11. EXCEPTION HANDLING Rocky K. C. Chang October 18, 2015 (Adapted from John Zelle’s slides)

Objectives To understand the idea of exception handling and be able to write simple exception handling code that catches standard Python run-time errors.

Exception Handling Various error messages can occur when executing Python programs. Such errors are called exceptions. An exception is a value (object) that is raised (“thrown”) signaling that an unexpected, or “exceptional,” situation has occurred. Python contains a predefined set of exceptions referred to as standard exceptions.

EXERCISE 11.1 Try List = [1,2,3]; List[3] file = open("unknown.py", "r") x = 3 + "4" in = 10 int("10.4") import maths

Standard exceptions in Python Source: Charles Dierbach Introduction to Computer Science Using Python. Wiley. See for all built-in exceptions

EXERCISE 11.2 Find a Python file that has functions. Try to make an error inside the function and observe the error messages reported to you.

Propagation of Raised Exceptions Source: Charles Dierbach Introduction to Computer Science Using Python. Wiley.

Propagation of Raised Exceptions An exception is either handled by the client code, or automatically propagated back to the client’s calling code, and so on, until handled. If an exception is thrown all the way back to the main module (and not handled), the program terminates displaying the details of the exception.

EXERCISE 11.3 Incur an error message from math.factorial(-1). Now try try: math.factorial(-1) except ValueError: print("Sorry, factorial() does not admit negative number.")

Catching and Handling Exceptions The try statement has the following form: try: except : Exceptions are caught and handled in Python by use of a try block and exception handler. When Python encounters a try statement, it attempts to execute the statements inside the body. If there is no error, control passes to the next statement after the try … except. Else, Python looks for an except clause with a matching error type. If one is found, the handler code is executed.

EXERCISE 11.4 Consider while True: number = eval(input("Enter a positive number: ")) if number >= 0: break # Exit loop if number is valid Use Try-Except block to catch the errors when the user is not entering a number. An error message will be printed out.

EXERCISE 11.5 Now we want to improve the codes for exercise If a user is not entering a number, in additional to the error message, he will be prompted to enter a positive integer until he enters a positive number.

EXERCISE 11.6 Now we impose a range of numbers (say from 0 to 9, inclusive) acceptable to the program in Exercise 11.5.

EXERCISE 11.7 Now you are given the function below: def getNumber(): number = eval(input("Enter a number between 0 and 9, inclusive: ")) if number 9: raise ValueError("The input must be between 0 and 9, inclusive.") return number Use this function to solve Exercise 11.6.

EXERCISE 11.8 If you have been using eval() all along, replace eval() with int(). Any difference between the error messages received here and those in the previous exercise?

EXERCISE 11.9 Replace your except handler with except ValueError as err_mesg: print(err_mesg) and use int(). Observe the difference.

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