Exception handling with try and except

try and except (spec 7.1.1)

Code that might fail goes in a try block. If an exception happens, Python jumps to a matching except block instead of crashing. Name the exception you expect; a bare except: catches everything, which can hide bugs.

for text in ["12", "twelve"]:
    try:
        n = int(text)
        print("Number", n)
    except ValueError:
        print("Not a whole number:", text)

Output:

Number 12
Not a whole number: twelve

In-built exceptions you must know

The Programming Language Subset lists these:

Several except blocks

Python checks except blocks in order and runs the first one that matches. Put specific exceptions before general ones such as Exception.

def safe_divide(a, b):
    try:
        return a / b
    except ZeroDivisionError:
        return "cannot divide by zero"
    except TypeError:
        return "numbers only"

print(safe_divide(9, 3))
print(safe_divide(9, 0))
print(safe_divide(9, "3"))

Output:

3.0
cannot divide by zero
numbers only

Robust input

Combine a while loop with try/except to keep asking until the user types a valid number:

valid = False
while not valid:
    try:
        qty = int(input("Quantity: "))
        valid = True
    except ValueError:
        print("Please type a whole number")

Worked example

Which exception does each line raise? (a) [10, 20, 30][3] (b) {"a": 1}["b"] (c) int("3.5")

1. (a) index 3 does not exist in a 3-item list: IndexError
2. (b) the key "b" is missing: KeyError
3. (c) "3.5" is a string but not a valid int: ValueError

Answer: IndexError, KeyError, ValueError

Check yourself

1. Which exception is raised when this code runs? ```python scores = [4, 8, 15] print(scores[3]) ```
   • KeyError
   • IndexError
   • ValueError
   • TypeError
   Show answer

   IndexError. Indexes run 0 to 2, so scores[3] raises IndexError.

2. Which exception is raised when this code runs? ```python total = 5 print(totl) ```
   • ValueError
   • KeyError
   • SyntaxError
   • NameError
   Show answer

   NameError. totl was never assigned, so Python raises NameError. Typos in identifiers usually cause this.

3. Which exception is raised when this code runs? ```python count = 0 def add_one(): count = count + 1 add_one() ```
   • NameError
   • UnboundLocalError
   • TypeError
   • IndexError
   Show answer

   UnboundLocalError. Assigning to count inside the function makes it local, but it is read before it has a local value, so Python raises UnboundLocalError.

4. What is displayed when this code runs? Type each line of output in order, separated by spaces. ```python try: x = 10 / 0 print("A") except ZeroDivisionError: print("B") print("C") ```
   Show answer

   B C. The division fails, so print("A") is skipped, the except block prints B, then the program continues with C: B, then C.

5. What is displayed when this code runs? ```python try: value = int("4x") except ZeroDivisionError: print("zero") except ValueError: print("value") except Exception: print("other") ```
   Show answer

   value. int("4x") raises ValueError; the first matching except is the ValueError one, so value is displayed.

6. What is displayed when this code runs? ```python try: n = int("42") print(n + 1) except ValueError: print("bad") ```
   Show answer

   43. No exception happens, so the except block is skipped and 43 is displayed.

7. This program should display 'missing' when the key is not in the dictionary. Which line completes it? ```python stock = {"pen": 4, "ruler": 2} try: print(stock["glue"]) __________ print("missing") ```
   • `except KeyError:`
   • `except IndexError:`
   • `except ValueError:`
   • `except NameError:`
   Show answer

   `except KeyError:`. Looking up a missing dictionary key raises KeyError.

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Sources

• Pearson Edexcel International Advanced Subsidiary/Advanced Level in Computer Science specification (XCP01/YCP01), first teaching 2026
• Pearson IAL Computer Science Unit 2 Programming Language Subset (PLS), Python 3
• Pearson IAL Computer Science IAS sample assessment materials (Unit 2 paper and mark scheme)

Last updated 8 November 2026. Spotted a mistake? Tell us.