Python: IndexError: list index out of range Caeser Cipher
<p>I am trying to create a caeser cipher python code and keep getting IndexError: list index out of range   when I type 'y'.  However there are no problems when I type something like 'hello'.  I am not necessarily looking for a solution but would love an explanation as I really don't understand it.</p>

<pre><code>    plaintext=raw_input("Enter message: ")
    convert=plaintext.lower()
    alphabet= ["a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z"]  
    key=2
    cipher=""

    for i in convert:
        if i in alphabet:
            cipher+=alphabet[(alphabet.index(i)+key%len(alphabet))] 
    print("Your decrypted message is: " +cipher)
</code></pre>