View source for Python-3.4.3/C4/Handling-Errors-and-Exceptions/English

'''Title of script''': '''Handling errors and exceptions'''

'''Author: Arun KP'''

'''Keywords: Python, Ipython, error handling, exceptions, debugging, video tutorial'''


{| style="border-spacing:0;"
| style="background-color:#ffffff;border:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| '''Visual Cue'''
| style="background-color:#ffffff;border:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| '''Narration'''

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| style="background-color:#ffffff;border:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| Show Slide title
| style="background-color:#ffffff;border:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| Welcome to the spoken tutorial on '''Handling errors and exceptions'''. 

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Objectives 



| style="background-color:#ffffff;border:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| In this tutorial, you will learn to, 

* Understand '''errors '''and''' exceptions''' 
* Handle '''errors''' and '''exceptions''' 

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System Specifications 
| style="background-color:#ffffff;border:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| To record this tutorial, I am using 

* '''Ubuntu Linux 16.04''' operating system
* '''Python 3.4.3'''
* '''IPython 5.1.0'''

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Prerequisite 
| style="background-color:#ffffff;border:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| To practise this tutorial, you should know how to 

* use''' functions '''and
* carry-out '''testing '''and '''debugging'''

If not, see the relevant '''Python''' tutorials on this website.

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| style="background-color:#ffffff;border:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| Slide Errors
| style="background-color:#ffffff;border:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| In '''Python''' there are two kinds of '''errors''': 

* '''''syntax errors'''''and 
* '''''exceptions'''''

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Syntax Error
| style="background-color:#ffffff;border-top:none;border-bottom:0.5pt solid #000001;border-left:0.5pt solid #000001;border-right:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| '''Syntax errors '''are caused by incorrect usages and these are detected by '''parser'''.


For example:

'''if True print'' inside brackets inside double quotes ''done'''


It will give '''Syntax Error''' since '''colon''' is missing after '''True'''.

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| style="background-color:#ffffff;border:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| Show Slide

Exceptions



| style="background-color:#ffffff;border:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| '''Exception''' is an '''error''' that occurs during '''execution''' of a '''program'''.


'''Python''' generates an '''exception''' that can be handled, which avoids the '''program''' to crash.


For example:

1/0


It will give '''ZeroDivisionError exception.'''

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| style="background-color:#ffffff;border:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| Open terminal
| style="background-color:#ffffff;border:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| Let us see these examples in action.


Open the '''terminal.''' 

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| style="background-color:#ffffff;border-top:none;border-bottom:0.5pt solid #000001;border-left:0.5pt solid #000001;border-right:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| Type,

'''ipython3'''
| style="background-color:#ffffff;border-top:none;border-bottom:0.5pt solid #000001;border-left:0.5pt solid #000001;border-right:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| Type '''ipython3 '''and press '''Enter.'''


From here onwards remember to press the '''Enter''' key after typing every '''command''' on the '''terminal'''.

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| style="background-color:#ffffff;border:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| Type,

'''if True print("done")'''


Highlight the arrow mark



| style="background-color:#ffffff;border:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| Now type

'''if True print '''''inside brackets inside double quotes''''' done'''


The output shows '''SyntaxError.'''


It displays an ‘arrow’ pointing at the earliest point in the line where the '''error''' was detected.


In our case, the error is caused by missing colon after '''True''' in '''If '''clause

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| style="background-color:#ffffff;border:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| Type,

'''1 / 0 '''
| style="background-color:#ffffff;border:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| Now type,

'''1 / 0'''


'''Python''' throws an '''exception''' called '''ZeroDivisionError.'''


Even though the expression is syntactically correct, we cannot divide a number by zero.

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| style="background-color:#ffffff;border:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| 
| style="background-color:#ffffff;border:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| '''Exception''' is a special kind of failure reported by the '''programming language'''. 

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| style="background-color:#ffffff;border:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| Type,

'''a = input("Enter a number: ")'''

'''Enter '''ac

'''num = int(a)'''
| style="background-color:#ffffff;border:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| Let's see how can we deal with '''Exceptions '''that occur in '''programs'''.


Type 

'''a = input''''' inside brackets inside double quotes '''''Enter an integer'''


I will enter''' ac '''as input.


Now type, '''num '''''is equal to''''' int '''''inside brackets '''''a'''

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| style="background-color:#ffffff;border:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| Point to the error
| style="background-color:#ffffff;border:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| When you run this code, it throws a ''''ValueError' Exception'''.


This is because, we are trying to convert the '''string''' to '''integer'''.

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| style="background-color:#ffffff;border:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| 
| style="background-color:#ffffff;border:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| So now we can ''''catch exceptions''' and write '''code''' to '''handle''' it. 


For this we have '''try''' and '''except clause''' in '''Python'''.

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| style="background-color:#ffffff;border:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| <nowiki>[slide]</nowiki>

try..except..else


'''try:'''

'''statement1'''

'''statement2'''
| style="background-color:#ffffff;border:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| Here is the syntax of '''''try....except...else'' blocks'''.


First, the '''statements''' between the '''try''' and '''except keywords''' is executed.

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'''except exception name:'''

'''exception handling statement(s)'''
| style="background-color:#ffffff;border:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| If the '''statements''' do not cause any '''exception''', the '''except clause''' is skipped.


