python Exception Handling

python

Exception Handling

  • Syntax Error

    • Grammatical error
    • IDE checks syntax error automatically
    • Becasue python’s grammar is simpler relatively, error occur rate is smaller and it is easy to delete errors

  • Exception

    • the cases having no syntax errors, the program can’t progress
    • Occuring exception, the program ends

    1. NameError (Usiung undefined variable)

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      4 + boo*3

    2. ZeroDivisionError (Dividing by 0)

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      a = 10
      b = 0
      c = a / b

    3. TypeError (ex: String + Number type)

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      '2' + 2

    4. IndexError (Using index over the range of index)

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      l = [1,2]
      print l[2]

    5. KeyError (Searching dictionary by unregisted key)

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      d = {"a":1, "b":2}
      print d['c']

    6. IOError (Opening not existing file)

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      a = open('aaa.txt')

How to dispose of exceptions

  1. Using try,except,else,finally statements

    • By expecting the situations making exceptions, we can control the whole code’s flow

    • try,except,else,finally

      • structure
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        try:
          (possible to make errors) usual statements
        except Exception:
          the statements working when error occurs
        else:
          the statements working when error doesn't occur
        finally:
          the statements working anyway no matter error

  • Doing exception handling, when error occurs the program doesn’t stop

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    a = 0
    try:
      print 1.0/a
    except ZeroDivisionError:
      print 'zero division error!!!'

  • msgvariable: the message that the person who defined ZeroDivisionError gives
    ex: float division by zero

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    def division():
      for n in range(0, 5):
        try:
          print 10.0 / n
        except ZeroDivisionError, msg
          print msg

    division()

  • It can handle the error from the called function in trystatement indirectly

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    try:
      spam()
    except NameError, msg:
      print 'Error -', msg

    , can be replaced with as

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    # same code with upper one
    try:
      spam()
    except NameError as msg:
      print 'Error -', msg

  • After except, not evincing any exceptions, it takes care of all exceptions

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    try:
      spam()
      print 1.0/0.0
    except:
      print 'Error'

  • About many exceptions, they can be taked care by each exceptstatement

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    b = 0.0
    name = 'aaa.txt'
    try:
      print 1.0 / b
      spam()
      f = open(name, 'r')
      '2' + 2
    except NameError:
      print 'NameError !!!'
    except ZeroDivisionError:
      print 'ZeroDivisionError !!!'
    excpet (TypeError, IOError):
      print 'TypeError or IOERror !!!'
    else:
      print 'No Exception'
    finally
      print 'Exit !!!'

Catching All exception in same kind

  • By using the hierarchy of Inheritance of exception class, It can take care of many exceptions by once

  • For example, there are FloatingPointError, OverflowError, ZeroDivisionError as sub-class of ArithmeticError so, about sub-class exceptions, ArithmeticError can take care of them

  • When an exception is catched by specific except, that one is not catched in other except

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    def dosomething():
      a = 1/0

    try:
      dosomething()
    except ZeroDivisionError:
      print "ZeroDivisionError occured"
    except ArithmeticError:
       print "ArithmeticError occured"
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