Exception Handling in Python: Handling Exception Using Try Except

While encountering an error a program of Python gets terminated. The errors are generally problems occurring in a program that stops its execution. The errors may be due to an error in syntax or might be an exception. Whenever an incorrect statement is detected by the parser, there is an occurrence of a syntax error.

However, when the code with the correct syntax generates an error, then it’s known as an exception. Various built-in exceptions are available in python. These are raised with the occurrence of internal events where the normal flow of a program gets changed.

Therefore exceptions may be defined as certain unusual program conditions that result in an interruption of the code and hence the program flow is aborted. 

The execution of a program stops as soon as it encounters an exception. This further stops the code to execute. Hence, exceptions are errors as a result of run-time that are unable to be handled by the python script.

For exception handling in python, the scripting language python provides a solution so that the execution of the code carries on and there aren’t any interruptions. The absence of exception handling stops executing the code that exists after the code that throws an exception.

Several built-in options are available in python that allows the execution of a program without any interruption including the common exceptions. Along with this, there is a provision of python custom exceptions.  For any python program, the common exceptions that can be thrown are:

  • ZeroDivisionError: This type of exception results when zero is used to divide a number.
  • NameError: Whenever a program fails to find a name be it global or local, this type of exception occurs.
  • IndentationError: Incorrect indentation gives rise to the indentationError.
  • IOError: Failure of an Input-Output operation results in IOError.
  • EOFError: It occurs in the continuous operation of a program  even when the 

file end is reached.

Check out our data science training to upskill yourself

Raising an exception

For throwing an exception under certain conditions the raise is used.
The execution of the program is halted and the associated exception is displayed on the screen. The display of the exceptions lets the users know what might be the underlying problem.

In python through the use of the raise clause, we can raise an exception. The application is useful in cases when the program needs to be stopped by raising an exception. 

For example: Supposedly a program needs around 1GB of memory for its execution and it tries occupying 1 GB, in that case, to stop executing the program an exception can be thrown.

Syntax for raising an exception:



  • ‘raise’ is used for raising an exception in a program.
  • A value can be provided to an exception which can be provided to the parenthesis.
  • Accessing the value can be done with the keyword ”as”. The value given to the exception can be stored in the reference variable denoted by “e”.
  • For specifying an exception type, the value can be passed to an exception.


An assertion can be made in python instead of letting the program crash. An assertion is made that a specific condition is met by the program. The program will continue running if the condition is true. Else an AssertionError exception is thrown by the program when the condition turns out to be false.

Handling exceptions through the try and except block

Exceptions thrown in python are caught and then handled by the try and except blocks in python.  The code within the try block is executed normally as the program’s part.  The other block includes statements that are executed in response to the exceptions thrown by the program in the try block.

The program throws an exception whenever it encounters an error in a syntactically correct code. If the exceptions thrown are not handled properly there will be a crash in the programs. In such scenarios, the except block determines the response of the program to that exception.

The application of the try and except clause will be best understood through the following example taken from the mentioned source.

In this case, whenever there is an occurrence of an exception, the program will continue running and will inform the user that the program was not successful rather than giving a blank output.

Checkout: Python Project Ideas & Topics

The program showed the error type that was thrown through the call of the function. However, the error thrown by the function can be caught to get an idea of what actually went wrong.

Running the code in a windows machine will generate the following

The message displayed first indicates an AssertionError  through which the user is informed that the function can be executed only on a system of Linux operating system. The second message further gives the information of which function was not able to get executed.
Non existence if the file.log will generate an output with the message “Could not open file.log”. 

Explore our Popular Data Science Online Certifications

The program will still continue running as it is a message that relays the information to the user. A lot of built-in exceptions are available in Python docs. One exception as described is shown below.

The non-existence of the file.log in this case will generate the following output.

Various exceptions can be caught through the use of more function calls in the try clause. However, on detecting an exception, the try clause statements will stop.

Top Data Science Skills You Should Learn

The else clause 

Use of the else clause of statements can aid in instructing the program for executing a block of statements only when there are no exceptions. 

