How can I get a loop to ignore non-letter elements in a list?

How can I get a loop to ignore non-letter elements in a list?

I have a string that contains letters and punctuation. I'm trying to
replace only the letters in this string with other letters. The function I
have developed only works for strings that contain letters. If numbers are
included it produces a logic error and if punctuation is included then it
produces a run-time error. Is there anyway that I can get my function to
ignore punctuation and leave it as is while only operating on the letters?
#Create a string variable, ABjumbler generates an alphabet shifted by x
units to the right
#ABshifter converts a string using one type to another
textObject = "g fmnc wms bgblr rpylqjyrc gr zw fylb. rfyrq ufyr amknsrcpq
ypc dmp. bmgle gr gl zw fylb gq glcddgagclr ylb rfyr'q ufw rfgq rcvr gq qm
jmle. sqgle qrpgle.kyicrpylq() gq pcamkkclbcb. lmu ynnjw ml rfc spj."
smalltext = 'abcde'
alphabet =
list(['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'])
def ABjumbler(alphabet, x):
freshset = []
i=0
j=0
while i<(len(alphabet)-x):
freshset.extend(alphabet[i+x])
i+=1
while j<x:
freshset.extend(alphabet[j]) #extend [0]
j+=1 #change j = to 1, extends by [1], then by [2], and then
terminates when it reaches x
alphabet = freshset
return alphabet
newAlphabet = ABjumbler(alphabet, 2)
def ABshifter(text, shiftedalphabet):
freshset = []
for letters in text:
position = text.index(letters)
freshset.extend(shiftedalphabet[position])
final = ''.join(freshset)
return final
print ABshifter(smalltext, newAlphabet)