Error and Exception Handling

When things go wrong

No matter how good a programmer you are, things will go wrong. A user might enter a string when you expected a number, or your code might try to open a file that doesn’t exist.

Without error handling, your program will “crash” (stop immediately and show an ugly error message). Today, we learn how to handle these mistakes gracefully.

Syntax Errors vs. Exceptions

• Syntax Errors: These are “typos” in your code. Python won’t even start running the program. (e.g., forgetting a colon).
• Exceptions: These occur while the program is running. The code is valid, but an operation failed. (e.g., dividing by zero).

The try...except Block

In Python, we handle exceptions using the try and except keywords.

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try:
    number = int(input("Enter a number: "))
    result = 10 / number
    print(f"Result is {result}")
except ZeroDivisionError:
    print("Error: You cannot divide by zero!")
except ValueError:
    print("Error: That wasn't a valid number.")

How it works:

1. Python tries to run the code in the try block.
2. If an error occurs, it jumps to the except block that matches that error type.
3. If no error occurs, the except blocks are skipped.

The finally Block

The finally block runs no matter what, whether an error occurred or not. It is typically used for “cleanup” (like closing a database connection).

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try:
    print("Connecting to database...")
    # Code that might fail
except:
    print("An error occurred.")
finally:
    print("Closing database connection.") # Always runs!

Handling “Any” Error

If you don’t know exactly what error might happen, you can use a generic except:

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try:
    # risky code
except Exception as e:
    print(f"Something went wrong: {e}")

Interactive Practice

Imagine a calculator app. If a user tries to divide by zero, instead of crashing, the app should show a friendly “Oops!” message.