The else clause got executed only because there were no exceptions. If the code contains exceptions, then the following will result.

upGrad’s Exclusive Data Science Webinar for you –

Watch our Webinar on The Future of Consumer Data in an Open Data Economy


Python custom exceptions

Python has a number of built-in exceptions that throw an error whenever there is something wrong in the program. However, the user needs to create some customized exceptions in cases where his purpose is to be served.

A new class can be created for defining the custom exceptions. Either directly or indirectly, these classes have to be derived from the class of built-in exceptions.

The user-defined class CustomError is created that is inherited from the class Exception. Similar to the other exceptions, these exceptions too are raised through the use of ‘raise’ with an error message that is optional.

The user-defined exceptions should be placed in a separate file whenever there is a development of a large python program. It is generally a good practice to do this and is followed by most of the standard modules where the exceptions are defined separately as or

The python custom exceptions are simple and as the normal classes implement everything they too follow the same. 

Syntax and Examples

Let us consider two variables which are a, and b. The input to the variables is taken from the user and the division of the numbers is performed. Now, if the denominator entered by the user is a zero.  

For handling these exceptions, the try-except blocks can be added to the program. For any type of code that leads to a suspicion of throwing exceptions. It should be placed in the try block of statements. 

Read our popular Data Science Articles

Syntax of a try block



except Exception1:    


except Exception2:    



It shows the use of try-except statements where the code is placed in the try block and gets executed when there are no exceptions in the code. 

Syntax of else statement with try-except


#try statements

Except Exception1



#execution of code when there is no exception

A few important points:

  • The exception is not to be specified through a statement of exception in python.
  • Multiple exceptions can be declared in a code through the use of a try block as there can be many statements inside a try block that can throw exceptions of different types.
  •  An else block can be specified with a try block which gets executed when there are no exceptions thrown by the try block.
  • The else block should contain statements that are not responsible for throwing exceptions.

Also Read: Top Reasons Why Python is Popular with Developers


In this article, we briefly discussed the concept of exception handling in python with some examples. Along with the built-in exceptions, the python custom exceptions are also defined briefly. Now, you can know the importance of exceptions and the handling of exceptions in python

If you are interested in learning more about the python language and its implementation in data science, you can check out the following course of upGrad “Executive PG Programme in Data Science”.

The online course is designed for all entry-level professionals who fall within the age group of 21 to 45 years of age. With over 20+ live sessions and practical knowledge of 14+ tools and programming languages, the course will guide you towards perfectness. Any queries could be messaged. Our team will help you.

How many types of errors are there in Python?

There are predominantly 2 distinguishable categories of errors in Python- Syntax Errors and Exceptions. Syntax Errors: Syntax errors are caused when any line of code has a faulty syntax. This is the most common error and is detected during the parsing of the program. The errors include missing an operator, improper indentation, keyword misspell, missing colon, leaving brackets, and other minute errors in the syntax. Exceptions: Exceptions are the errors that are detected during the execution of the program. After the program succeeds in the syntax test, it goes through a check of logical errors. Python has a rich collection of built-in exceptions. You can also create custom user-defined exceptions.

What is the key difference between a syntax error and an exception?

A syntax error occurs when the code or a line of code is syntactically wrong. When a syntax error occurs in a code, the program terminates. A syntax error could occur by missing a colon or even misspelling a keyword as well.
However, exceptions are quite different. An exception can be considered as an anomaly that disrupts the flow of the program. Even if the program is syntactically correct, the exception may occur. Unlike in an error, the code executes up to the line where the exception occurs.

What is the role of the raise keyword in Python?

The raise keyword is used for raising an exception in Python. The type of error can also be raised along and you can also add text to be displayed at the time of exception raising. It comes out very handy when you need to check your code for inputs.

Want to share this article?

Prepare for a Career of the Future

Leave a comment

Your email address will not be published. Required fields are marked *

Leave a comment

Your email address will not be published. Required fields are marked *

Get Free career counselling from upGrad experts!
Book a session with an industry professional today!
No Thanks
Let's do it
Get Free career counselling from upGrad experts!
Book a Session with an industry professional today!
Let's do it
No Thanks