Code Review Skill

Code Review Skill

Tech Stack: Python 3.11+, pytest, AWS Lambda, Aurora MySQL

Source: Extracted from CLAUDE.md code quality principles and testing patterns.

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When to Use This Skill

Use the code-review skill when:

• ✓ Reviewing pull requests
• ✓ After implementing significant features
• ✓ Before merging to main/​dev
• ✓ Auditing legacy code for issues
• ✓ Post-incident code review

DO NOT use this skill for:

• ✗ Simple typo fixes (no review needed)
• ✗ Documentation-only changes
• ✗ Automated dependency updates (unless major version)

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Quick Review Decision Tree

What type of change?
├─ Security-sensitive? (auth, permissions, data access)
│  └─ Review SECURITY.md checklist
│
├─ Performance-critical? (Lambda, DB queries, API calls)
│  └─ Review PERFORMANCE.md checklist
│
├─ Distributed system? (Lambda, Aurora, S3, SQS, Step Functions)
│  └─ Review Boundary Verification section + execution-boundaries.md
│
├─ Error handling? (try/​catch, validation, failures)
│  └─ Review DEFENSIVE.md checklist
│
├─ Tests added/​modified?
│  └─ Check testing-workflow skill anti-patterns
│
└─ General code quality?
   └─ Review all sections

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Core Review Principles

Principle 1: Fail Fast and Visibly

From CLAUDE.md:

"Fail fast and visibly when something is wrong. Silent failures hide bugs."

Review Checklist:

# ❌ REJECT: Silent failure
def store_report(symbol, data):
    try:
        result = db.execute(query, params)
        return True  # Always returns True!
    except Exception as e:
        logger.warning(f"DB error: {e}")  # Logged at WARNING
        return True  # ❌ Still returns True!

# ✅ APPROVE: Explicit failure
def store_report(symbol, data):
    rowcount = db.execute(query, params)
    if rowcount == 0:
        logger.error(f"INSERT affected 0 rows for {symbol}")
        return False  # ✅ Explicit failure signal
    return True

Questions to Ask:

• Does this function check operation outcomes (not just exceptions)?
• Are errors logged at ERROR level (not WARNING)?
• Does the function return explicit success/​failure indicators?

Principle 2: Validate Prerequisites

From CLAUDE.md:

"Before executing a workflow node, validate that all prerequisite data exists and is non-empty."

Review Checklist:

# ❌ REJECT: Assumes upstream succeeded
def analyze_technical(state: AgentState) -> AgentState:
    result = analyzer.calculate_indicators(state['ticker_data'])
    state['indicators'] = result  # Might be {} if ticker_data was empty!
    return state

# ✅ APPROVE: Validates prerequisites
def analyze_technical(state: AgentState) -> AgentState:
    # VALIDATION GATE
    if not state.get('ticker_data') or len(state['ticker_data']) == 0:
        logger.error(f"Cannot analyze: ticker_data is empty for {state['ticker']}")
        state['error'] = "Missing ticker data"
        return state

    # Safe to proceed
    result = analyzer.calculate_indicators(state['ticker_data'])
    state['indicators'] = result
    return state

Questions to Ask:

• Does this function validate inputs before processing?
• Does it check that prerequisite data exists AND is non-empty?
• Does it fail explicitly if prerequisites are missing?

Principle 3: No Silent None Propagation

From CLAUDE.md:

"Functions that return None on failure create cascading silent failures."

Review Checklist:

# ❌ REJECT: Returns None hides failures
def fetch_ticker_data(ticker: str):
    hist = yf.download(ticker, period='1y')
    if hist is None or hist.empty:
        logger.warning(f"No data for {ticker}")
        return None  # ✗ Caller doesn't know WHY it failed

# ✅ APPROVE: Raises explicit exception
def fetch_ticker_data(ticker: str):
    hist = yf.download(ticker, period='1y')
    if hist is None or hist.empty:
        error_msg = f"No historical data for {ticker}"
        logger.error(error_msg)
        raise ValueError(error_msg)  # ✓ Explicit failure

Questions to Ask:

• Does this function return None on error?
• Would an exception be more appropriate?
• Does the caller know HOW to handle None?

