python - altering a word encoder and decoder to give recognize spaces and punctuation -

the 2nd function encodes word phase , 3rd 1 decodes same word function doesn't skip on spaces , punctuation.

def buildcipher(key):     alpha="abcdefghijklmnopqrstuvwxyz"     rest = ""     letter in alpha:       if not(letter in key):         rest = rest + letter     print key+rest  def encode(string,keyletters):     alpha="abcdefghijklmnopqrstuvwxyz"     secret = ""     letter in string:       index = alpha.find(letter)       secret = secret+keyletters[index]        print secret  def decode(secret,keyletters):     alpha="abcdefghijklmnopqrstuvwxyz"     clear = ""     letter in secret:       index = keyletters.find(letter)       clear = clear+alpha[index]  encode("this zest!!!" , "earthbcdfgijklmnopqsuvwxyz") #gives me sdfqfqzhqs #need give me sdfq fq zhqs!!! decode("tdfq fq zhqs!!!" , "earthbcdfgijklmnopqsuvwxyz") 

at moment space character isn't in either alpha or keyletters - if don't want space encrypted add in same position in both.

note code ignores fact space in string encode not in keyletters. idea explicit in code - check letter present rather ignoring error.