Foundations Updated 2026-09 View as Markdown

Exception Handling

When programming, errors happen. It’s just a fact of life. Perhaps the user gave bad input. Maybe a network resource was unavailable.

When programming, errors happen. It’s just a fact of life. Perhaps the user gave bad input. Maybe a network resource was unavailable. Maybe the program ran out of memory. Or the programmer may have even made a mistake!

Python’s solution to errors are exceptions. You might have seen an exception before.

print(a)

#error
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'a' is not defined

Oops! Forgot to assign a value to the ‘a’ variable.

But sometimes you don’t want exceptions to completely stop the program. You might want to do something special when an exception is raised. This is done in a try/except block.

Here’s a trivial example: Suppose you’re iterating over a list. You need to iterate over 20 numbers, but the list is made from user input, and might not have 20 numbers in it. After you reach the end of the list, you just want the rest of the numbers to be interpreted as a 0. Here’s how you could do that:

def do_stuff_with_number(n):
    print(n)

def catch_this():
    the_list = (1, 2, 3, 4, 5)

    for i in range(20):
        try:
            do_stuff_with_number(the_list[i])
        except IndexError: # Raised when accessing a non-existing index of a list
            do_stuff_with_number(0)

catch_this()

There, that wasn’t too hard! You can do that with any exception. For more details on handling exceptions, look no further than the Python Docs

actor = {"name": "John Cleese", "rank": "awesome"}

def get_last_name():
    return actor["name"].split()[1]

get_last_name()
print("All exceptions caught! Good job!")
print("The actor's last name is %s" % get_last_name())