Try and Except

Introduction

The try/except block lets you catch exceptions and handle them gracefully instead of letting the program crash.

Basic Syntax

try:
    # code that might raise an exception
except ExceptionType:
    # code to run if that exception occurs

Example

try:
    age = int(input("Enter your age: "))
    print(f"In 10 years you will be {age + 10}")
except ValueError:
    print("Please enter a valid number.")

Catching Multiple Exceptions

try:
    value = int(input("Enter a number: "))
    result = 100 / value
    print(result)
except ValueError:
    print("That is not a valid number.")
except ZeroDivisionError:
    print("Cannot divide by zero.")

Catching Multiple in One Except

try:
    ...
except (ValueError, TypeError):
    print("Invalid input.")

The else Clause

The else block runs only if no exception was raised.

try:
    value = int(input("Enter a number: "))
except ValueError:
    print("Not a number.")
else:
    print(f"You entered {value}")   # only runs if no exception

The finally Clause

finally always runs, whether or not an exception occurred. Use it for cleanup — closing a file, releasing a resource.

try:
    f = open("data.csv", "r")
    content = f.read()
except FileNotFoundError:
    print("File not found.")
finally:
    print("Done.")   # always runs

In practice, the with statement handles file cleanup automatically — you rarely need finally for files.

Accessing the Exception

Use as to capture the exception object and read its message.

try:
    result = int("not a number")
except ValueError as e:
    print(f"Error: {e}")   # Error: invalid literal for int() with base 10: 'not a number'

A Practical Pattern: Validating User Input

def get_integer(prompt):
    """Keep asking until the user enters a valid integer."""
    while True:
        try:
            return int(input(prompt))
        except ValueError:
            print("Invalid input. Please enter a whole number.")

age = get_integer("Enter your age: ")
print(f"Age: {age}")

Practice Exercises

• Write a function safe_divide(a, b) that returns a / b or prints a message and returns None if b is zero.
• Write a function read_file(path) that returns the file contents or a clear error message if the file does not exist.
• Use a try/except/else/finally block — print a different message in each clause and observe the output both when an error occurs and when it does not.
• Write a loop that keeps asking for a valid positive integer, rejecting anything that is not a number or is less than 1.