When an error occurs in your code, Python stops executing your code and generates an error message. In Python, an error is called an exception.
Here is a quick example to demonstrate this:
The variable doesn’t exist, so Python throws a NameError exception. Our program stops so it never prints the “Testing exceptions” message. Here is one more example:
Above, we see we get a TypeError exception because you can’t concatenate a string and an integer. Python has a list of built-in exceptions.
The two examples above deal with exceptions in our own code. We could easily fix these, preventing those exceptions in the first place. This is not always possible though. For example, when you connect with an external network device through SSH or an API, it’s possible that the connection sometimes fails. We have to take this into account when we write our code.
We can use a
try/except block. We add the code we want to run under the
try block. When it fails, the
except block handles the exception. Let me show you an example:
The variable we try to print doesn’t exist, so our code hits the
In the example above we handled our exception but it was a “catch-all”
except block. It’s also possible to deal with exceptions based on the exception error. Let me show you an example:
In the code above, we have an
except block for NameError and the general
except block. It hits our first
except block because we ran into a NameError exception.
If you want to do something when no exceptions are raised, you can add an additional
else statement. Here is an example:
try block didn’t run into exceptions and we executed the
else statement. Here is one more example:
The code above will raise an exception because the variable I specify under the
try block doesn’t exist. The
else statement doesn’t run, because we hit the
We can also run something in the end, independent from whether we hit the
try or except block. We do this with the
finally statement. Here is an example:
In the example above, we hit the
try block and we print something because of the
finally statement. Here is one more example:
This time we hit the
except block but we still run whatever code we have after the
When you create your own programs, it’s also possible to generate your own exception error messages with the
raise keyword. Here’s how:
This raises a general exception with our message. It’s better to include the exact exception though. For example:
This helps someone who uses your program to understand what went wrong.
You have now learned how to deal with exceptions in your Python code.
- When an error occurs in your code, Python generates an exception and stops executing your code.
- We can deal with exceptions with a
try/exceptblock. Python attempts to run the code under the
tryblock. When it fails, it runs the code under the
- Code under the
elseblock runs when Python successfully executed the code under the
- Code under the
finallyblock is always executed.
- You can raise your own exceptions with the
I hope you enjoyed this lesson. If you have any questions feel free to leave a comment!