FastAPI Custom Validators

A FastAPI project demonstrating advanced Pydantic validation techniques including field validators, model validators, and cross-field validation rules.

🎯 Project Overview

This project showcases professional validation patterns in FastAPI using Pydantic's validation features. Built as part of my journey to becoming a Python developer, this API implements comprehensive validation for a Todo management system.

✨ Features

• Field Validation: String length constraints, numeric ranges
• Custom Validators: Whitespace trimming, empty string handling
• Cross-Field Validation: Business rules across multiple fields
• Proper HTTP Status Codes: 201, 404, 422 responses
• Comprehensive Testing: 100% test coverage with pytest
• Automatic API Documentation: Interactive docs at /docs

🛠️ Technologies Used

• FastAPI: Modern Python web framework
• Pydantic: Data validation using Python type hints
• Uvicorn: ASGI server
• pytest: Testing framework
• Python 3.10+

📋 Validation Rules Implemented

Field-Level Validation

• Title:
  • Required field
  • 1-100 characters
  • Cannot be only whitespace
  • Automatically trimmed
• Description:
  • Optional field
  • Max 500 characters
  • Empty strings converted to None
  • Automatically trimmed
• Priority:
  • Required integer
  • Must be between 1-5

Model-Level Validation

• Urgent Todos: Priority 5 (urgent) todos must have a description

🚀 Installation & Setup

Prerequisites

• Python 3.10 or higher
• pip

Install Dependencies

# Clone the repository
git clone https://github.com/YOUR_USERNAME/fastapi-custom-validators.git
cd fastapi-custom-validators

# Install required packages
pip install fastapi uvicorn pytest

💻 Running the Application

# Start the server
uvicorn fastapi_custom_validators:app --reload

The API will be available at:

• API: http://127.0.0.1:8000
• Interactive docs: http://127.0.0.1:8000/docs
• Alternative docs: http://127.0.0.1:8000/redoc

🧪 Running Tests

# Run all tests
pytest test_fastapi_custom_validators.py -v

# Run with coverage
pytest test_fastapi_custom_validators.py -v --cov

📡 API Endpoints

Create Todo

POST /todos
Content-Type: application/json

{
  "title": "Buy groceries",
  "description": "Milk, eggs, bread",
  "priority": 3
}

Response (201 Created):

{
  "id": 1,
  "title": "Buy groceries",
  "description": "Milk, eggs, bread",
  "priority": 3,
  "completed": false
}

Get All Todos

GET /todos

Response (200 OK):

[
  {
    "id": 1,
    "title": "Buy groceries",
    "description": "Milk, eggs, bread",
    "priority": 3,
    "completed": false
  }
]

Get Single Todo

GET /todos/{todo_id}

Response (200 OK):

{
  "id": 1,
  "title": "Buy groceries",
  "description": "Milk, eggs, bread",
  "priority": 3,
  "completed": false
}

Response (404 Not Found):

{
  "detail": "Todo not found"
}

⚠️ Validation Examples

Valid Requests ✅

// Minimal valid todo
{"title": "Buy milk", "priority": 3}

// With description
{"title": "Fix bug", "description": "Critical production issue", "priority": 5}

// Whitespace is trimmed
{"title": "  Task  ", "priority": 2}
// Saved as: "Task"

Invalid Requests ❌

// Empty title (422 Error)
{"title": "", "priority": 3}

// Whitespace-only title (422 Error)
{"title": "   ", "priority": 3}

// Title too long (422 Error)
{"title": "x".repeat(101), "priority": 3}

// Priority out of range (422 Error)
{"title": "Task", "priority": 6}

// Urgent todo without description (422 Error)
{"title": "Critical task", "priority": 5}
// Error: "Urgent todos must have a description"

📚 What I Learned

• Pydantic Field Constraints: Using Field() for min/max length, numeric ranges
• Custom Field Validators: @field_validator for custom validation logic
• Model Validators: @model_validator for cross-field business rules
• Proper Error Handling: HTTPException with appropriate status codes
• Professional Testing: Comprehensive test coverage including edge cases
• API Documentation: Automatic OpenAPI/Swagger documentation

🔄 Project Evolution

This project is part of my Python learning roadmap:

• Day 91: Basic FastAPI application (fastapi-hello-api)
• Day 92: CRUD operations (fastapi-todo-crud)
• Day 93: Advanced validation (this project) ⭐

📝 Future Enhancements

• Database integration (SQLite/PostgreSQL)
• User authentication with JWT
• Update and Delete endpoints
• Pagination for GET /todos
• Filtering and sorting
• Deployment to production

🤝 Contributing

This is a learning project, but feedback and suggestions are welcome! Feel free to open an issue or submit a pull request.

📄 License

This project is open source and available under the MIT License.

👨‍💻 Author

Your Name

• GitHub: @jandaghi14
• LinkedIn: [ali-jandaghi-9a3188b1]
• Learning Journey: Python Developer Roadmap (Days 1-180)

🙏 Acknowledgments

Built as part of my self-taught Python developer journey. Special focus on professional coding practices, testing, and API design patterns.

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Day 93 of 180 - Advanced Pydantic Validation ✅