Python Programming Essentials - M21 - Exception Handling

Python Programming Essentials - M21 - Exception Handling

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  http://www.skillbrew.com /Skillbrew Talent brewed bythe industry itself Exception Handling in python Pavan Verma @YinYangPavan Founder, P3 InfoTech Solutions Pvt. Ltd. 1 Python Programming Essentials
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  © SkillBrew http://skillbrew.com Contents Exceptions  Throwing and catching exceptions  try except  try except else  try except finally  Common python exceptions 2
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  © SkillBrew http://skillbrew.com Whatis an Exception?  An exception is an error that happens during execution of a program  If an exception is not caught the program is terminated  In Python, exceptions are triggered automatically on errors, and they can be triggered and intercepted by your code 3
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  © SkillBrew http://skillbrew.com Exceptions Exception handling has two steps: • Raising or Throwing • Catching  Python provides 3 keywords to deal with exceptions : • raise • try • except 4
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  © SkillBrew http://skillbrew.com Exceptions(2) Program to divide a constant by a number def divide(num): print 100/num if __name__ == '__main__': divide(0) OUPUT: ZeroDivisionError: integer division or modulo by zero 5
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  © SkillBrew http://skillbrew.com Exceptionpropagation 6 >>> f1(0) Traceback (most recent call last): File "<pyshell#10>", line 1, in <module> f1(0) File "<pyshell#8>", line 2, in f1 return f2(num) File "<pyshell#5>", line 2, in f2 return f3(num) File "<pyshell#2>", line 3, in f3 return constant/num ZeroDivisionError: integer division or modulo by zero >>> def f3(num): constant = 100 return constant/num >>> def f2(num): return f3(num) >>> def f1(num): return f2(num) >>> f1(10) 10 >>> f1(0)
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  © SkillBrew http://skillbrew.com Whyuse exceptions  Error handling: Python raises an exception whenever it detects errors in program at runtime. You can catch and respond to errors in the code or Python’s default behavior kicks in, stops the program and prints the error message.  Event notification: exceptions can also be used to signal valid conditions without you having to pass result flags around a program 7
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  © SkillBrew http://skillbrew.com Throwingand Catching Exceptions 8
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  © SkillBrew http://skillbrew.com Throwingan exception 9 def avg(seq): result = 0 for val in seq: result += convert(val) return result/len(seq) def convert(val): try: val = int(val) except ValueError: raise ValueError('val type is not int') return val print avg([1, 2, 4, 5]) Output: 3
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  © SkillBrew http://skillbrew.com Throwingan exception 10 print avg([1, 'two', 4, 5]) Output: Traceback (most recent call last): File "exceptions1.py", line 15, in <module> print avg([1, 'two', 4, 5]) File "exceptions1.py", line 4, in avg result += convert(val) File "exceptions1.py", line 11, in convert raise ValueError('val type is not int') ValueError: val type is not int
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  © SkillBrew http://skillbrew.com HandlingExceptions (try except block)  In order to handle exceptions wrap the code in try except def divide(num): try: print 100/num except ZeroDivisionError: print("division by Zero not allowed") if __name__=='__main__': divide(0) Output: division by Zero not allowed 11
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  © SkillBrew http://skillbrew.com tryexcept else 12 try: # do something except: # handle exception else: # executed only when there is no exception The code in else block is only executed if there is no exception
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  © SkillBrew http://skillbrew.com tryexcept else (2) def divide(num): try: result = 100/num except ZeroDivisionError: print('division by Zero not allowed') else: print “Result is %d" % (result) if __name__ == '__main__': divide(10) Output: Result is 10 13
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  © SkillBrew http://skillbrew.com tryexcept finally 14 try: # do something except: # handle exception finally: # always executed The code in finally block is always executed, no matter what
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  © SkillBrew http://skillbrew.com tryexcept finally (2) 15 def divide(num): try: result = 100/num except ZeroDivisionError: print('division by Zero not allowed') finally: print “Input was %d" % (num) if __name__ == '__main__': divide(0) Output: Division by Zero not allowed Your input was 0
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  © SkillBrew http://skillbrew.com CommonPython Exceptions Exception Description IOError If the file cannot be opened ImportError If python cannot find the module ValueError Raised when a built-in operation or function receives an argument that has the right type but an inappropriate value KeyError Raised when a mapping (dictionary) key is not found in the set of existing keys IndentationError raised due to incorrect indentation SyntaxError Raised when the parser encounters a syntax error 16
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  © SkillBrew http://skillbrew.com Customexceptions class MyException(Exception): pass def divide(num): try: return 100/num except ZeroDivisionError: raise MyException('Cannot divide by 0') 17
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  © SkillBrew http://skillbrew.com Resources http://doughellmann.com/2009/06/pyth on-exception-handling-techniques.html 18