Как выполнить пошаговую отладку?

#!/usr/bin/env python
#coding: utf-8
 
 
def crypto(fin, fout, flag='code'):
  total=[]
  info ={'lines':0, 'countsym':0 }
  for line in file(fin, 'rb', 1):
    buffer=[]
    info['lines'] +=1
    for key in line:
      if flag is 'code':
        buffer.append(chr(ord(key)+1))
      elif flag is 'decode':
        buffer.append(chr(ord(key)-1))
      else:
        print "Wrong key. Abort\n"
        exit(0)
      info['countsym'] +=1
    total.extend(buffer)
  with file(fout, 'wb', 1) as f:
    for line in total:
      f.write(line)
  print 'Total lines: %3d\nTotal syms: %4d\n' %(info['lines'], info['countsym'])

  crypto(uyygyj,ivh,utf-8)
  print(crypto)

fin = 'simple.txt'
fout = 'simple.code'
fdecode = 'simple.decode'
# шифруем
crypto(fin, fout, flag='code')
# дешифруем
crypto(fout, fdecode, flag='decode')