Python Exception Handling

As we start writing programs, we may encounter errors which are not basically a syntax error but run time errors, which cannot be predicted easily. So, there needs to be a mechanism to handle these kinds of errors so that the program doesn’t crash during the runtime. Errors that occur during the execution are known as exceptions.

What happens when we do not handle exceptions in Python?

Let us have a look at the following example, where we take two input numbers from users and divide the first number by the second number:-

a = int(input("Enter first number: "))
b = int(input("Enter second number: "))
c = a / b
print("a/b = %d" % c)

print("Rest of the program")

So here, you can see ZeroDivisionError is the Exception being thrown. The program halts and the “Rest of the program” are not executed. To provide the user with a clearer message and make the program work after the exception too, we need to handle this with Python Exception Handling methods.

How do we handle Exception in Python?

Python provides try … except block to handling exception. The syntax for the exception handling is:-

Here the code inside try block is what we expect to produce an exception. The operations are done in the try block and if the exception occurs, the program moves to except block. Here, we can use multiple exceptions to provide an appropriate message to the user and also use else block if the exception is not caught.

Let us have an example to make it more clear on how can we handle these kind of exception in the above program:-

a = int(input("Enter first number: "))
b = int(input("Enter second number: "))
try:
    c = a / b
    print("a/b = %d" % c)
except ZeroDivisionError:
    print("can't divide by zero")
else:
    print("I am else block")

print("Rest of the program")

The output of the above program is:-

How to define the exception clause with no exceptions in Python?

We can use except block only if we want to catch all the exceptions.

The except Clause with Multiple Exceptions

The except keyword can handle multiple exceptions too using comma separated values of exceptions.

What is finally block in Python for exception handling?

You can use a finally: block along with a try: block. The finally block is a place to put any code that must execute, whether the try-block raised an exception or not.

What are the built-in exceptions in Python?

How do we raise exceptions in Python?

To raise an exception during the program execution, we use raise keyword in Python. We can pass arguments to exception using a comma. The syntax to use the raise keyword is:-

try:
    age = int(input("Enter the age: "))
    if age < 18:
        raise ValueError
    else:  
        print("The age is valid")

except ValueError:
    print("The age is not valid")

How to create a user-defined exception in Python?

We can create user-defined custom exception in Python by creating a class that extends the Exception class. Let us create a custom class for defining our exception.

class ErrorInCode(Exception):
    def __init__(self, data):
        self.data = data

    def __str__(self):
        return repr(self.data)


try:
    raise ErrorInCode(1300)
except ErrorInCode as ae:
    print("Received error:", ae.data)    

The output is:-