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What is Exception Handling in Python?

Error in Python can be of two types i.e. Syntax errors and Exceptions. Errors are the problems in a program due to which the program will stop the execution. On the other hand, exceptions are raised when some internal events occur which changes the normal flow of the program.

An exception can be defined as an unusual condition in a program resulting in the interruption in the flow of the program.

Whenever an exception occurs, the program stops the execution, and thus the further code is not executed. Therefore, an exception is the run-time errors that are unable to handle to Python script. An exception is a Python object that represents an error

Python provides a way to handle the exception so that the code can be executed without any interruption. If we do not handle the exception, the interpreter doesn't execute all the code that exists after the exception.

Python has many built-in exceptions that enable our program to run without interruption and give the output.

The difference between Syntax Error and Exceptions

Syntax Error: As the name suggests this error is caused by the wrong syntax in the code. It leads to the termination of the program.

File name : index.py

# initialize the amount variable
amount = 10000

# check that You are eligible to
# purchase Dsa Self Paced or not
if(amount>2999)
print("You are eligible to purchase Dsa Self Paced")

Exceptions:

Exceptions are raised when the program is syntactically correct, but the code resulted in an error. This error does not stop the execution of the program, however, it changes the normal flow of the program.

File name : index.py

# initialize the amount variable
marks = 10000

# perform division with 0
a = marks / 0
print(a)

Common Exceptions

Python provides the number of built-in exceptions, but here we are describing the common standard exceptions. A list of common exceptions that can be thrown from a standard Python program is given below.

ZeroDivisionError: Occurs when a number is divided by zero.
NameError: It occurs when a name is not found. It may be local or global.
IndentationError: If incorrect indentation is given.
IOError: It occurs when Input Output operation fails.
EOFError: It occurs when the end of the file is reached, and yet operations are being performed.

File name : index.py


The problem without handling exceptions

Suppose we have two variables a and b, which take the input from the user and perform the division of these values. What if the user entered the zero as the denominator? It will interrupt the program execution and through a ZeroDivision exception. Let's see the following example.

File name : index.py

a = int(input("Enter a:"))
b = int(input("Enter b:"))
c = a/b
print("a/b = %d" %c)

#other code:
print("Hi I am other part of the program")


Output:


Enter a:10
Enter b:0
Traceback (most recent call last):
File "exception-test.py", line 3, in <module>
c = a/b;
ZeroDivisionError: division by zero

Exception handling

The try block lets you test a block of code for errors.

The except block lets you handle the error.

The finally block lets you execute code, regardless of the result of the try- and except blocks.

File name : index.py

try:
#block of code

except Exception1:
#block of code

except Exception2:
#block of code

#other code

Example :-

File name : index.py

try:
a = int(input("Enter a:"))
b = int(input("Enter b:"))
c = a/b
except:
print("Can't divide with zero")


Output:


Enter a:10
Enter b:0
Can't divide with zero

File name : index.py

try :- Run this code

Except :- Run this code if exception occurs.

else :- Run this code if no exception occurs


Example

File name : index.py

try:
a = int(input("Enter a:"))
b = int(input("Enter b:"))
c = a/b
print("a/b = %d"%c)
# Using Exception with except statement. If we print(Exception) it will return exception class
except Exception:
print("can't divide by zero")
print(Exception)
else:
print("Hi I am else block")


Output:


Enter a:10
Enter b:0
can't divide by zero
<class 'Exception'>

Example

File name : index.py

try:
a = int(input("Enter a:"))
b = int(input("Enter b:"))
c = a/b;
print("a/b = %d"%c)
except:
print("can't divide by zero")
else:
print("Hi I am else block")

Example

File name : index.py

try:
a = int(input("Enter a:"))
b = int(input("Enter b:"))
c = a/b
print("a/b = %d"%c)
# Using exception object with the except statement
except Exception as e:
print("can't divide by zero")
print(e)
else:
print("Hi I am else block")



Output:



Enter a:10
Enter b:0
can't divide by zero
division by zero

Declaring Multiple Exceptions

File name : index.py

try:
#block of code

except (<Exception 1>,<Exception 2>,<Exception 3>,...<Exception n>)
#block of code

else:
#block of code

File name : index.py

try:
a=10/0;
except(ArithmeticError, IOError):
print("Arithmetic Exception")
else:
print("Successfully Done")


Output:


Arithmetic Exception

The try...finally block

File name : index.py

try:
# block of code
# this may throw an exception
finally:
# block of code
# this will always be executed

Example

File name : index.py

try:
fileptr = open("file2.txt","r")
try:
fileptr.write("Hi I am good")
finally:
fileptr.close()
print("file closed")
except:
print("Error")


Output:


file closed
Error

Raising exceptions

An exception can be raised forcefully by using the raise clause in Python. It is useful in in that scenario where we need to raise an exception to stop the execution of the program. For example, there is a program that requires 2GB memory for execution, and if the program tries to occupy 2GB of memory, then we can raise an exception to stop the execution of the program.


Points to remember

To raise an exception, the raise statement is used. The exception class name follows it.
An exception can be provided with a value that can be given in the parenthesis.
To access the value "as" keyword is used. "e" is used as a reference variable which stores the value of the exception.
We can pass the value to an exception to specify the exception type.

File name : index.py

try:
age = int(input("Enter the age:"))
if(age<18):
raise ValueError
else:
print("the age is valid")
except ValueError:
print("The age is not valid")

Output:


Enter the age:17
The age is not valid

example

File name : index.py

try:
num = int(input("Enter a positive integer: "))
if(num <= 0):
# we can pass the message in the raise statement
raise ValueError("That is a negative number!")
except ValueError as e:
print(e)


Output:


Enter a positive integer: -5
That is a negative number!

Custom Exception

The Python allows us to create our exceptions that can be raised from the program and caught using the except clause. However, we suggest you read this section after visiting the Python object and classes.

File name : index.py

class ErrorInCode(Exception):
def __init__(self, data):
self.data = data
def __str__(self):
return repr(self.data)

try:
raise ErrorInCode(2000)
except ErrorInCode as ae:
print("Received error:", ae.data)


Output:


Received error: 2000





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