try, except, else, finally

Big idea

try / except catches an exception so your program can handle it instead of crashing. else runs when nothing went wrong; finally runs no matter what.

The idea

try:
    # code that might fail
except SomeError:
    # what to do if it does

text = input()
try:
    number = int(text)
    print("Double is", number * 2)
except ValueError:
    print(text, "isn't a whole number")
print("Still running")

input
abc

output
abc isn't a whole number
Still running

Change the input to 21 and run again. When the try block succeeds, the except block is skipped.

Catch the specific error

Name the exception you expect. You can have several except blocks, and as e gives you the exception's message:

for a, b in [(10, 2), (5, 0), (7, "x")]:
    try:
        print(a / b)
    except ZeroDivisionError:
        print("can't divide by zero")
    except TypeError as e:
        print("type problem:", e)

5.0
can't divide by zero
type problem: unsupported operand type(s) for /: 'int' and 'str'

A bare except: (with no type) catches everything, including typos in your own code — which hides bugs. Always name what you expect.

else and finally

def read_age(text):
    try:
        age = int(text)
    except ValueError:
        print("Not a number")
    else:
        print("Age is", age)
    finally:
        print("Checked", repr(text))

read_age("42")
read_age("forty")

output
Age is 42
Checked '42'
Not a number
Checked 'forty'

• else — runs only if the try block raised nothing. Keep the risky part in try and the "it worked" part in else.
• finally — always runs: success, failure, even a return. Used for cleanup, like closing a file.

Ask again until it's right

A common pattern: keep asking until the input is valid.

while True:
    try:
        n = int(input())
    except ValueError:
        print("Try again")
print("Got", n)

input
ten
10

output
Try again
Got 10

When int(...) succeeds nothing leaves the loop, so it reads past the end of the input. Add break after n = int(input()) (or in an else: block).

Watch it

Think of it like

A trapeze act with a safety net. try is the jump. except is the net — it catches you if you fall. finally is taking a bow at the end, whatever happened.

Key terms

• try / except — Run code, and handle a named exception if it's raised.
• else — Runs when the try block raised nothing.
• finally — Always runs; used for cleanup.