Manejo de errores en Python

Forma "tradicional" de manejar errores:

if os.path.exists("noexistis.csv"):
  fh=open("noexistis.csv")
  linea=fh.readline()
  fh.close()
  if "\t" in linea:
      valor=linea[:linea.index("\t")]
      if valor.isdigit():
          newvalue=int(valor)*.2
          if os.access("/home/sbassi/out.txt",os.W_OK):
              fw=open("out.txt","w")
              fw.write(str(newvalue))
              fw.close()
          else:
              print "Output file is not writable"
      else:
          print "It can't be converted to int"
  else:
      print "No tiene TAB"
else:
  print "The file doesn't exist"

En Python podemos usar try/except para no tener que "deformar" el código en función de los posibles errores o situaciones no deseadas:

try:
  fh=open("noexistis.csv")
  linea=fh.readline()
  fh.close()
  valor=linea[:linea.index("\t")]
  newvalue=int(valor)*.2
  fw=open("/home/sbassi/out.txt","w")
  fw.write(str(newvalue))
  fw.close()
except IOError, (errno,errtext):
  if errno==13:
      print "Can't write to out.txt"
  elif errno==2:
      print "File not exist"
except ValueError, strerror:
  if "substring not found" in strerror[0]:
      print "There is no tab"
  elif "invalid literal for int()" in strerror[0]:
      print "It can't be converted to int"