3. Runtime Errors¶

The second type of error is a runtime error, so called because the error does not appear until you run the program. These errors are also called exceptions because they usually indicate that something exceptional (and bad) has happened.

Runtime errors are rare in the simple programs you will see in the first few chapters, so it might be a while before you encounter one. To see a runtime error in the activecode below, run it and enter “0” when prompted for input.

Check your understanding

Which of the following is a runtime error?
• Attempting to divide by 0.
• Python cannot reliably tell if you are trying to divide by 0 until it is executing your program (e.g., you might be asking the user for a value and then dividing by that value—you cannot know what value the user will enter before you run the program).
• Forgetting a colon at the end of a statement where one is required.
• This is a problem with the formal structure of the program. Python knows where colons are required and can detect when one is missing simply by looking at the code without running it.
• Forgetting to divide by 100 when printing a percentage amount.
• This will produce the wrong answer, but Python will not consider it an error at all. The programmer is the one who understands that the answer produced is wrong.