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Review Checklists by Category

Security Review

See SECURITY.md for:

• Authentication/​authorization
• SQL injection prevention
• XSS prevention
• Secrets management
• Input validation

Performance Review

See PERFORMANCE.md for:

• Lambda cold start optimization
• Database query patterns
• Caching strategies
• External API efficiency

Defensive Programming Review

See DEFENSIVE.md for:

• Error handling patterns
• Validation gates
• Boundary type checking
• Silent failure detection

Boundary Verification Review

When: Code changes affect distributed systems (Lambda, Aurora, S3, SQS, Step Functions)

Checklist:

• WHERE: Execution environment identified (Lambda, EC2, local)?
• WHAT env: Environment variables verified (Terraform/​Doppler)?
• WHAT services: External systems identified (Aurora schema, S3 bucket)?
• WHAT properties: Entity configuration verified (timeout, memory, concurrency)?
• WHAT intention: Usage matches designed purpose (sync vs async)?
• Contract verification: Boundaries verified through ground truth (not just code inspection)

Common boundary failures to catch:

• ❌ Missing env var (code expects, Terraform doesn't provide)
• ❌ Schema mismatch (code INSERT vs Aurora columns)
• ❌ Permission denied (IAM role vs resource policy)
• ❌ Timeout mismatch (code needs 120s, Lambda configured 30s)
• ❌ Intention violation (sync Lambda for async workload)

Quick checks:

# Verify Lambda configuration matches code requirements
aws lambda get-function-configuration \
  --function-name <name> \
  --query '{Timeout:Timeout, Memory:MemorySize}'

# Verify Aurora schema matches code
mysql> SHOW COLUMNS FROM table_name;

# Verify IAM permissions
aws iam get-role-policy --role-name <role> --policy-name <policy>

See: Execution Boundary Checklist for comprehensive verification

Related: Principle #20 (CLAUDE.md) - Execution Boundary Discipline

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Common Code Smells

Smell 1: God Object

Symptom: Single class/​module with >500 lines doing many things.

# ❌ BAD: God object
class ReportService:
    """1200 lines - does everything!"""

    def fetch_data(self): pass
    def analyze_technical(self): pass
    def analyze_fundamental(self): pass
    def fetch_news(self): pass
    def generate_report(self): pass
    def score_report(self): pass
    def send_to_user(self): pass
    def log_to_analytics(self): pass
    # ... 20 more methods

# ✅ GOOD: Separated concerns
class DataFetcher:
    def fetch_ticker_data(self): pass

class TechnicalAnalyzer:
    def analyze(self): pass

class ReportGenerator:
    def generate(self): pass

Review Action: Request refactoring if class > 500 LOC or > 10 methods.

Smell 2: Magic Numbers

# ❌ BAD: Magic numbers
if rsi > 70:  # What does 70 mean?
    return "Overbought"

if len(price_history) < 30:  # Why 30?
    raise ValueError("Insufficient data")

# ✅ GOOD: Named constants
RSI_OVERBOUGHT_THRESHOLD = 70
MIN_PRICE_HISTORY_DAYS = 30

if rsi > RSI_OVERBOUGHT_THRESHOLD:
    return "Overbought"

if len(price_history) < MIN_PRICE_HISTORY_DAYS:
    raise ValueError(f"Need {MIN_PRICE_HISTORY_DAYS} days of data")

Smell 3: Long Parameter List

# ❌ BAD: 7 parameters
def generate_report(ticker, price, sma20, sma50, rsi, macd, volume):
    pass

# ✅ GOOD: Parameter object
from dataclasses import dataclass

@dataclass
class MarketData:
    ticker: str
    price: float
    sma20: float
    sma50: float
    rsi: float
    macd: dict
    volume: int

def generate_report(data: MarketData):
    pass

Review Action: Request refactoring if function has > 4 parameters.

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Testing Review

Anti-Pattern 1: The Liar (Can't Fail)

# ❌ REJECT: Test that can't fail
def test_store_report(self):
    mock_client = MagicMock()
    service.store_report('NVDA19', 'report')
    mock_client.execute.assert_called()  # Only checks "was it called?"

# ✅ APPROVE: Test that can actually fail
def test_store_report_detects_failure(self):
    mock_client = MagicMock()
    mock_client.execute.return_value = 0  # Simulate failure
    result = service.store_report('NVDA19', 'report')
    assert result is False

Review Question: "If I break the code, does this test fail?"

Anti-Pattern 2: Happy Path Only

# ❌ REJECT: Only success tested
def test_fetch_ticker(self):
    mock_yf.download.return_value = sample_dataframe
    result = service.fetch('NVDA')
    assert result is not None

# ✅ APPROVE: Both success AND failure
def test_fetch_ticker_success(self):
    mock_yf.download.return_value = sample_dataframe
    result = service.fetch('NVDA')
    assert len(result) > 0

def test_fetch_ticker_handles_empty(self):
    mock_yf.download.return_value = pd.DataFrame()
    with pytest.raises(ValueError):
        service.fetch('INVALID')

Review Question: "Are failure paths tested?"

