Exception Handling with Copilot Prompts

When writing Python code, errors (called exceptions) can happen at any time. For example, dividing by zero or trying to open a file that doesn’t exist.

Instead of letting your program crash, you can handle exceptions to make your code safer and more user-friendly.

What is an Exception?

An exception is an error that stops the normal flow of a program. Examples include:

• Dividing by zero (ZeroDivisionError)
• Trying to access an item in a list that doesn’t exist (IndexError)
• Converting a string that isn’t a number (ValueError)

Try-Except Blocks

The try-except block lets you try some code that might fail and catch the error to handle it gracefully.

Prompt

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# Write a function that handles division by zero

Copilot might suggest:

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def safe_divide(a, b):
    try:
        result = a / b
        return result
    except ZeroDivisionError:
        return "Cannot divide by zero"

Explanation

• try: Code inside this block is attempted first.
• except ZeroDivisionError: This block runs only if dividing by zero happens.
• return: Gives a clear message instead of crashing the program.

Example Usage

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print(safe_divide(10, 2))  # Output: 5.0
print(safe_divide(5, 0))   # Output: Cannot divide by zero

Why handle exceptions?

Without a try-except block, dividing by zero would stop your whole program with an error.

Finally Block

Sometimes, you want to run certain code no matter what happens, whether there was an error or not.

The finally block always runs after try-except.

Prompt

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# Show how to use finally block after try-except

Copilot might suggest:

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def read_file(filename):
    try:
        f = open(filename, 'r')
        data = f.read()
        return data
    except FileNotFoundError:
        return "File not found."
    finally:
        print("Execution completed. Closing resources if needed.")

Explanation

• try: Tries to open and read a file.
• except FileNotFoundError: Runs if the file doesn’t exist.
• finally: Runs regardless of success or failure (e.g., to close files or show a message).

Example Usage

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print(read_file("my_file.txt"))

If the file does not exist, it prints:

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Execution completed. Closing resources if needed.
File not found.

If it exists, it prints:

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Execution completed. Closing resources if needed.
<file contents>

Custom Exceptions

Sometimes, you want to define your own errors to make your code more descriptive.

Prompt

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# Create a custom exception called InvalidAgeError

Copilot might suggest:

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class InvalidAgeError(Exception):
    def __init__(self, message="Age must be between 0 and 120."):
        self.message = message
        super().__init__(self.message)

def check_age(age):
    if age < 0 or age > 120:
        raise InvalidAgeError()
    else:
        return "Valid age"

Explanation

• class InvalidAgeError(Exception): Defines a new error type that inherits from Python’s built-in Exception.
• __init__: Sets a default error message.
• raise InvalidAgeError(): Raises the error when conditions aren’t met.

Example Usage

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try:
    print(check_age(25))   # Valid age
    print(check_age(-5))   # This will raise our custom exception
except InvalidAgeError as e:
    print(e)               # Output: Age must be between 0 and 120.

Why Use Custom Exceptions?

• Makes your code more clear and specific.
• Helps other developers understand what went wrong.
• Provides better error messages to users.

Key Takeaways

• Use try-except blocks to handle errors without crashing your program.
• Use finally to run code that should execute no matter what.
• Create custom exceptions to make your code clearer and safer.
• Copilot can suggest full exception-handling code if you provide detailed prompts.

Mini Challenge

Prompt:

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# Create a function that opens a file and reads its content. If the file is missing, return "File not found." Always print "Done reading file" at the end.

Practice Exercises

1. Write a function to convert a string to an integer. If it fails, return "Invalid number."
2. Create a class NegativeNumberError and raise it if a number is negative.
3. Write a function that divides two numbers and always prints "Operation finished" whether or not an exception occurs.

Extra Tips

• Always read Copilot's suggestions carefully before accepting.
• Add comments explaining what each try-except block does.
• Start with small examples, then build up to more complex error handling.