Introduction to Programming

Exceptions and errors

Exceptions and errors are unexpected events in a program. Here are some errors you are already familiar with:

3+y = x

for in range(5):
print("hello") # tab missing at start of line

print(x) # x not initialized

s = 'hello' + 3
# perhaps the programmer meant 'hello' + str(3)

int("hello") # Can't convert this string to an int

a_list = [1, 2]
a_list[5] = 9

open('no_such_file.txt','r')

Run-time errors can be detected and processed as part of the program. This means that you can write code to address the error and recover from it gracefully, rather than the program just crashing. This is done with a try-except code block. Place the code that has the chance of producing a runtime exception within the try part of a try-except block, then process the potential exception in the except block. The except block is only executed in case of the runtime exception named. Here's an example of how to process a user input error:

try:
    age = int(input("Input your age: "))
except ValueError:
    print('Error: Age must be a whole number')

Here's a sample output showing with the user sees:

Input your age: Sue
Error: Age must be a whole number

In the above code, instead of the program crashing with the ValueError, the error is caught by the try block and handled by the except block. It is common to put a try-except block within a loop, giving the opportunity for the code to repeat execution until no error occurs.

age = None # initialize age to an invalid value
# Keep trying to get valid input until finally
# age is a valid integer
while age is None:
    try:
        age = int(input("Input your age: "))
    except ValueError:
        print('Error: Age must be a whole number.')
print(f'Your age is {age}'

Sample output:

Input your age: Sue
Error: Age must be a whole number.
Input your age: 27.5
Error: Age must be a whole number.
Input your age: 27
Your age is 27

It is very common practice to put opening of a file within a try-except block.

file_name = input("What is the name of the file? ")
try:
    with open(file_name) as a_file:
        # code to process contents of file
except FileNotFoundError:
    print(f'Error: could not open "{file_name}"')

Best practices for when to handle exceptions

Catching errors during standard running of a progam is good programming practice, since unhandled exceptions cause the program to crash.

From now on, you should be using try-except blocks to handle errors that legitimately occur during the running of programs, in particular to respond to invalid input entered by the user. However, do not use try-except blocks to mask errors in your own code. For example, if your program has a bug in which you traverse a list past its end and crash with an IndexError, never stop your program from crashing by wrapping the incorrect lines of code within a try-except block. Instead always fix the underlying bug.