Fastapi API Patterns

fastapi-api-patterns

Instructions

This skill provides comprehensive REST API design patterns and implementation templates for FastAPI applications. It covers CRUD operations, pagination, filtering, request/response models, error handling, and API organization following modern best practices.

1. CRUD Endpoint Patterns

Create, Read, Update, Delete endpoints using FastAPI routers:

# Use CRUD template to generate complete endpoint set
cp ./skills/fastapi-api-patterns/templates/crud_endpoint.py app/routers/items.py

# Customize for your model
# - Replace Item model with your Pydantic model
# - Update database operations
# - Add authentication dependencies

What This Provides:

• POST /items/ - Create new item
• GET /items/{item_id} - Read single item by ID
• GET /items/ - Read multiple items with pagination
• PUT /items/{item_id} - Update entire item
• PATCH /items/{item_id} - Partial update
• DELETE /items/{item_id} - Delete item

Router Structure:

from fastapi import APIRouter, HTTPException, Depends, status
from typing import List

router = APIRouter(
    prefix="/items",
    tags=["items"],
    responses={404: {"description": "Not found"}},
)

2. Pagination and Filtering

Implement pagination with query parameters:

# Use pagination template
cp ./skills/fastapi-api-patterns/templates/pagination.py app/utils/pagination.py

Pagination Strategies:

1. Offset-Based Pagination (Simple):

@router.get("/items/")
async def list_items(skip: int = 0, limit: int = 10):
    return items[skip : skip + limit]

2. Cursor-Based Pagination (Performance):

@router.get("/items/")
async def list_items(cursor: str | None = None, limit: int = 10):
    # Use last item ID as cursor for next page
    # Better for large datasets

3. Page-Based Pagination (User-Friendly):

@router.get("/items/")
async def list_items(page: int = 1, page_size: int = 10):
    skip = (page - 1) * page_size
    return items[skip : skip + page_size]

Filtering Patterns:

@router.get("/items/")
async def list_items(
    skip: int = 0,
    limit: int = 10,
    category: str | None = None,
    min_price: float | None = None,
    max_price: float | None = None,
    search: str | None = None,
):
    # Apply filters before pagination
    filtered_items = apply_filters(items, category, min_price, max_price, search)
    return filtered_items[skip : skip + limit]

Sorting:

from enum import Enum

class SortBy(str, Enum):
    name = "name"
    price = "price"
    created_at = "created_at"

@router.get("/items/")
async def list_items(
    sort_by: SortBy = SortBy.created_at,
    order: Literal["asc", "desc"] = "desc",
):
    # Sort before returning

3. Request and Response Models

Define clear Pydantic models for type safety and validation:

Base Models:

from pydantic import BaseModel, Field, validator
from datetime import datetime

class ItemBase(BaseModel):
    """Shared properties"""
    name: str = Field(..., min_length=1, max_length=100)
    description: str | None = Field(None, max_length=500)
    price: float = Field(..., gt=0)
    category: str

class ItemCreate(ItemBase):
    """Properties required for creation"""
    pass

class ItemUpdate(BaseModel):
    """Properties that can be updated"""
    name: str | None = None
    description: str | None = None
    price: float | None = Field(None, gt=0)
    category: str | None = None

class ItemInDB(ItemBase):
    """Properties stored in database"""
    id: int
    created_at: datetime
    updated_at: datetime

class Item(ItemInDB):
    """Properties returned to client"""
    class Config:
        from_attributes = True

Response Models with Metadata:

from typing import Generic, TypeVar, List
from pydantic import BaseModel

T = TypeVar('T')

class PaginatedResponse(BaseModel, Generic[T]):
    items: List[T]
    total: int
    page: int
    page_size: int
    pages: int

@router.get("/items/", response_model=PaginatedResponse[Item])
async def list_items(page: int = 1, page_size: int = 10):
    total = len(items)
    pages = (total + page_size - 1) // page_size
    skip = (page - 1) * page_size

    return PaginatedResponse(
        items=items[skip : skip + page_size],
        total=total,
        page=page,
        page_size=page_size,
        pages=pages,
    )