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PR Review Process

Step 1: High-Level Review (5 minutes)

# Check PR description
# - What problem does this solve?
# - Why this approach?
# - Any breaking changes?

# Check files changed
gh pr diff 123

# Size check
LINES_CHANGED=$(gh pr diff 123 --stat | tail -1)
# If > 500 lines, ask to split PR

Questions:

• Is PR description clear?
• Are changes focused (not mixing features)?
• Is PR size reasonable (< 500 lines)?

Step 2: Security Review (10 minutes)

See SECURITY.md for detailed checklist.

Quick checks:

• No secrets hardcoded?
• Input validation present?
• SQL injection safe?
• XSS prevention?

Step 3: Logic Review (20 minutes)

# Read the code, ask:

# 1. Does it handle errors?
try:
    result = risky_operation()
except SpecificException as e:  # ✅ Specific exception
    logger.error(f"Failed: {e}")
    raise  # ✅ Re-raise or return False

# 2. Does it validate inputs?
if not ticker or len(ticker) > 10:
    raise ValueError("Invalid ticker")

# 3. Does it check outcomes?
rowcount = db.execute(query)
if rowcount == 0:
    return False  # Explicit failure

Step 4: Test Review (10 minutes)

# Run tests locally
gh pr checkout 123
pytest

# Check test coverage
pytest --cov=src/​module tests/​test_module.py

# Review test quality (not just quantity)

Questions:

• Are new features tested?
• Are edge cases covered?
• Do tests follow patterns (see testing-workflow skill)?

Step 5: Performance Review (5 minutes)

See PERFORMANCE.md for detailed checklist.

Quick checks:

• N+1 query pattern avoided?
• Caching appropriate?
• Lambda timeout reasonable?

Step 6: Boundary Verification (If distributed system changes)

When to apply: PR modifies Lambda, Aurora, S3, SQS, Step Functions, or cross-service integrations

Quick assessment:

# Check if PR touches distributed system boundaries
git diff main...PR-branch | grep -E "lambda|aurora|s3|sqs|stepfunctions"

If YES, verify boundaries:

• Execution environment: WHERE does modified code run?
• Environment variables: Code expectations vs Terraform reality?
• External services: Schema/​format contracts verified?
• Entity configuration: Timeout/​memory match code requirements?
• Usage intention: Sync/​async pattern matches entity design?

Example checks:

# Lambda timeout matches code execution time?
aws lambda get-function-configuration --function-name <name> \
  --query 'Timeout'
# Compare with code's longest execution path

# Aurora schema matches INSERT queries?
mysql> SHOW COLUMNS FROM table_name;
# Compare with code's INSERT/​UPDATE queries

# IAM permissions allow code's operations?
aws iam get-role-policy --role-name <role> --policy-name <policy>
# Compare with code's aws sdk calls

See: Execution Boundary Checklist for comprehensive verification

Skip if: PR only touches frontend, documentation, or single-process logic

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Approval Checklist

Before approving PR, verify ALL of these:

Correctness

• Logic is sound
• Edge cases handled
• Error handling present
• Tests pass

Security

• No hardcoded secrets
• Input validated
• SQL injection safe
• XSS safe

Performance

• No obvious bottlenecks
• Caching used appropriately
• DB queries optimized

Code Quality

• Follows project style
• Functions < 50 LOC
• Classes < 500 LOC
• Clear naming

Testing

• New code tested
• Both success and failure paths
• No test anti-patterns

Documentation

• Docstrings present
• Complex logic commented
• README updated if needed

Boundary Verification (Distributed Systems)

• Execution boundaries identified (WHERE code runs)
• Environment variables verified (Terraform/​Doppler match code)
• External service contracts verified (Aurora schema, S3 format, API payload)
• Entity configuration verified (timeout, memory, concurrency)
• Usage intention verified (sync/​async pattern matches design)
• Progressive evidence applied (verified through ground truth, not just code)

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Quick Reference

Review Time Budget

Common Rejection Reasons

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File Organization

.claude/​skills/​code-review/
├── SKILL.md          # This file (entry point)
├── SECURITY.md       # Security review checklist
├── PERFORMANCE.md    # Performance review patterns
└── DEFENSIVE.md      # Defensive programming review

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Next Steps

• For security review: See SECURITY.md
• For performance review: See PERFORMANCE.md
• For defensive review: See DEFENSIVE.md
• For test patterns: See testing-workflow skill

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References

• Google Engineering Practices: Code Review
• The Art of Readable Code
• Clean Code
• Code Review Best Practices