PoojaMoolya: Created page with "{|border=1 |Time |Narration |- |00:01 |Welcome to the spoken tutorial on '''Handling errors and exceptions.''' |- |00:07 |In this tutorial, you will learn to, Understand ''..."

2019-06-10T11:56:33Z

Created page with "{|border=1 |Time |Narration |- |00:01 |Welcome to the spoken tutorial on '''Handling errors and exceptions.''' |- |00:07 |In this tutorial, you will learn to, Understand ''..."

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{|border=1
|Time
|Narration

|-
|00:01
|Welcome to the spoken tutorial on '''Handling errors and exceptions.'''

|-
|00:07
|In this tutorial, you will learn to, Understand '''errors''' and '''exceptions'''

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|00:14
|Handle '''errors''' and '''exceptions'''

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|00:17
|To record this tutorial, I am using '''Ubuntu Linux 16.04''' operating system

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|00:25
| '''Python 3.4.3''' and '''IPython 5.1.0'''

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|00:32
|To practise this tutorial, you should know how to use '''functions''' and carry-out '''testing''' and '''debugging.'''

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|00:43
|If not, see the relevant '''Python''' tutorials on this website.

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|00:48
|In '''Python''' there are two kinds of errors: '''syntax errors''' and '''exceptions'''

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|00:57
|'''Syntax errors''' are caused by incorrect usages and these are detected by '''parser. '''

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|01:04
|For example: '''if True print inside brackets inside double quotes done'''

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|01:11
|It will give '''Syntax Error''' since '''colon''' is missing after '''True.'''

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|01:17
|'''Exception''' is an '''error''' that occurs during '''execution''' of a '''program. '''

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|01:23
|'''Python''' generates an '''exception''' that can be handled, which avoids the '''program''' to crash.

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|01:30
|For example: 1/0

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

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|01:37
|Let us see these examples in action. Open the '''terminal.'''

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|01:43
|Type '''ipython3''' and press '''Enter.'''

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|01:48
|From here onwards remember to press the '''Enter''' key after typing every '''command''' on the '''terminal.'''

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|01:55
|Now type '''if True print''' inside brackets inside double quotes '''done. '''

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|02:03
|The output shows '''SyntaxError.'''

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|02:06
|It displays an ‘arrow’ pointing at the earliest point in the line where the '''error''' was detected.

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|02:13
|In our case, the error is caused by missing colon after '''True''' in '''If''' clause

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|02:20
|Now type, '''1 / 0'''

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

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|02:29
|Even though the expression is syntactically correct, we cannot divide a number by zero.

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|02:36
|'''Exception''' is a special kind of failure reported by the '''programming language.'''

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|02:42
|Let us see how can be deal with the '''Exceptions''' that occured in '''Programs'''

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|02:48
|Type '''a''' = '''input''' inside brackets inside double quotes '''Enter an integer'''

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|02:57
|I will enter '''ac''' as input. Now type, '''num''' is equal to '''int''' inside brackets '''a'''

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|03:07
|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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|03:17
|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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|03:27
|Here is the syntax of '''try....except...else blocks'''. First, the '''statements''' between the '''try''' and '''except keywords''' is executed.

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|03:38
|If the '''statements''' do not cause any '''exception''', the '''except clause''' is skipped.

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|03:44
| If the '''statements''' cause any '''exception''', then '''except clause''' is executed if '''exception''' name matches.

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|03:52
|And the '''execution''' continues after the '''try statement'''

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|03:57
|The '''code''' in the '''else-block executes''' if the '''code''' in the '''try: block''' does not raise an '''exception.'''

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|04:05
|Type as shown. Give '''ac''' as '''input.'''

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|04:12
|Now we will type the '''try''' and '''except blocks.'''

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|04:16
|Press '''Enter''' twice to get the '''output.'''

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|04:20
|In the previous example, We encountered a problem because we tried to convert the '''string ‘ac’ ''' to '''integer.'''

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|04:28
|Here conversion of '''string value''' to an integer is given inside '''try block.'''

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|04:34
|'''ValueError exception''' is raised and so the '''output''' is displayed as '''Wrong input.'''

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|04:41
|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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|04:53
|Next we will see another '''case''' in '''try except statement ''' with '''else clause.'''

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|04:59
|Lets change our previous '''code''' slightly. Type as shown.

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

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|05:10
|Now type as shown. This '''try… except statement''' has an optional '''else clause.'''

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|05:20
|It is useful if the '''try clause''' does not raise an '''exception.'''

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|05:25
|Lets see another example for '''debugging'''. Create a file '''mymodule.py''' with the following '''code.'''

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|05:34
|The '''variable ‘i’ ''' is iterated from 0 to 9.

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|05:39
|Let us '''run''' this '''code''' in '''Ipython. '''

Type, '''from mymodule import test'''

'''test ''' open and close brackets

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|05:52
|'''Interpreter''' gives us '''ZeroDivisionError''' because there is a '''division by zero error.'''

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|05:59
|To find the value which caused the '''error''', type Percentage '''debug'''

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|06:06
|The '''code''' which caused the '''error''' is shown by an arrow.

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|06:11
|The '''prompt''' has changed to '''ipdb''' which is the '''ipython debugger mode.'''

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|06:18
|Using this '''debugger''' here, you can access '''variables''' in the previous '''code block.'''

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|06:24
| We can check values of '''variables''' to inspect what went wrong.

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|06:29
|For example, on typing ‘i’, we get 5 as '''output.'''

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|06:36
|This means that the '''error''' was caused when the value of i became 5.

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|06:42
|To exit from the '''ipdb prompt''', press q and press '''Enter.'''

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|06:48
|This brings us to the end of this tutorial. Let us summarize.

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|06:54
| In this tutorial, we have learnt about '''Errors''' and '''exceptions.'''

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|07:00
| Handling '''exception''' using '''try''' and '''except'''. Using '''percentage debug''' for '''debugging''' in '''ipython.'''

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|07:09
|Here is a self assessment question for you to solve

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

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|07:17
|And the answer, We start the '''debugger''' on '''ipython''' by saying, '''percentage debug'''

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|07:25
|Please post your timed queries in this forum.

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|07:29
|Please post your general queries on Python in this forum.

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|07:34
|FOSSEE team coordinates the TBC project.

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|07:38
|Spoken Tutorial Project is funded by NMEICT, MHRD, Govt. of India. For more details, visit this website.

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|07:49
|This is Priya from IIT Bombay signing off. Thank you.

Python-3.4.3/C4/Handling-Errors-and-Exceptions/English-timed - Revision history

PoojaMoolya: Created page with "{|border=1 |Time |Narration |- |00:01 |Welcome to the spoken tutorial on '''Handling errors and exceptions.''' |- |00:07 |In this tutorial, you will learn to, Understand ''..."