Assertions and Try/Except

Introduction to Python Exceptions Darren Jones 05:44

In this lesson, you’ll learn how assertions are made and how to handle exceptions using try and except. You’ll see that assertions follow this general pattern:

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assert (condition), "Optional message if condition not met"

When assertions fail, they raise an AssertionErrorException. The second part of the lesson shows how to handle assertion exceptions when they come up using the try and except keywords. You’ll also see how to handle other types of exceptions and how to provide useful information to the user about the errors.

00:00 Next, you’ll look at the AssertionError exception. The assert statement checks to see if the condition in the brackets is True.

00:09 If it isn’t, then an AssertionError will be raised and the program will stop. Note that it’s possible to put an optional message after the assert statement which will be printed out alongside the AssertionError.

00:22 So, instead of waiting for a program to crash, you can start by making an assertion. You can assert that a particular condition is met. If it’s True, that’s fine and the program will carry on. But if it’s False, we can have the program raise an AssertionError and stop.

00:40 Here’s a program to demonstrate that. We’re going to set the value of name to be 'quaid'. Then we’re going to assert for the name is equal to 'quaid'.

00:59 Finally, let’s print something out to show that that’s what’s happened.

01:10 Let’s run the program. You can see, as expected, because the assertion is True it didn’t have any other effect on the program and line 3 gets carried out.

01:27 But what happens if the assertion is False? Let’s change the value of name to be something else. And now let’s see what happens.

01:41 You can see an AssertionError is generated because we asserted that the name was 'quaid' and it wasn’t, so we get an AssertionError.

01:50 And just like raise, we can clarify what’s happening by adding some texts after it which will be printed out when the assertion is generated. So here I’m going to add a string which will clarify what’s happened.

02:11 I’m running the program again. You can see the assertion is clearer, name is hauser, it should be quaid. Next, you’ll look at try and except and handling exceptions. Here you see the form. The try keyword is followed by a block of code, which is attempted to run. Next is an except statement.