Nancyvarkey at 06:14, 25 October 2018

2018-10-25T06:14:48Z

Nancyvarkey at 03:36, 23 October 2018

2018-10-23T03:36:48Z

Priyacst: Created page with "'''Title of script''': '''Handling errors and exceptions''' '''Author: Arun KP''' '''Keywords: Python, Ipython, error handling, exceptions, debugging, video tutorial''' {|..."

2018-10-15T09:58:15Z

Created page with "'''Title of script''': '''Handling errors and exceptions''' '''Author: Arun KP''' '''Keywords: Python, Ipython, error handling, exceptions, debugging, video tutorial''' {|..."

New page

'''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'''

|-
| 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'''.

|-
| 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

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'''

|-
| 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

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'''

|-
| 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

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.

|-
| 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'''''

|-
| 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;"| Show Slide

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'''.

|-
| 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.'''

|-
| 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.'''

|-
| 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.

|-
| 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

|-
| 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.

|-
| 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'''.

|-
| 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;"| Lets 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'''

|-
| 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.

|-
| 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'''.

|-
| 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'''

'''except exception name:'''

'''exception handling statement(s)'''

'''else:'''

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

'''after statement(s)'''

Highlight according to narration

| 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 [https://docs.python.org/3/reference/compound_stmts.html#try try] and [https://docs.python.org/3/reference/compound_stmts.html#except except] keywords is executed.

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.

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

|-
| 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.'''

|-
| 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'''.

|-
| 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.

|-
| 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.

|-
| 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''' with the following '''code'''.

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

Save it as '''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;"| 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 an '''ZeroDivisionError''' because there is a''' division by zero error'''.

|-
| 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.'''

|-
| 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 variable 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'''.

|-
| 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

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'''.

|-
| 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

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

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

|-
| 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

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, p''ercentage '''''debug'''

|-
| 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 Forum
| style="background-color:#ffffff;border:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| Please post your timed queries in this forum.

|-
| 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 Fossee Forum
| style="background-color:#ffffff;border:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| Please post your general queries on Python in this forum.

|-
| style="background-color:#ffffff;border:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| Slide Textbook Companion
| style="background-color:#ffffff;border:0.5pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.097cm;padding-right:0.191cm;"| FOSSEE team coordinates the TBC project.

|-
| 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;"| Show Slide

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.

|-
| 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.

|}

← Older revision		Revision as of 06:14, 25 October 2018
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	Highlight ipdb>		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~~	+	\| 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

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

Nancyvarkey at 06:14, 25 October 2018

Nancyvarkey at 03:36, 23 October 2018

Priyacst: Created page with "'''Title of script''': '''Handling errors and exceptions''' '''Author: Arun KP''' '''Keywords: Python, Ipython, error handling, exceptions, debugging, video tutorial''' {|..."