CS50's Introduction to Programming with Python - Week on Exceptions

Instructor: David Malan

Overview of Exceptions

Definition: Exceptions refer to problems in code that arise when something goes wrong.
Syntax Errors: Errors in the code that must be fixed by the programmer; they prevent the program from running.
- Example: Forgetting to close a string literal results in a syntax error.

Example of Syntax Error

Written code: print("Hello, world" (missing closing quote)
Terminal output: SyntaxError: unterminated string literal
Correction: Close the string literal.

Runtime Errors vs Syntax Errors

Runtime Errors: Errors that occur while the program is running, often due to unexpected user input.
Defensive Programming: Write code that anticipates errors and handles them gracefully.

Example of Handling User Input

Code to prompt user for an integer:

x = int(input("What's x? "))
print(f"x is {x}")

Issue: If user inputs a non-integer (e.g., "cat"), it raises a ValueError.
Terminal output example: ValueError: invalid literal for int() with base 10: 'cat'

Handling Value Errors with Try-Except

Using try and except keywords to handle errors:

try:
    x = int(input("What's x? "))
except ValueError:
    print("x is not an integer")

Indentation is crucial; only the indented code under try is attempted.
except block executes only if an error occurs.

Improving User Experience

Use loops to continually prompt the user until valid input is provided:

while True:
    try:
        x = int(input("What's x? "))
        break
    except ValueError:
        print("x is not an integer")

Allows for repeated attempts until valid input.

Enhancements to Code Structure

Define functions to encapsulate behavior, improving reusability:

def get_int(prompt):
    while True:
        try:
            return int(input(prompt))
        except ValueError:
            print("Not an integer!")

Call function in main:

x = get_int("What's x? ")
print(f"x is {x}")

Code Refinements

Passing Parameters: Function can accept different prompts:
```
def get_int(prompt):
    # code...
```
Raising Exceptions: Programmers can define custom exceptions using raise keyword.

Conclusion

Errors are inevitable in programming, but knowing how to handle them is essential:
- Syntax Errors: Must be fixed to run the program.
- Value Errors: Handled using try-except to provide user feedback.
Understanding and using exceptions will enhance your programming skills.

Understanding Python Exceptions and Error Handling