python Exception Raise

python

Exception raise

Occuring exception by raise

  • raisekeyword can make an exception in specific situation

  • IndexError occurs when __getitem__method(which implements indexing when sequance type class is made) goes out of the index range

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    class SquareSeq:
      def __init__(self, n):
        self.n = n
      def __getitem__(self,k):
        if k >= self.n or k < 0:
          raise IndexError # out of index range, IndexError occurs
        return k * k
      def __len_(self):
        return self.n

    s = SquareSeq(10)
    print s[2], s[4]
    for x in s: # repeating until IndexError occurs
       print x,
    print s[20] # out of the index range

Customized exception class

  • Generally It is implemented by extending Exceptionclass
    (Other exceptions are unusual)
  • How to raise the exception
    • Same to built-in exception. Using raise [class instance]
  • How to catch the exception from Customized exception class
    • Using the class’ name like except [class name]

  • In the example below, except Big catches Bigand Small exception
    -> Because Smallis Big‘s sub-class’

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    class Big(Exception):
      pass

    class Small(Big):
      pass

    def dosomething1():
      x = Big()
      raise x # x is exception object, it can use raise

    def dosomething2():
      raise Small()

    for f in (dosomething1, dosomething2):
      try:
        f()
      except Big:
        print "Exception occurs!"

Passing exception

  • After raisekeyword, with exception, additional message can follow

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    def f():
      raise Exception, 'message!!!'

    try:
      f()
    except Exception, a:
      print a

  • When except keyword is used, exception message in initializer can be taken as 2nd parameter.

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    a = 10
    b = 0
    try:
      if b == 0:
        raise ArithmeticError('you are dividing by 0')
        a/b
    except ArithmeticError, v:
      print v
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