##Say that I am creating a function meant to 
##shift every character in a text document up 
##one character, ie. 'b' would turn to 'c'.
text=moby_dick.txt
shift=1
def shift_text(text, shift):
  shifted=''
  for ch in text:
      shifted+=shifted.join(chr(97+((ord(ch)-96+shift)%26)))
    else:
      shifted+=ch
  return shifted
##For some reason, this function doesn't work 
##when I run it. To debug this, I decide to just 
##manipulate the string 'How are you?'. That 
##would look like this:
text='How are you?'
shift=1
def shift_text(text, shift):
  shifted=''
  for ch in text:
      shifted+=shifted.join(chr(97+((ord(ch)-96+shift)%26)))
    else:
      shifted+=ch
  return shifted
##This lets me see that '?' characters are being 
##changed, which is not something that I want. 
##Thus, I now know I need to exclude non-letter
##characters:
def shift_text(text, shift):
  shifted=''
  for ch in text:
    if 'a'<=ch and 'z'>=ch:
      shifted+=shifted.join(chr(97+((ord(ch)-96+shift)%26)))
    else:
      shifted+=ch
  return shifted
##Now my code works, through the use of mocking.