fastapi-validation

# FastAPI Validation

Master FastAPI validation with Pydantic for building type-safe APIs
with comprehensive request and response validation.

## Pydantic BaseModel Fundamentals

Core Pydantic patterns with Pydantic v2.

```python
from pydantic import BaseModel, Field, ConfigDict
from typing import Optional
from datetime import datetime

# Basic model
class User(BaseModel):
    id: int
    name: str
    email: str
    created_at: datetime

# With defaults and optional fields
class UserCreate(BaseModel):
    name: str
    email: str
    age: Optional[int] = None
    is_active: bool = True

# With Field constraints
class Product(BaseModel):
    name: str = Field(..., min_length=1, max_length=100)
    price: float = Field(..., gt=0, le=1000000)
    quantity: int = Field(default=0, ge=0)
    description: Optional[str] = Field(None, max_length=500)

    model_config = ConfigDict(
        str_strip_whitespace=True,
        validate_assignment=True,
        json_schema_extra={
            'example': {
                'name': 'Widget',
                'price': 29.99,
                'quantity': 100,
                'description': 'A useful widget'
            }
        }
    )
```

## Request Body Validation

Validating complex request bodies.

```python
from fastapi import FastAPI, HTTPException
from pydantic import BaseModel, EmailStr, Field
from typing import List

app = FastAPI()

# Simple request validation
class CreateUserRequest(BaseModel):
    username: str = Field(..., min_length=3, max_length=50)
    email: EmailStr
    password: str = Field(..., min_length=8)
    age: int = Field(..., ge=13, le=120)

@app.post('/users')
async def create_user(user: CreateUserRequest):
    # user is automatically validated
    return {'username': user.username, 'email': user.email}

# Nested models
class Address(BaseModel):
    street: str
    city: str
    state: str = Field(..., min_length=2, max_length=2)
    zip_code: str = Field(..., pattern=r'^\d{5}(-\d{4})?$')

class UserPro