalad!
fhvduvdodgr

Of course, now punctuation is taken along. Last revision, now only shifting letters:

def shifttext(text, shift):
    a = ord('a')
    return ''.join(
        chr((ord(char) - a + shift) % 26 + a) if 'a' <= char <= 'z' else char
        for char in text.lower())

and we get:

>>> print shifttext(raw_input('Input text here: '), 3)
Input text here: Ceasarsalad!
fhdvduvdodg!

Looks you're doing cesar-cipher encryption, so you can try something like this:

strs = 'abcdefghijklmnopqrstuvwxyz'      #use a string like this, instead of ord() 
def shifttext(shift):
    inp = raw_input('Input text here: ')
    data = []
    for i in inp:                     #iterate over the text not some list
        if i.strip() and i in strs:                 # if the char is not a space ""  
            data.append(strs[(strs.index(i) + shift) % 26])    
        else:
            data.append(i)           #if space the simply append it to data
    output = ''.join(data)
    return output

output:

In [2]: shifttext(3)
Input text here: how are you?
Out[2]: 'krz duh brx?'

In [3]: shifttext(3)
Input text here: Fine.
Out[3]: 'Flqh.'

strs[(strs.index(i) + shift) % 26]: line above means find the index of the character i in strs and then add the shift value to it.Now, on the final value(index+shift) apply %26 to the get the shifted index. This shifted index when passed to strs[new_index] yields the desired shifted character.

Martijn's answer is great. Here is another way to achieve the same thing:

import string

def shifttext(text, shift):
    shift %= 26 # optional, allows for |shift| > 26 
    alphabet = string.lowercase # 'abcdefghijklmnopqrstuvwxyz' (note: for Python 3, use string.ascii_lowercase instead)
    shifted_alphabet = alphabet[shift:] + alphabet[:shift]
    return string.translate(text, string.maketrans(alphabet, shifted_alphabet))

print shifttext(raw_input('Input text here: '), 3)