Testing & Debugging

Types of Errors

Programs can have three main types of errors. Understanding each type helps you fix problems more efficiently.

Common Syntax Errors

• Missing colons after if, for, while, def
• Mismatched brackets or parentheses
• Incorrect indentation
• Misspelled keywords (pritn instead of print)
• Missing quotes around strings

Common Runtime Errors

• Division by zero
• Using a variable before it's defined
• Index out of range (accessing list item that doesn't exist)
• Type errors (adding string to number)
• File not found

Common Logic Errors

• Wrong operator (= instead of ==)
• Off-by-one errors in loops
• Incorrect order of operations
• Wrong condition in if statements
• Forgetting to update a variable in a loop

Testing

Testing is the process of running your program with various inputs to check if it produces the correct outputs.

Test Cases

A test case includes the input values you'll use and the expected output. Good test cases cover:

• Normal cases - Typical, expected inputs
• Boundary cases - Edge values (minimum, maximum, zero)
• Error cases - Invalid or unexpected inputs

Example: Testing a Grade Calculator

Debugging Strategies

Debugging is the process of finding and fixing errors (bugs) in your code.

1. Read the Error Message

Error messages tell you:

• The type of error
• The line number where it occurred
• A description of what went wrong

Traceback (most recent call last):
  File "program.py", line 5, in <module>
    print(x)
NameError: name 'x' is not defined

# This tells us:
# - Error is on line 5
# - It's a NameError
# - The variable 'x' doesn't exist

2. Print Debugging

Add print statements to see what's happening inside your code:

# Add prints to trace execution
def calculate_average(numbers):
    print(f"Input: {numbers}")  # Debug print

    total = 0
    for num in numbers:
        total = total + num
        print(f"Running total: {total}")  # Debug print

    average = total / len(numbers)
    print(f"Final average: {average}")  # Debug print
    return average

3. Rubber Duck Debugging

Explain your code line-by-line to someone (or a rubber duck!). Often, you'll find the problem while explaining.

4. Divide and Conquer

Comment out sections of code to isolate where the error is occurring. Narrow down until you find the problematic line.

5. Check Your Assumptions

• What type is this variable? (Use type() to check)
• What value does it actually have? (Use print())
• Is this condition actually True or False?
• Is this code even being executed?

Trace Tables

A trace table tracks how variables change as a program executes, step by step. This helps find logic errors.

Example: Finding the Bug

# This code should count down from 5 to 1
# But it has a bug!

count = 5
while count > 0:
    print(count)
    count = count - 1

Let's trace it:

The trace shows the loop works correctly, printing 5, 4, 3, 2, 1 and stopping when count becomes 0.

Debugging Checklist

1. Read the error message carefully
2. Check the line number mentioned
3. Look for common errors (spelling, brackets, indentation)
4. Add print statements to see variable values
5. Create a trace table for logic errors
6. Test with different inputs
7. Take a break and come back with fresh eyes
8. Ask someone else to look at it