If the '''statements''' cause any '''exception''', then '''except clause''' is executed if '''exception''' name matches.

And the '''execution''' continues after the '''try statement'''.

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| style="background-color:#ffffff;border:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"|'''else:'''

'''statement(s) when no exception'''


'''after statement(s)'''
| style="background-color:#ffffff;border:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| The '''code''' in the '''else-block executes''' if the '''code''' in the '''try: block''' does not raise an '''exception'''.

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| style="background-color:#ffffff;border:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| Type,

'''a = input("Enter an integer: ") '''

'''ac'''

'''try:'''

'''num = int(a)'''

'''except ValueError:'''

'''print ("Wrong input")'''

'''Press Enter twice'''

'''output:'''

'''Wrong input'''
| style="background-color:#ffffff;border:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| Type as shown.


Give '''ac''' as '''input'''.

Now we will type the '''try''' and '''except blocks.'''


Press '''Enter''' twice to get the '''output'''.


In the previous example,

We encountered a problem because we tried to convert the '''string ‘ac’''' to '''integer'''. 


Here conversion of '''string value''' to an '''integer''' is given inside '''try block'''.


'''ValueError exception''' is raised and so the '''output''' is displayed as '''Wrong input.'''

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| style="background-color:#ffffff;border:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| 
| style="background-color:#ffffff;border:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| In the previous example, we found out what caused the '''error''' and then resolved to get a solution for it. 


This whole process is called '''debugging'''. 

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| style="background-color:#ffffff;border:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| 
| style="background-color:#ffffff;border:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| Next we will see another '''case''' in '''try except statement''' with '''else clause'''.

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| style="background-color:#ffffff;border:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| '''b = input("Enter an integer: ") '''

'''23'''

'''try:'''

'''num = int(b)'''

'''except ValueError:'''

'''print ("Wrong input")'''

'''else:'''

'''print(“No exception “)'''

'''Press Enter twice'''


Highlight '''No exception '''
| style="background-color:#ffffff;border:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| Lets change our previous '''code''' slightly. 


Type as shown.


I will give '''input''' as 23.


Now type as shown.


This''' try… except statement''' has an optional '''else clause'''.


It is useful if the '''try clause''' does not raise an '''exception'''.

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| style="background-color:#ffffff;border:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| Open a '''text editor''' and type the following '''code'''. 


'''def test():'''

'''prod = 1'''

'''for i in range(0, 10):'''

'''prod *= i / (i - 5)'''

'''print(prod)'''

Save it as file '''mymodule.py''' 
| style="background-color:#ffffff;border:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| Lets see another example for '''debugging'''. 


Create a file '''mymodule.py''' with the following '''code'''.


The '''variable ‘i’''' is iterated from 0 to 9.

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| style="background-color:#ffffff;border:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| Type,

'''from mymodule import test'''

'''test()'''



| style="background-color:#ffffff;border:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| Let us '''run''' this '''code''' in '''Ipython'''.


Type,

'''from mymodule import test'''

'''test '''''open and close brackets''


'''Interpreter''' gives us '''ZeroDivisionError''' because there is a''' division by zero error'''. 

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| style="background-color:#ffffff;border:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| Type,

'''%debug''' 


Highlight the arrow mark


Highlight ipdb>
| style="background-color:#ffffff;border:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| To find the value which caused the '''error''', type this


''Percentage '''''debug'''


The '''code''' which caused the '''error''' is shown by an arrow.


The '''prompt''' has changed to '''ipdb '''which is the '''ipython debugger mode.'''

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| style="background-color:#ffffff;border:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| <nowiki>[IPython Terminal]</nowiki>


'''ipdb> i'''

'''5'''
| style="background-color:#ffffff;border:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| Using this '''debugger''' here, you can access '''variables''' in the previous '''code block'''.


We can check values of '''variables''' to inspect what went wrong.


For example, on typing ‘i’, we get 5 as '''output'''.


This means that the '''error''' was caused when the value of '''i''' became 5.


To exit from the '''ipdb prompt''', press '''q '''and press '''Enter'''.

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Summary 
| style="background-color:#ffffff;border:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| This brings us to the end of this tutorial. Let us summarize.


In this tutorial, we have learnt about

* '''Errors '''and''' exceptions'''.
* Handling '''exception''' using '''try''' and '''except'''. 
* Using '''''percentage ''debug''' for '''debugging''' in '''ipython'''. 

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Self assessment questions
| style="background-color:#ffffff;border:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| Here is a self assessment question for you to solve 

#How do you start the''' debugger '''on''' ipython?''' 

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Solutions
| style="background-color:#ffffff;border:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| And the answer,

# We start the '''debugger''' on '''ipython''' by saying, '''percentage ''debug'''

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Acknowledgment



| style="background-color:#ffffff;border-top:none;border-bottom:0.5pt solid #000001;border-left:0.5pt solid #000001;border-right:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| Spoken Tutorial Project is funded by NMEICT, MHRD, Govt. of India.

For more details, visit this website.

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| style="background-color:#ffffff;border:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| Previous slide
| style="background-color:#ffffff;border:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| This is Priya from IIT Bombay signing off.

Thank you.

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