Python Exceptions

The Exception is an object in Python that represents an error. As an error occurs in a program, Python generates an exception. To avoid crashing a program, this error can be handled.

This chapter explains how exceptions are handled by using Python try except statements.

How to raise an exception

If you feel your code can produce an error you can raise an exception in Python. In Python, raising an exception is quite simple. You can use Python raise exception statement that will break the current code execution and may return after it is handled.

Syntax of using try except in Python

Following is the general structure of Python except with try:

Try:

                Code lines to be executed

Except:

                Error handler statements

Where Python try block is the code of statements where you feel it can produce an error. It also allows to use multiple Python except statements as explained in the section below.

An exception example of Python

In the example below, we will use a very basic program with variables a, b, c to demonstrate exception handling. Where a is an int variable, b is a string and c is the sum of a and b. Let us now check how interpreter gives the output:

Output without try except block

The output will be an error as follow:

Traceback (most recent call last):

  File “C:PythonProgramshello.py”, line 3, in <module>

    c = (a + b)

TypeError: unsupported operand type(s) for +: ‘int’ and ‘str’

Example with try and except

The output will be:

An error occurred

As you can see, the error is generated in both cases, as b is a string variable. With an exception handler, the error is more descriptive and the program is not crashed. This can happen as you are taking input from the user, where you only accept numbers while user entered the strings. In that case, you can throw an error and show the user a descriptive error message without crashing the program.

try except Python with user input example

The following example takes user input to generate an exception that will be handled in the try-except block.

The output:

After running this code, the program will ask to enter two numbers. If 2 numbers are entered it will produce the sum of two. If a character is entered, an error will be generated that is handled in the error handler.

Python try except with else for multiple exceptions

You can use multiple exception handlers or Python try except blocks to manage more errors in the same program. First of all, you have to use a try block, followed by Except handler 1, except handler 2 and so on. Finally, you can also place the else statement that will execute if no error occurs.

Following is the structure of multiple exceptions handling in Python.

Try:

                Code lines to be executed

Except handler1:

                Error handler statements

Except handler 2

                Error handler statements

Else:

                Execute if no exceptions here

 

Note that, the else block will be executed if no errors are found.