Debugging Skill

Objectives

Provide systematic methodology for debugging code:

• Identify bugs efficiently
• Find root causes
• Fix without side effects
• Prevent regression

Debugging Strategies

1. Binary Search Debugging

When bug location is unknown:

1. Identify range where bug exists
2. Test midpoint
3. Narrow range based on result
4. Repeat until isolated

2. Trace-Based Debugging

For flow-related bugs:

1. Add logging at key points
2. Trace execution path
3. Compare expected vs actual flow
4. Identify divergence point

3. State Inspection

For data-related bugs:

1. Capture state at key points
2. Compare with expected state
3. Identify corruption point
4. Trace data source

4. Regression Testing

When something "used to work":

1. Identify when it broke (git bisect)
2. Compare working vs broken code
3. Isolate the change
4. Understand why change broke it

Bug Categories

Logic Errors

• Off-by-one errors
• Incorrect conditionals
• Wrong operator precedence
• Missing edge cases

Data Errors

• Null/undefined references
• Type mismatches
• Invalid state transitions
• Race conditions

Integration Errors

• API contract violations
• Protocol mismatches
• Version incompatibilities
• Configuration issues

Performance Bugs

• Memory leaks
• Inefficient algorithms
• Blocking operations
• Resource exhaustion

Debugging Tools Usage

Logging

# Strategic logging
logger.debug(f"Function entry: {func.__name__}, args={args}")
logger.debug(f"State: {vars(self)}")
logger.debug(f"Exit with result: {result}")

Assertions

# Catch invalid states early
assert data is not None, "Data should not be None at this point"
assert len(items) > 0, "Items list should not be empty"

Breakpoints

• Set at suspected locations
• Conditional breakpoints for specific cases
• Watch expressions for key variables

Common Patterns

The Scientific Method

1. Observe: Gather data about the bug
2. Hypothesize: Form theory about cause
3. Predict: What would prove/disprove hypothesis
4. Test: Execute debugging action
5. Analyze: Evaluate results
6. Iterate: Refine hypothesis

Rubber Duck Debugging

1. Explain the code line by line
2. Describe expected behavior
3. Describe actual behavior
4. Identify discrepancy

Output Specification

Debugging reports should include:

• Clear problem statement
• Steps taken to investigate
• Findings and evidence
• Root cause identification
• Proposed fix
• Verification approach