4. Error Handling Strategies

Implement consistent error handling:

# Use error handling template
cp ./skills/fastapi-api-patterns/templates/error_handling.py app/utils/errors.py

HTTP Exception Patterns:

from fastapi import HTTPException, status

# 404 Not Found
if item is None:
    raise HTTPException(
        status_code=status.HTTP_404_NOT_FOUND,
        detail=f"Item with id {item_id} not found"
    )

# 400 Bad Request
if price < 0:
    raise HTTPException(
        status_code=status.HTTP_400_BAD_REQUEST,
        detail="Price must be positive"
    )

# 409 Conflict
if item_exists:
    raise HTTPException(
        status_code=status.HTTP_409_CONFLICT,
        detail="Item with this name already exists"
    )

# 403 Forbidden
if not is_owner:
    raise HTTPException(
        status_code=status.HTTP_403_FORBIDDEN,
        detail="Not authorized to modify this item"
    )

Custom Exception Handlers:

from fastapi import Request
from fastapi.responses import JSONResponse

class ItemNotFoundError(Exception):
    def __init__(self, item_id: int):
        self.item_id = item_id

@app.exception_handler(ItemNotFoundError)
async def item_not_found_handler(request: Request, exc: ItemNotFoundError):
    return JSONResponse(
        status_code=404,
        content={
            "error": "not_found",
            "message": f"Item {exc.item_id} not found",
            "item_id": exc.item_id
        }
    )

Validation Error Customization:

from fastapi.exceptions import RequestValidationError
from fastapi.responses import JSONResponse

@app.exception_handler(RequestValidationError)
async def validation_exception_handler(request: Request, exc: RequestValidationError):
    return JSONResponse(
        status_code=422,
        content={
            "error": "validation_error",
            "message": "Invalid request data",
            "details": exc.errors()
        }
    )

5. Dependency Injection for Common Logic

Use dependencies for authentication, database sessions, and validation:

from fastapi import Depends, Header, HTTPException

# Authentication dependency
async def verify_token(x_token: str = Header(...)):
    if x_token != "secret-token":
        raise HTTPException(status_code=401, detail="Invalid token")
    return x_token

# Database session dependency
async def get_db():
    db = SessionLocal()
    try:
        yield db
    finally:
        db.close()

# Pagination dependency
async def pagination_params(
    skip: int = 0,
    limit: int = 10,
    max_limit: int = 100
):
    if limit > max_limit:
        limit = max_limit
    return {"skip": skip, "limit": limit}

# Use in endpoints
@router.get("/items/")
async def list_items(
    token: str = Depends(verify_token),
    db: Session = Depends(get_db),
    pagination: dict = Depends(pagination_params),
):
    return db.query(Item).offset(pagination["skip"]).limit(pagination["limit"]).all()

6. API Router Organization

Structure APIs with APIRouter for modularity:

# app/routers/items.py
from fastapi import APIRouter

router = APIRouter(
    prefix="/items",
    tags=["items"],
    dependencies=[Depends(verify_token)],
    responses={404: {"description": "Not found"}},
)

# app/main.py
from fastapi import FastAPI
from app.routers import items, users

app = FastAPI()

app.include_router(items.router)
app.include_router(users.router, prefix="/api/v1")

7. OpenAPI Documentation Enhancement

Generate better API documentation:

# Generate enhanced OpenAPI docs
bash ./skills/fastapi-api-patterns/scripts/generate-openapi-docs.sh

Endpoint Documentation:

@router.post(
    "/items/",
    response_model=Item,
    status_code=status.HTTP_201_CREATED,
    summary="Create a new item",
    description="Create a new item with the provided data",
    response_description="The created item",
    responses={
        201: {"description": "Item created successfully"},
        400: {"description": "Invalid input data"},
        409: {"description": "Item already exists"},
    }
)
async def create_item(item: ItemCreate):
    """
    Create a new item with all the information:

