This document discusses exception handling in Python. It explains that exceptions are errors that occur during program execution and can be handled using try, except, and raise keywords. The document covers throwing exceptions, catching exceptions using try/except blocks, propagating exceptions, using try/except/else and try/except/finally, and some common built-in Python exceptions. It also provides an example of defining a custom exception class.

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Python Programming Essentials - M21 - Exception Handling

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  http://www.skillbrew.com/SkillbrewTalent brewed bythe industry itselfException Handling in pythonPavan Verma@YinYangPavanFounder, P3 InfoTech Solutions Pvt. Ltd.1Python Programming Essentials
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  © SkillBrew http://skillbrew.comContentsExceptions Throwing and catching exceptions try except try except else try except finally Common python exceptions2
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  © SkillBrew http://skillbrew.comWhatis an Exception? An exception is an error that happensduring execution of a program If an exception is not caught the programis terminated In Python, exceptions are triggeredautomatically on errors, and they can betriggered and intercepted by your code3
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  © SkillBrew http://skillbrew.comExceptionsException handling has two steps:• Raising or Throwing• Catching Python provides 3 keywords to deal withexceptions :• raise• try• except4
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  © SkillBrew http://skillbrew.comExceptions(2)Program to divide a constant by a numberdef divide(num):print 100/numif __name__ == '__main__':divide(0)OUPUT:ZeroDivisionError: integer division or modulo by zero5
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  © SkillBrew http://skillbrew.comExceptionpropagation6>>> f1(0)Traceback (most recent call last):File "<pyshell#10>", line 1, in<module>f1(0)File "<pyshell#8>", line 2, in f1return f2(num)File "<pyshell#5>", line 2, in f2return f3(num)File "<pyshell#2>", line 3, in f3return constant/numZeroDivisionError: integer divisionor modulo by zero>>> def f3(num):constant = 100return 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.comWhyuse exceptions Error handling: Python raises an exception wheneverit detects errors in program at runtime. You can catchand respond to errors in the code or Python’s defaultbehavior kicks in, stops the program and prints theerror message. Event notification: exceptions can also be used tosignal valid conditions without you having to passresult flags around a program7
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  © SkillBrew http://skillbrew.comThrowingand Catching Exceptions8
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  © SkillBrew http://skillbrew.comThrowingan exception9def avg(seq):result = 0for 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 valprint avg([1, 2, 4, 5])Output:3
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  © SkillBrew http://skillbrew.comThrowingan exception10print 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 avgresult += convert(val)File "exceptions1.py", line 11, in convertraise ValueError('val type is not int')ValueError: val type is not int
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  © SkillBrew http://skillbrew.comHandlingExceptions (try except block) In order to handle exceptions wrap the code in tryexceptdef divide(num):try:print 100/numexcept ZeroDivisionError:print("division by Zero not allowed")if __name__=='__main__':divide(0)Output:division by Zero not allowed11
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  © SkillBrew http://skillbrew.comtryexcept else12try:# do somethingexcept:# handle exceptionelse:# executed only when there is no exceptionThe code in else block is only executed if there is noexception
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  © SkillBrew http://skillbrew.comtryexcept else (2)def divide(num):try:result = 100/numexcept ZeroDivisionError:print('division by Zero not allowed')else:print “Result is %d" % (result)if __name__ == '__main__':divide(10)Output:Result is 1013
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  © SkillBrew http://skillbrew.comtryexcept finally14try:# do somethingexcept:# handle exceptionfinally:# always executedThe code in finally block is always executed, nomatter what
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  © SkillBrew http://skillbrew.comtryexcept finally (2)15def divide(num):try:result = 100/numexcept ZeroDivisionError:print('division by Zero not allowed')finally:print “Input was %d" % (num)if __name__ == '__main__':divide(0)Output:Division by Zero not allowedYour input was 0
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  © SkillBrew http://skillbrew.comCommonPython ExceptionsException DescriptionIOError If the file cannot be openedImportError If python cannot find the moduleValueError Raised when a built-in operation orfunction receives an argument that hasthe right type but an inappropriate valueKeyError Raised when a mapping (dictionary) key isnot found in the set of existing keysIndentationError raised due to incorrect indentationSyntaxError Raised when the parser encounters asyntax error16
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  © SkillBrew http://skillbrew.comCustomexceptionsclass MyException(Exception):passdef divide(num):try:return 100/numexcept ZeroDivisionError:raise MyException('Cannot divideby 0')17
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  © SkillBrew http://skillbrew.comResourceshttp://doughellmann.com/2009/06/python-exception-handling-techniques.html18