Handling Exceptions

Exceptions can be handled by three ways

• Handling a raised Exception
• Raise Exception
• Use of assert to handle Exception

Handling a raised Exception

The try and except block in python is used to catch and handle exceptions. Python executes the code following try statement as a normal part of the program. The code that follows the except statement is he program’s response to any exceptions in the preceding try clause

Working of try and except

try:
	run this code
except:
	Execute this code when there is an exception

• When syntactically correct code runs into an error, Python will throw an exception error
• The exception error will crash the program if it is unhandled.
• The except clause determines how your program responds to exceptions

Use of try and except

Syntax

try:
	# Block of code that causes exception
except:
	#Execute this code when there is an exception in try block

Example

try:
	print(x)
except:
	print("An exception occured")

Output

Description

Since x is not defined or it does not have any value assigned to it, the statement in try block raises an exception. Since try block gives an error, except block is executed

Multiple exceptions (more than one except block)

Many exceptions block can be given to handle different type of errors

Syntax

try:
	# Block of code that causes exception
except <Error 1>:
	# Block of code if the statement in try produces Error1
except <Error 2>:
	# Block of code if the statement in try produces Error 2
….
except <Error n>:
	# Block of code if the statement in try produces Error n

except:
	#Execute this code when there is an exception in try block

Example

try:
	x = '2' + 2
except NameError:
	print("Variable z is not defined")
except TypeError:
    print("TypeError")

Output

Description

Since x is assigned with adding string and integer it leads to error. The error in try block is TypeError which raises an exception and thus the except block intended for TypeError is executed

Use of try, except, else

Syntax

try:
	# Block of code that causes exception
except <Error 1>:
	# Block of code if the statement in try produces Error1
except <Error 2>:
	# Block of code if the statement in try produces Error 2
….
except <Error n>:
	# Block of code if the statement in try produces Error n
else:
	#block of code to be executed when no error is raised by try block 

except:
	#Execute this code when there is an exception in try block

Example

try:
	print("I like Python")
except NameError:
	print("Variable z is not defined")
except TypeError:
    print("TypeError")
else:
	print("Execute when no error in try block")

Output

Description

When the try block does not produce any error, then else part will be executed.

Use of try, except, finally

The finally block will be executed regardless of try block raises an exception or not. This can be useful to close objects and clean resources

Syntax

try:
	# Block of code that causes exception
except <Error 1>:
	# Block of code if the statement in try produces Error1
except <Error 2>:
	# Block of code if the statement in try produces Error 2
….
except <Error n>:
	# Block of code if the statement in try produces Error n
else:
	#block of code to be executed when no error is raised by try block 
finally:
	#block of code that executes always regardless of whether try block raises an exception or not

except:
	#Execute this code when there is an exception in try block

Example

try:
	print(z)
except NameError:
	print("Variable z is not defined")
except TypeError:
    print("TypeError")
else:
	print("Execute when no error in try block")
finally:
	print("I ll execute always regardless of try block raises an exception or not")

Output

Description

When the try block does not produce any error, then else part will be executed.

Regardless of try block produces an error or not, always finally will be executed. Here try block produces name error as z is undefined, then also the statement in finally will be executed

Raising an Exception

An expression can be raised is a condition occurs. Keyword raise is used to define the exception. Followed by raise specify error type and text to show to the user

Syntax

if (condition):
	raise <Error Type>

Example

mark1 = int(input("Enter mark:"))
if(mark1 < 0 or mark1 > 100):
	raise ValueError("Mark is invalid")
else:
	print("Mark is valid")

Output 1

Output 2

Use of assert to handle Exception

• An assertion is like a checkpoint in a program that can be turned on or off while testing the program
• Assertions are carried out by assert statement
• When an assert statement is executed, python evaluates the accompanying expression which is true. If the expression is evaluated to false then python raises an AssertionError exception

Syntax

assert Expression[,Arguments]

• If the assertion fails, Python uses argument as the AssertionError.
• AssertionError exceptions can be caught and handled like any other exception using try – except statement but if not handled they will terminate the program and produce a traceback

Example

mark1 = int(input("Enter mark:"))
assert(mark1 >= 0 and mark1 <= 100), "Mark is Invalid"

Output

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