Advertisement

---

Top

< Prev

Next >

In order to handle an exception raised in a Python program, we can use a try-except block, which is made up of two individual blocks - a try block and an except block.

try block


Preceded by the try keyword, a try block encloses a portion of a program in which an exception may occur during the runtime. If an exception is raised in the portion of a program within a try block, the raised exception is thrown to the associated except block.

Note: Sometimes, the try block may also be referred to as try statement.






except block

Preceded by the except keyword, an except block declares a type of exception that it is going to catch and handle. If the type of exception declared in an except block is same as the type of exception raised by its associated try block, the exception is caught within this except block and the program continues its normal execution without ending abruptly.

Note: Sometimes, the except block may also be referred to as except statement.

How to catch an exception using try-except blocks?

Exception raised has a certain type. In order to catch an exception of a specific type, we have to make sure that our except block has declared the same type of exception that is raised by its try block.


Let's understand this by an example -:

# Python - try-except block

a,b= 10,0
try:
	c=a/b   			# If a is divided by b(which has a value 0)
	print("The result of a/b is : ", c)
		
except ZeroDivisionError:        	# A matching type of exception is declared in except block
	print("An exception is caught") # A message related to caught exception is printed in except block
	
print("Outside try-except block")

Output :

An exception is caught
Outside try-except block

As you can see in the program, an exception of type ZeroDivisionError is raised when a statement enclosed within the try block is executed. This exception is caught by the except block because it is declared with a matching exception of type ZeroDivisionError and a message of our choice "An exception is caught" is printed.

As the raised exception of type ZeroDivisionError is caught by the except block, hence, the program does not end abruptly and it continues to successfully execute the statement outside the try-except blocks, which prints a message - "Outside try-catch block".

Advertisement

Except block with the "as" keyword

When declaring an except statement, we can optionally use the as keyword, which helps us declare an argument, using which we could print a useful message associated with the caught exception.

Let's understand this by an example -:

# Python - try-except blocks example

a,b= 10,0
try:
	c=a/b   			  # If a is divided by b(which has a value 0)
	print("The result of a/b is : ", c)
		
except ZeroDivisionError as arg:           # An except block is declared with the as keyword
	print("An exception caught : ",arg) # A message related to caught exception is printed
	
print("Outside try-except block") 		

Output :

An exception caught :  division by zero
Outside try-except block

• As you can see in the program above, an exception of type ZeroDivisionError is raised when a statement enclosed within the try block is executed. This exception is caught by the except block because it is declared with a matching exception of type ZeroDivisionError.


• With the except statement, we have declared an argument using the as keyword, by using this argument, a built-in message associated with the caught exception i.e. "division by zero", is printed.


• As the raised exception of type ZeroDivisionError is caught by the except block, hence, the program does not end abruptly and it continues to successfully execute the statement outside the try-except block, which prints a message - "Outside try-catch block".

Mismatch between exception raised and exception declared in except block

• If an exception is raised within a try block of an executing method, but the associated except block declares a mismatching type of exception, then the exception remains uncaught and such except block is not executed.


• Next, this uncaught exception is thrown down to the next method in the call stack. If a matching except block is found in the next method, the exception is caught, otherwise, the exception is again thrown down to the next method, down the call stack.


• The process continues until the raised exception reaches the statement in the module which called the first method (where it all began). If this statement is enclosed in a try block but there is no matching except block, the call stack explodes and the program terminates abruptly.

Let us understand this by an example here.

class TryCatch2:
	def meth(self):
		try:
			print(100/0); 	#ZeroDivisionError exception is raised here
			print("Hello from meth function")
		except IOError :	#Non-matching exception-type is declared with this except statement		
			print("We have caught an exception")
		
		print("Outside try-except blocks")

ob = TryCatch2()
try:
	ob.meth() 			#enclosing the call to meth() method in try block
except ZeroDivisionError as e:
	print("An exception is caught - ", e)

Output:

An exception is caught -  division by zero

• In the last program, an exception of type ZeroDivisionError is raised within the try block of meth() method. This raised exception is not caught by the except block defined in the meth() method, because it has declared a non-matching type of exception i.e. IOError.


• This uncaught exception is thrown down to the statement in the module which called the meth() method. This statement is enclosed within a try-except block and it does have a matching except block to catch the exception raised by meth() function of type ZeroDivisionError, hence the exception is finally caught.
  
  
  

  Note :

  
  Within the meth() function, the print statement with a message "Hello from meth function" didn't execute because the control had already exited the meth() function when the exception was raised down the call stack.








Points to remember when declaring a try-except block



• There should be no statement between a try-block and its associated except block. The except-block should begin right after the end of try-block, otherwise there will a syntax error i.e. Invalid Syntax, as shown below.



try:
    f = open("File.txt", 'w');
    f.write('Hello there from DecodeJava')
print("Statement between try-except");	#This statement is outside the try block statements as it is not indented
except Exception :
	print("An exception is caught") 





• We cannot declare a try block alone. It has to be declared with either a except block or a finally block, otherwise, there will be a syntax error i.e. Unexpected EOF while parsing.



try:
        f = open('File.txt','w')
        f.write('Hello there')



In one of our next tutorials, we will talk about the finally block(also used with the try-except block), but before that, we will talk about how we could use multiple except blocks in our next tutorial.





Please share this article -







< Prev
Next >
< Python Exception Propagation
Python multiple except Block >



Advertisement