Exception Handling In Python | Exceptions In Python | Python Programming Tutorial | Edureka

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Agenda
Exception Handling In Python

Agenda
Introduction 01
Why need Exception
Handling?
Getting Started 02
Concepts 03
Practical Approach 04
What are exceptions?
Looking at code to
understand theory
Process of Exception Handling

Why Need Exception Handling?

Dividing by zero
Kid
Programmer

Dividing by zero
Beautiful Error Message!
Traceback (most recent call last):
File, line 1, in <module>
ZeroDivisionError: integer division or modulo by zero

What Is Exception Handling?

What Is Exception Handling?
An exception is an event, which occurs during the execution of a program, that disrupts
the normal flow of the program's instructions
Definition of Exception
Process of responding to the occurrence, during computation, of exceptional conditions requiring
special processing – often changing the normal flow of program execution
Exception Handling
What is an exception?
What is exception handling?
Let us begin!

Everything is fixable!How?
Handle the Exception
User Finds Anomaly Python Finds Anomaly
User Made Mistake
Fixable?
Yes No
Find Error
Take Caution
Fix Error “Catch”
“Try”
If you can’t,
Python will!
I love Python!

Important Terms
Keyword used to keep the code segment under checktry
Segment to handle the exception after catching itexcept
Run this when no exceptions existelse
No matter what run this code if/if not for exceptionfinally

Visually it looks like this!

Coding In Python

Coding In Python
Python code for Exception Handling
>>> print( 0 / 0 ))
File "<stdin>", line 1
print( 0 / 0 ))
^
SyntaxError: invalid syntax
Syntax Error
>>> print( 0 / 0)
File "<stdin>", line 1, in <module>
ZeroDivisionError: integer division or modulo by zero
Exception

Raising An Exception
We can use raise to throw an exception if a condition occurs
x = 10
if x > 5:
raise Exception('x should not exceed 5. The value of x was: {}'.format(x))
File "<input>", line 4, in <module>
Exception: x should not exceed 5. The value of x was: 10

AssertionError Exception
Instead of waiting for a program to crash midway, you can also start by making an assertion in Python
import sys
assert ('linux' in sys.platform), "This code runs on Linux only."
File "<input>", line 2, in <module>
AssertionError: This code runs on Linux only.

Try and Except Block

Try And Except Block
The try and except block in Python is used to catch and handle exceptions
def linux_interaction():
assert ('linux' in sys.platform), "Function can only run on Linux systems."
print('Doing something.')
try:
linux_interaction()
except:
pass
Try it out!

The program did not crash!
try:
linux_interaction()
except:
print('Linux function was not executed')
Linux function was not executed

Function can only run on Linux systems.
The linux_interaction() function was not executed
Example where you capture the AssertionError and output message
try:
linux_interaction()
except AssertionError as error:
print(error)
print('The linux_interaction() function was not executed')

Could not open file.log
Another example where you open a file and use a built-in exception
try:
with open('file.log') as file:
read_data = file.read()
except:
print('Could not open file.log')
If file.log does not exist, this block of code will output the following:

Exception FileNotFoundError
Raised when a file or directory is requested but doesn’t exist.
Corresponds to errno ENOENT.
In the Python docs, you can see that there are a lot of built-in exceptions that you can use
try:
except FileNotFoundError as fnf_error:
print(fnf_error)
[Errno 2] No such file or directory: 'file.log'
If file.log does not exist, this block of code will output the following:

Another Example Case
try:
linux_interaction()
print(fnf_error)
print(error)
print('Linux linux_interaction() function was not executed')
If the file does not exist, running this code on a Windows machine will output the following:
Linux linux_interaction() function was not executed

Key Takeaways
• A try clause is executed up until the point where the first exception is
encountered.
• Inside the except clause, or the exception handler, you determine how the
program responds to the exception.
• You can anticipate multiple exceptions and differentiate how the program
should respond to them.
• Avoid using bare except clauses.

The else Clause

The else Clause
Using the else statement, you can instruct a program to execute a certain
block of code only in the absence of exceptions

The else Clause
Example Case
try:
linux_interaction()
print(error)
else:
print('Executing the else clause.')
Doing something.
Executing the else clause.

The else Clause
You can also try to run code inside the else clause and catch possible exceptions there as well:
try:
linux_interaction()
print(error)
else:
try:
print(fnf_error)
Doing something.

The finally Clause

The finally Clause
finally is used for cleaning up!

The finally Clause
finally is used for cleaning up!
try:
linux_interaction()
print(error)
else:
try:
print(fnf_error)
finally:
print('Cleaning up, irrespective of any exceptions.')
Cleaning up, irrespective of any exceptions.

Summary

Summary
In this session, we covered the following topics:
• raise allows you to throw an exception at any time.
• assert enables you to verify if a certain condition is met and throw an
exception if it isn’t.
• In the try clause, all statements are executed until an exception is
encountered.
• except is used to catch and handle the exception(s) that are encountered in
the try clause.
• else lets you code sections that should run only when no exceptions are
encountered in the try clause.
• finally enables you to execute sections of code that should always run, with
or without any previously encountered exceptions.

Summary
Python Programming, yay!

Exception Handling In Python | Exceptions In Python | Python Programming Tutorial | Edureka

Exception Handling In Python | Exceptions In Python | Python Programming Tutorial | Edureka

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