Python: Exception Handling

Exceptions are errors in our program that causes Python to stop and display an error message. Exceptions are handled using the try-except-finally block.

Exception Handling

As mentioned before, we handle exceptions using the try-except-finally block. The try block is where you put the code that could potentially throw an exception. The except block is where you can gracefully perform an action if an exception was indeed thrown. Lastly, the finally block allows you to run code no matter if an exception was thrown or not.

Let's look at a very simple exception:

try:
    print(myVar)
except:
    print("An exception occurred.")

An exception occurred.

In our piece of code, the variable myVar was purposely not defined. This threw an exception which we handled inside our except block.

Notice how we didn't use a finally block because they are entirely optional. However, we could use one anyway like this:

try:
    print(myVar)
except:
    print("An exception occurred.")
finally:
    print("Moving on!")

An exception occurred.
Moving on!

Multiple Exceptions

You can handle multiple exceptions by chaining the except blocks and specifying what error you want to handle.

Here's how that looks:

try:
    print(myVar)
except OverflowError:
    print("An overflow occurred.")
except:
    print("An exception occurred.")
finally:
    print("Moving on!")

An exception occurred.
Moving on!

Else

You can use the else block to run code when no exceptions were thrown. This is similar to the finally block except that the finally block runs no matter what whereas the else block runs only when the code was exception-free.

try:
    print("Hello!")
except OverflowError:
    print("An overflow occurred.")
except:
    print("An exception occurred.")
else:
    print("Great, no errors!")
finally:
    print("Moving on!")

Hello!
Great, no errors!
Moving on!

Raising Exceptions

When the situation arises, you might want to raise an exception yourself. This is done using the raise keyword:

raise Exception("Sorry, this is an exception!")

Traceback (most recent call last):
File "C:\Users\Nilan\example.py", line 1, in <module>
raise Exception("Sorry, this is an exception!")
Exception: Sorry, this is an exception!

This is useful especially if you're writing your own module because it allows users of your module to then handle your exceptions themselves and handle it however they want.

Python Docs - Errors and Exceptions