Fundamentals
8 min read
Updated January 2025
Complete Guide to Code Review
Master the fundamentals of effective code review. Learn proven processes, techniques, and best practices that top development teams use to maintain code quality and knowledge sharing.
What is Code Review?
Code review is the systematic examination of code changes by peers before they're merged into the main codebase. Here's what actually happens:
Typical Review Flow
git push origin feature-branch
Developer pushes changes
Create Pull Request
Request peer review
Reviewer examines diff
Line-by-line analysis
What Reviewers Check
- Logic correctness
- Edge case handling
- Security vulnerabilities
- Performance implications
- Code style consistency
Review Outcomes
- Approve: Code is ready to merge
- Request Changes: Issues need fixing
- Comment: Suggestions, no blocking
Why Code Review Matters
Bug Prevention: Real Example
# Developer's original code:
def calculate_shipping(weight, distance):
base_rate = 5.00
weight_rate = weight * 0.5
distance_rate = distance / 100
return base_rate + weight_rate + distance_rate
Reviewer spotted: "What happens if distance is 0? Division by zero will cause an error."
# Fixed version:
distance_rate = max(distance, 1) / 100 # Minimum 1 mile
Knowledge Sharing: Learning Opportunity
Junior Developer: "I used a for loop to filter the list."
active_users = []
for user in all_users:
if user.is_active:
active_users.append(user)
Senior Reviewer: "Great logic! Python has a more concise way to do this:"
active_users = [user for user in all_users if user.is_active]
# List comprehension - more Pythonic and faster
Security: Vulnerability Caught
# Original code (vulnerable):
@app.route('/user/<user_id>')
def get_user(user_id):
query = f"SELECT * FROM users WHERE id = {user_id}"
return db.execute(query).fetchone()
Security Review: "SQL injection risk! User input goes directly into query. Use parameterized queries."
# Secure version:
query = "SELECT * FROM users WHERE id = ?"
return db.execute(query, (user_id,)).fetchone()
Effective Review Process
📝 For Code Authors
1. Write Clear PR Descriptions
Example PR Description:
What: Add user authentication to API endpoints
Why: Security requirement for v2.0 release
How: JWT tokens with 24h expiration, middleware validation
Testing: Added unit tests for auth middleware, manual testing with Postman
Risks: Breaking change - clients need to include Authorization header
2. Keep Changes Focused
❌ Bad: Too Many Changes
• Add user auth
• Refactor database layer
• Update CSS styling
• Fix unrelated bug
Files changed: 47
• Refactor database layer
• Update CSS styling
• Fix unrelated bug
Files changed: 47
✅ Good: Single Purpose
• Add JWT authentication
• Update auth middleware
• Add auth tests
Files changed: 4
• Update auth middleware
• Add auth tests
Files changed: 4
👀 For Code Reviewers
Review Checklist
🔍 Logic & Functionality
- • Does the code do what it's supposed to?
- • Are edge cases handled?
- • Will it handle unexpected input?
- • Are error conditions covered?
🛡️ Security & Performance
- • Input validation present?
- • No SQL injection risks?
- • Sensitive data encrypted?
- • Expensive operations optimized?
How to Give Good Feedback
❌ Vague Feedback
"This doesn't look right"
"Bad performance here"
"Fix this"
"Bad performance here"
"Fix this"
✅ Specific Feedback
"This function doesn't handle null input. Add validation."
"N+1 query problem. Consider using JOIN or batch loading."
"Missing error handling for API timeout scenarios."
"N+1 query problem. Consider using JOIN or batch loading."
"Missing error handling for API timeout scenarios."
Common Code Review Pitfalls
Nitpicking on Style
Problem:
Spending time on formatting instead of logic
Solution:
Use automated formatters (Prettier, Black, gofmt)
Example:
Don't review: 'Use single quotes' | Do review: 'Missing null check'
Review Fatigue
Problem:
Large PRs get rubber-stamp approvals
Solution:
Limit PR size to <400 lines of changes
Example:
Break 'User Management System' into 'Add User', 'Edit User', 'Delete User'
Lack of Context
Problem:
Reviewers don't understand the business need
Solution:
Include ticket links and business context in PR
Example:
Link to requirements doc, explain user story
Personal Preferences
Problem:
Arguments about subjective code style
Solution:
Document team standards, focus on functionality
Example:
Team style guide resolves 'map() vs for loop' debates
Tools & Workflow Integration
Platform Features
GitHub
- • Pull request templates
- • Required reviewers
- • Branch protection rules
- • Inline comments
GitLab
- • Merge request approvals
- • Approval rules
- • Code quality reports
- • Security scanning
Automation Helpers
Pre-commit Checks
# .pre-commit-config.yaml
- repo: local
hooks:
- id: tests
name: run tests
entry: pytest
language: system
CI/CD Integration
- • Automated test runs
- • Code coverage reports
- • Security scans
- • Performance benchmarks
