Debugging

When to use this skill

• Encountering runtime errors or exceptions
• Code produces unexpected output or behavior
• Performance degradation or memory issues
• Intermittent or hard-to-reproduce bugs
• Understanding unfamiliar error messages
• Post-incident analysis and prevention

Instructions

Step 1: Gather Information

Collect all relevant context about the issue:

Error details:

• Full error message and stack trace
• Error type (syntax, runtime, logic, etc.)
• When did it start occurring?
• Is it reproducible?

Environment:

• Language and version
• Framework and dependencies
• OS and runtime environment
• Recent changes to code or config

# Check recent changes
git log --oneline -10
git diff HEAD~5

# Check dependency versions
npm list --depth=0  # Node.js
pip freeze          # Python

Step 2: Reproduce the Issue

Create a minimal, reproducible example:

# Bad: Vague description
"The function sometimes fails"

# Good: Specific reproduction steps
"""
1. Call process_data() with input: {"id": None}
2. Error occurs: TypeError at line 45
3. Expected: Return empty dict
4. Actual: Raises exception
"""

# Minimal reproduction
def test_reproduce_bug():
    result = process_data({"id": None})  # Fails here
    assert result == {}

Step 3: Isolate the Problem

Use binary search debugging to narrow down the issue:

Print/Log debugging:

def problematic_function(data):
    print(f"[DEBUG] Input: {data}")  # Entry point

    result = step_one(data)
    print(f"[DEBUG] After step_one: {result}")

    result = step_two(result)
    print(f"[DEBUG] After step_two: {result}")  # Issue here?

    return step_three(result)

Divide and conquer:

# Comment out half the code
# If error persists: bug is in remaining half
# If error gone: bug is in commented half
# Repeat until isolated

Step 4: Analyze Root Cause

Common bug patterns and solutions:

Pattern	Symptom	Solution
Off-by-one	Index out of bounds	Check loop bounds
Null reference	NullPointerException	Add null checks
Race condition	Intermittent failures	Add synchronization
Memory leak	Gradual slowdown	Check resource cleanup
Type mismatch	Unexpected behavior	Validate types

Questions to ask:

1. What changed recently?
2. Does it fail with specific inputs?
3. Is it environment-specific?
4. Are there any patterns in failures?

Step 5: Implement Fix

Apply the fix with proper verification:

# Before: Bug
def get_user(user_id):
    return users[user_id]  # KeyError if not found

# After: Fix with proper handling
def get_user(user_id):
    if user_id not in users:
        return None  # Or raise custom exception
    return users[user_id]

Fix checklist:

• Addresses root cause, not just symptom
• Doesn't break existing functionality
• Handles edge cases
• Includes appropriate error handling
• Has test coverage

Step 6: Verify and Prevent

Ensure the fix works and prevent regression:

# Add test for the specific bug
def test_bug_fix_issue_123():
    """Regression test for issue #123: KeyError on missing user"""
    result = get_user("nonexistent_id")
    assert result is None  # Should not raise

# Add edge case tests
@pytest.mark.parametrize("input,expected", [
    (None, None),
    ("", None),
    ("valid_id", {"name": "User"}),
])
def test_get_user_edge_cases(input, expected):
    assert get_user(input) == expected

Examples

Example 1: TypeError debugging

Error:

TypeError: cannot unpack non-iterable NoneType object
  File "app.py", line 25, in process
    name, email = get_user_info(user_id)

Analysis:

# Problem: get_user_info returns None when user not found
def get_user_info(user_id):
    user = db.find_user(user_id)
    if user:
        return user.name, user.email
    # Missing: return None case!

# Fix: Handle None case
def get_user_info(user_id):
    user = db.find_user(user_id)
    if user:
        return user.name, user.email
    return None, None  # Or raise UserNotFoundError

Example 2: Race condition debugging

Symptom: Test passes locally, fails in CI intermittently

Analysis:

# Problem: Shared state without synchronization
class Counter:
    def __init__(self):
        self.value = 0

    def increment(self):
        self.value += 1  # Not atomic!

# Fix: Add thread safety
import threading

class Counter:
    def __init__(self):
        self.value = 0
        self._lock = threading.Lock()

    def increment(self):
        with self._lock:
            self.value += 1

Example 3: Memory leak debugging

Tool: Use memory profiler

from memory_profiler import profile

@profile
def process_large_data():
    results = []
    for item in large_dataset:
        results.append(transform(item))  # Memory grows
    return results

# Fix: Use generator for large datasets
def process_large_data():
    for item in large_dataset:
        yield transform(item)  # Memory efficient

Best practices

1. Reproduce first: Never fix what you can't reproduce
2. One change at a time: Isolate variables when debugging
3. Read the error: Error messages usually point to the issue
4. Check assumptions: Verify what you think is true
5. Use version control: Easy to revert and compare changes
6. Document findings: Help future debugging efforts
7. Write tests: Prevent regression of fixed bugs

Debugging Tools

Language	Debugger	Profiler
Python	pdb, ipdb	cProfile, memory_profiler
JavaScript	Chrome DevTools	Performance tab
Java	IntelliJ Debugger	JProfiler, VisualVM
Go	Delve	pprof
Rust	rust-gdb	cargo-flamegraph

References

• Debugging: The 9 Indispensable Rules
• How to Debug
• Rubber Duck Debugging