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Lesson Content

Error Handling: Dealing with Problems

Errors happen in programming. Learning to handle them gracefully makes your programs more robust and user-friendly.

Common Error Types

# SyntaxError - Code syntax is wrong
# if x = 5:  # Error! Should be ==

# NameError - Variable doesn't exist
# print(undefined_var)  # Error!

# TypeError - Wrong data type
# "5" + 3  # Error! Can't add string and int

# ValueError - Wrong value
# int("hello")  # Error! Can't convert to int

# IndexError - List index out of range
# my_list = [1, 2, 3]
# print(my_list[10])  # Error!

# KeyError - Dictionary key doesn't exist
# my_dict = {"a": 1}
# print(my_dict["b"])  # Error!

The try-except Block

Handle errors gracefully:

# Basic error handling
try:
    number = int(input("Enter a number: "))
    print(f"You entered: {number}")
except ValueError:
    print("That's not a valid number!")

# Multiple exceptions
try:
    result = 10 / 0
except ZeroDivisionError:
    print("Cannot divide by zero!")
except TypeError:
    print("Wrong data type!")

try-except-else-finally

try:
    number = int(input("Enter a number: "))
except ValueError:
    print("Invalid input!")
else:
    print(f"Success! You entered: {number}")
finally:
    print("This always runs, even if there's an error")

Handling Multiple Exceptions

try:
    # Some code that might fail
    value = my_list[index] / divisor
except (IndexError, ZeroDivisionError):
    print("Either index is wrong or division by zero!")
except Exception as e:
    print(f"An error occurred: {e}")

Raising Exceptions

Create your own errors:

def check_age(age):
    if age < 0:
        raise ValueError("Age cannot be negative!")
    if age > 150:
        raise ValueError("Age seems unrealistic!")
    return age

try:
    check_age(-5)
except ValueError as e:
    print(f"Error: {e}")

Best Practices

✅ Be specific with exception types
✅ Don't catch all exceptions unless necessary
✅ Provide helpful error messages
✅ Use finally for cleanup code
✅ Log errors for debugging

Common Patterns

# Safe division
def safe_divide(a, b):
    try:
        return a / b
    except ZeroDivisionError:
        return "Cannot divide by zero"

# Safe input
def get_number(prompt):
    while True:
        try:
            return int(input(prompt))
        except ValueError:
            print("Please enter a valid number!")

Example Code

Create a safe calculator that handles errors.

# Safe calculator function
def safe_calculate(a, b, operation):
    try:
        if operation == "add":
            return a + b
        elif operation == "divide":
            return a / b
        else:
            return "Unknown operation"
    except ZeroDivisionError:
        return "Error: Cannot divide by zero"
    except Exception as e:
        return f"Error: {e}"

# Test the function
print(safe_calculate(10, 2, "divide"))
print(safe_calculate(10, 0, "divide"))

Expected Output:

5.0
Error: Cannot divide by zero

Study Tips

•Read the theory content thoroughly before practicing
•Review the example code to understand key concepts
•Proceed to the Practice tab when you're ready to code

Error Handling and Exceptions