class Networkerror(RuntimeError):
  def __init__(self, arg):
   self.args = arg

try:
  raise Networkerror("Bad hostname")
except Networkerror,e:
  print e.args

>>> class MyError(Exception):
...   def __init__(self, value):
...     self.value = value
...   def __str__(self):
...     return repr(self.value)
...
>>> try:
...   raise MyError(2*2)
... except MyError as e:
...   print 'My exception occurred, value:', e.value
...
My exception occurred, value: 4
>>> raise MyError, 'oops!'
Traceback (most recent call last):
 File "<stdin>", line 1, in ?
__main__.MyError: 'oops!'

class Error(Exception):
  """Base class for exceptions in this module."""
  pass

class InputError(Error):
  """Exception raised for errors in the input.

  Attributes:
    expression -- input expression in which the error occurred
    message -- explanation of the error
  """

  def __init__(self, expression, message):
    self.expression = expression
    self.message = message

class TransitionError(Error):
  """Raised when an operation attempts a state transition that's not
  allowed.

  Attributes:
    previous -- state at beginning of transition
    next -- attempted new state
    message -- explanation of why the specific transition is not allowed
  """

  def __init__(self, previous, next, message):
    self.previous = previous
    self.next = next
    self.message = message