Exceptions

CMSC 201

Overview

Exceptions are run-time errors, especially ones

that the programmer cannot predict.

example 1: division by zero

example 2: user enters "garbage" data

example 3: disk full

Vocabulary

When some piece of code causes a run-time error,

we say that the code throws an exception or that it

raises an exception.

The part of a program that deals with the run-time

error catches the exception or handles the

exception.

Divide by Zero

totalBill = 67

n = int(input("Number of people? "))

share = totalBill / n

print("Share of the bill is ", share)

If user enters 0, Python complains and terminates:

Traceback (most recent call last):

File "divide_by_zero.py", line 3, in <module>

share = totalBill / n

ZeroDivisionError: division by zero

Exception Handling

try:

totalBill = 67

n = int(input("Number of people? "))

share = totalBill / n

except ZeroDivisionError :

print("Customers ran away")

else:

print("Share of the bill is ", share)

Syntax for Exceptions

try:

block of code that might causeone or more types of exceptions

except ExceptionType1 :block of code to handle ExceptionType1

except ExceptionType2 :block of code to handle ExceptionType2

...else:

block of code to execute when no exceptions found

Exception Types

How to find exception types?

1. Read the friendly manual (RTFM)

2. Google (really, same as RTFM)

3. Make Python tell you:

Traceback (most recent call last):

File "divide_by_zero.py", line 3, in <module>

share = totalBill / n

ZeroDivisionError: division by zero

Exception Types

>>> userInput = input("Enter a number: ")

Enter a number: abc

>>> n = int(userInput)

Traceback (most recent call last):

File "<stdin>", line 1, in <module>

ValueError: invalid literal for int() with base 10: 'abc'

>>>

Exception Types

>>> userInput = input("Enter a number: ")

Enter a number: ^D

Traceback (most recent call last):

File "<stdin>", line 1, in <module>

EOFError

Badgering the user for input

done = False

while not done: try: userInput = input("Enter a number: ") n = int(userInput) except ValueError: print("That's not an integer! Try again.") else: print("Thank you!") done = True

print("n is ", n)

Badgering the user for input

done = False

while not done: try: userInput = input("Enter a number: ") n = int(userInput) except ValueError: print("That's not an integer! Try again.") else: print("Thank you!") done = True

print("n is ", n)

Badgering the user for input

done = False

while not done: try: userInput = input("Enter a number: ") n = int(userInput) except ValueError: print("That's not an integer! Try again.") except EOFError: print("Please type something! Try again.") else: print("Thank you!") done = True

print("n is ", n)

Raising an ExceptionYou can write code that raises exceptions:

try: raise ZeroDivisionError

except ZeroDivisionError: print("Did someone divide by zero?")

else: print("Everything is hunky-dory")

More useful later when we look at functions

BaseException• The BaseException type matches all

exceptions, even ones you don't know about.

• Use this very carefully! Might not be a good

idea.

• What can you do if you catch a BaseException?

o exit the program slightly more gracefully.

o return to home state (if this is possible).

o re-throw the exception (requires more syntax

and not clear what is accomplished).