    - **name**: Item name (required, 1-100 characters)
    - **description**: Item description (optional, max 500 characters)
    - **price**: Item price (required, must be positive)
    - **category**: Item category (required)
    """
    pass

Examples

Example 1: Complete CRUD API for Chat Messages

# Copy chat API example
cp ./skills/fastapi-api-patterns/examples/chat_api.py app/routers/chat.py

Features:

• Create chat messages
• List messages with pagination and filtering
• Get single message by ID
• Update message content
• Delete messages
• Search messages by content
• Filter by user, channel, date range

Result: Production-ready chat message API with full CRUD operations

Example 2: User Management API

# Copy user management example
cp ./skills/fastapi-api-patterns/examples/user_management.py app/routers/users.py

Features:

• User registration with validation
• User authentication (simulated)
• Profile retrieval and updates
• Password change endpoint
• List users with role filtering
• User deactivation (soft delete)

Result: Complete user management system with security best practices

Example 3: Memory/Context Endpoints for AI Applications

# Copy memory endpoints example
cp ./skills/fastapi-api-patterns/examples/memory_endpoints.py app/routers/memory.py

Features:

• Store conversation context
• Retrieve context by session ID
• Update context with new messages
• Clear old contexts
• Search contexts by keywords
• Pagination for large context histories

Result: API for managing AI conversation memory and context

Requirements

Dependencies:

• FastAPI 0.100+
• Pydantic 2.0+
• Python 3.10+

Optional Dependencies:

• SQLAlchemy (for database operations)
• python-jose (for JWT authentication)
• passlib (for password hashing)
• python-multipart (for file uploads)

Project Structure:

app/
├── main.py
├── routers/
│   ├── __init__.py
│   ├── items.py
│   ├── users.py
│   └── chat.py
├── models/
│   ├── __init__.py
│   └── schemas.py
├── utils/
│   ├── pagination.py
│   └── errors.py
└── dependencies.py

Best Practices

1. Use Response Models:

• Always specify response_model to control what's returned
• Use separate models for create, update, and read operations
• Never expose sensitive data (passwords, tokens)

2. Consistent Error Responses:

• Use standard HTTP status codes
• Return structured error objects with error, message, and details
• Include request ID for debugging

3. Pagination Everywhere:

• Never return unbounded lists
• Default to reasonable page sizes (10-50 items)
• Include total count and page metadata

4. Validation and Documentation:

• Use Pydantic Field validators for complex validation
• Document all endpoints with descriptions and examples
• Use response examples in OpenAPI schema

5. Dependencies for Reusability:

• Extract common logic into dependencies
• Use dependency injection for auth, DB, pagination
• Keep endpoints thin, move logic to services

6. Versioning:

• Use prefix-based versioning (/api/v1/items)
• Keep old versions running during migration
• Document breaking changes clearly

Validation Script

Validate endpoint structure and best practices:

# Validate all endpoints in a router file
bash ./skills/fastapi-api-patterns/scripts/validate-endpoints.sh app/routers/items.py

# What it checks:
# - Response models defined
# - Status codes specified
# - Error handling present
# - Documentation strings
# - Proper HTTP methods
# - Path parameter validation

Performance Considerations

Database Queries:

• Use pagination to limit query size
• Add indexes on frequently filtered fields
• Use database-level filtering, not Python filtering
• Implement query result caching for expensive operations

Response Size:

• Exclude unnecessary fields from responses
• Support field selection via query params
• Compress responses with gzip middleware
• Use streaming for large responses

Request Validation:

• Set reasonable limits on request sizes
• Validate early and fail fast
• Use background tasks for heavy processing
• Implement rate limiting on expensive endpoints

Plugin: fastapi-backend Version: 1.0.0 Category: API Development Skill Type: REST API Patterns