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In Python, you may use the raise keyword when you explicitly want to raise/throw an exception. The raise keyword is followed by specifying the type of exception that you want to raise, which is specified by either an object of exception class or the name of exception class.

By using the raise keyword, an exception can be explicitly raised from within the two places -

• A try -block or,
• An except-block.

• Syntax to use the "raise" keyword


Let us take a look at the syntax to use the raise keyword to explicitly raise an exception.



raise type-of-exception(argument, traceback)



The raise keyword has three attributes :

Exception Class	Description
type-of-exception	This is type of an exception to be raised explicitly. It could be defined with either the name of exception class or with an object of exception class, that you are raising.
argument	This gives us a string message associated with the exception which we are going to raise explicitly. This is an optional attribute and if it is not defined then its value is None i.e. no value.
traceback	This is an object which gives us the stack trace from the point of an exception handler, down the call stack, to the point where the exception was raised. This is also an optional attribute.








• Raising an exception from try block by using an object of exception class


In the upcoming example, we are going to raise an exception out of a try block by declaring an object of a built-in exception class i.e. ZeroDivisionError (you could declare an object of any built-in exception class of your choice).



#Python - Using raise keyword to raise an exception from try block by using an object of exception class


import sys

# Variables a and b are initialized with 10
a,b = 10,10 				   
try:
    
    numerator = a+b
    denominator = a-b
    if denominator==0:
        #Creating an object of type ZeroDivisionError exception
        ob=ZeroDivisionError('A number could be divided by zero')

        #raising an exception of type ZeroDivisionError through its object
        raise ob
    else:
        c = numerator/denominator		
except ZeroDivisionError as arg:           # A matching exception of type ZeroDivisionEror is declared in except block
    print("An exception caught : ",arg)    # A message related to caught exception is printed
    print(sys.exc_info())		   # This function gives the name of caught exception which was thrown by try block
print("Outside try-catch block")





Output

An exception caught :  A number could be divided by zero
(<class 'ZeroDivisionError'>, ZeroDivisionError('A number could be divided by zero'), <traceback object at 0x0000000002EF8248>)
Outside try-catch block






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• Raising an exception from try block by using the name of exception class


In the upcoming program, we are going to detect a situation which could raise the ZeroDivisionError exception and use the raise keyword to raise this exception from a try block(by using the name of an exception class). This raised exception will be caught by a matching exception declared in an except block.



#Python - Using raise keyword to raise an exception from try block by using the name of exception class


import sys

a,b = 10,10 				   # Variables a and b are initialized with 10
try:
    
    numerator = a+b
    denominator = a-b
    if denominator==0:
        raise ZeroDivisionError('A number could be divided by zero')
    else:
        c = numerator/denominator		
except ZeroDivisionError as arg:           # A matching exception of type ZeroDivisionEror is declared in except block
    print("An exception caught : ",arg)    # A message related to caught exception is printed
    print(sys.exc_info())		   # This function gives the name of caught exception which was thrown by try block
print("Outside try-catch block")





Output is -:




An exception caught :  A number could be divided by zero
(<class 'ZeroDivisionError'>, ZeroDivisionError('A number could be divided by zero'), <traceback object at 0x0000000002EF83C8>)
Outside try-catch block


As you can see in the code above, a situation is detected that could raise an exception of type ZeroDivisionError, hence this exception is explicitly raised using the raise keyword. This exception is then caught in the folliwing except block which is declared with a matching exception-type, i.e. ZeroDivisionError and the program does not end abruptly.






• Raising an already caught exception from except block


In the upcoming program, we are going to use the raise keyword to raise an exception from an except block, which has already been caught within it.

Note : We are going to raise an exception by using the name of an exception class, though we could have raised it using an object of any built-in exception class as well.



# Python - Using raise keyword to raise an exception from except block.

import sys

try:
	i = 'one'
	ab = i + 2
except TypeError as e:
    print("Exception caught - ",e)
    raise TypeError





Output -




Exception caught -  can only concatenate str (not "int") to str
Traceback (most recent call last):
  File "D:/Python Programs/exp_raise.py", line 7, in <module>
    ab = i + 2
TypeError: can only concatenate str (not "int") to str

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "D:/Python Programs/exp_raise.py", line 10, in <module>
    raise TypeError
TypeError




• In this code, an exception of type TypeError is raised from try block, when an integer value is concatenated with a string value.


• Next to try block, an except block is declared with a matching exception class i.e. TypeError, hence, the except block catches the TypeError exception and it explicitly raises it again but this time, this raised exception remains uncaught.






• Raising a different exception than the one caught in except block


In the upcoming program, we are going to use the raise keyword to raise a different exception from the one already caught in an except block.

Note : We are going to raise an exception using the object of an exception class, though we could have raised it using the name of any built-in exception class as well.





# Python - Using the raise keyword to raise a different exception from what is already caught by except block

import sys


try:
    i
except NameError as e:
	print("Exception caught - ",e)
	ob = BufferError('Raising an exception of type BufferError')
	raise ob





Output -




Exception caught -  name 'i' is not defined
Traceback (most recent call last):
  File "D:/Python Programs/exp_raise3.py", line 7, in <module>
    i
NameError: name 'i' is not defined

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "D:/Python Programs/exp_raise3.py", line 11, in <module>
    raise ob
BufferError: Raising an exception of type BufferErrorr




• In this code, an exception of type NameError is raised from try block, because we have declared a variable named i without defining it.


• Next to this try block, an except block is declared with matching Exception class, which is a superclass of every built-in exception class in Python(except system exiting exception classes). Hence, the except block catches the exception NameError but, it also raises a different exception i.e. BufferError, by using the raise keyword.






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