Debugging

Debugging Skill

This skill provides a systematic methodology for debugging software issues.

When to Use

• Application crashes or throws errors
• Unexpected behavior or wrong output
• Performance issues or slowdowns
• Intermittent failures
• Understanding unfamiliar code behavior

Debugging Methodology

1. Reproduce the Issue

Before debugging, ensure you can reliably reproduce the problem:

• Document exact steps to reproduce
• Note the environment (OS, versions, config)
• Identify if it's consistent or intermittent

2. Gather Information

Collect relevant data:

• Error messages and stack traces
• Log files
• System state at time of failure
• Recent changes to the codebase

3. Form a Hypothesis

Based on the information:

• What component is likely failing?
• What could cause this specific behavior?
• What assumptions might be wrong?

4. Test the Hypothesis

Validate your theory:

• Add logging or print statements
• Use a debugger to inspect state
• Write a minimal test case
• Check boundary conditions

5. Fix and Verify

Once you find the root cause:

• Implement the fix
• Verify the original issue is resolved
• Check for regressions
• Add tests to prevent recurrence

Common Debugging Techniques

Binary Search Debugging

When you have a large codebase or long history:

1. Find a known good state
2. Find the bad state
3. Check the middle point
4. Repeat until you isolate the change

Rubber Duck Debugging

Explain the problem out loud:

1. Describe what the code should do
2. Walk through what it actually does
3. The discrepancy often reveals the bug

Print/Log Debugging

Strategic logging:

print(f"DEBUG: variable={variable}, type={type(variable)}")
print(f"DEBUG: entering function with args={args}")
print(f"DEBUG: condition result={condition}")

Divide and Conquer

1. Comment out half the code
2. Does the problem persist?
3. If yes, bug is in remaining code
4. If no, bug is in commented code
5. Repeat

Common Bug Patterns

Questions to Ask

• What changed recently?
• Does it fail in all environments?
• What are the exact inputs that cause failure?
• What does the error message actually say?
• Have you checked the obvious things?