Try / Except

Sometimes things go wrong. The user types a word when we expect a number. We try to access a list index that doesn't exist. We divide by zero. These things are called errors or exceptions, and they crash the program

number = int("abc")

Run it. ValueError. Python can't convert "abc" to a number

names = ["Tommy", "Lance"]
print(names[10])

Run it. IndexError. There's no element at position 10

print(10 / 0)

Run it. ZeroDivisionError. Math says no

Until now, when an error happened, the program crashed and that was it. But in a real program, we don't want a crash. We want to handle the error gracefully and keep going

This is where try and except come in

try:
    number = int("abc")
    print(number)
except:
    print("That's not a valid number")

Output That's not a valid number

How it works: Python tries to run the code inside the try block. If everything goes well, it continues normally. If an error happens, instead of crashing, Python jumps to the except block and runs that code instead

try:
    age = int(input("Your age: "))
    print(f"Next year you'll be {age + 1}")
except:
    print("Please enter a number, not text!")

If the user types 18, everything works. If they type hello, we get a friendly message instead of a crash

We can be specific about which error we want to catch

try:
    number = int(input("Enter a number: "))
    result = 100 / number
    print(result)
except ValueError:
    print("That's not a number!")
except ZeroDivisionError:
    print("Can't divide by zero!")

If the user types text → ValueError is caught. If they type 0 → ZeroDivisionError is caught. Each error gets its own message. This is better than a generic except because we know exactly what went wrong

A common pattern: keep asking until the user gives valid input

while True:
    try:
        age = int(input("Your age: "))
        break
    except ValueError:
        print("That's not a number, try again")

print(f"Your age is {age}")

The while True loop keeps running. If int() succeeds, we break out of the loop. If it fails, we print a message and the loop continues. This is a pattern you'll use a lot

We can also use else (runs only if no error occurred) and finally (runs no matter what)

try:
    number = int("42")
except ValueError:
    print("Error!")
else:
    print("No errors, great!")
finally:
    print("This always runs")

Output

No errors, great!
This always runs

else and finally are optional. For now, try and except are the important ones

Mission: Sensor Cleanup

The station receives a list of sensor readings as strings, stored in a variable named readings, but some are corrupted: they aren't numbers at all. Go through the list and, using try/except, turn each one into an integer:

• if it converts, add it to a running total
• if int() raises a ValueError, count it as corrupted and move on (no crash)

At the end, print:

• Total: then the sum of the valid readings
• Corrupted: then how many readings failed

Output

Total: 162
Corrupted: 2

The valid readings are 42, 100, 7, and 13 (sum 162); x9 and bad are the two corrupted ones.