API Endpoint Implementation Pattern

This skill defines how to implement FastAPI REST endpoints following established patterns.

Endpoint Structure

Basic Endpoint

from fastapi import FastAPI, status
from fastapi.responses import JSONResponse
from datetime import UTC, datetime

@app.get("/api/items")
async def list_items() -> JSONResponse:
    """List all items.

    Returns:
        JSONResponse with status 200 and list of items
    """
    # Implementation
    items = get_items()  # Your business logic
    return JSONResponse(
        status_code=status.HTTP_200_OK,
        content={
            "items": items,
            "count": len(items),
            "timestamp": datetime.now(UTC).isoformat().replace("+00:00", "Z")
        }
    )

Endpoint with Request Body

from pydantic import BaseModel

class CreateItemRequest(BaseModel):
    """Request model for creating an item."""
    name: str
    description: str | None = None
    price: float = Field(ge=0, description="Price must be non-negative")

class ItemResponse(BaseModel):
    """Response model for item operations."""
    id: str
    name: str
    description: str | None
    price: float
    created_at: datetime

@app.post("/api/items")
async def create_item(request: CreateItemRequest) -> JSONResponse:
    """Create a new item.

    Args:
        request: Item creation request

    Returns:
        JSONResponse with created item data
    """
    # Validate and process
    item = create_item_logic(request)  # Your business logic
    return JSONResponse(
        status_code=status.HTTP_201_CREATED,
        content=ItemResponse(
            id=item.id,
            name=item.name,
            description=item.description,
            price=item.price,
            created_at=item.created_at
        ).model_dump()
    )

Request/Response Models

Request Models

Define Pydantic models for request validation:

from pydantic import BaseModel, Field, field_validator

class CreateItemRequest(BaseModel):
    """Request model for creating an item."""

    name: str = Field(min_length=1, max_length=100, description="Item name")
    description: str | None = Field(None, max_length=500, description="Optional description")
    price: float = Field(ge=0, description="Price must be non-negative")
    tags: list[str] = Field(default_factory=list, description="Item tags")

    @field_validator("name")
    @classmethod
    def validate_name(cls, v: str) -> str:
        """Validate item name."""
        if not v.strip():
            raise ValueError("Name cannot be empty")
        return v.strip()

Response Models

class ItemResponse(BaseModel):
    """Response model for item operations."""

    id: str
    name: str
    description: str | None
    price: float
    tags: list[str]
    created_at: datetime
    updated_at: datetime

class ErrorResponse(BaseModel):
    """Standard error response model."""

    status: str = "failure"
    error_code: str
    message: str
    timestamp: datetime
    details: dict | None = None

Error Handling

Standard Error Response

Define error codes in your project (e.g., errors.py or constants):

# Define error codes (e.g., in errors.py)
E_INVALID_REQUEST = "E_INVALID_REQUEST"
E_RESOURCE_NOT_FOUND = "E_RESOURCE_NOT_FOUND"
E_RESOURCE_CONFLICT = "E_RESOURCE_CONFLICT"
E_SERVICE_NOT_READY = "E_SERVICE_NOT_READY"
E_INTERNAL_ERROR = "E_INTERNAL_ERROR"

@app.post("/api/items")
async def create_item(request: CreateItemRequest) -> JSONResponse:
    """Create item endpoint."""
    try:
        # Validate business rules
        if item_exists(request.name):
            return JSONResponse(
                status_code=status.HTTP_409_CONFLICT,
                content={
                    "status": "failure",
                    "error_code": "E_RESOURCE_CONFLICT",
                    "message": f"Item with name '{request.name}' already exists",
                    "timestamp": datetime.now(UTC).isoformat().replace("+00:00", "Z"),
                    "details": {"field": "name", "value": request.name}
                }
            )

        # Create item
        item = create_item_logic(request)
        return JSONResponse(status_code=201, content=item.model_dump())
    except ValueError as e:
        return JSONResponse(
            status_code=status.HTTP_400_BAD_REQUEST,
            content={
                "status": "failure",
                "error_code": "E_INVALID_REQUEST",
                "message": str(e),
                "timestamp": datetime.now(UTC).isoformat().replace("+00:00", "Z"),
                "details": None
            }
        )

Error Code to HTTP Status Mapping

Map error codes to appropriate HTTP status codes:

Error Code	HTTP Status	Use Case
E_INVALID_REQUEST	400 Bad Request	Invalid input data, validation failures
E_RESOURCE_NOT_FOUND	404 Not Found	Resource doesn't exist
E_RESOURCE_CONFLICT	409 Conflict	Resource already exists, state conflict
E_UNAUTHORIZED	401 Unauthorized	Authentication required
E_FORBIDDEN	403 Forbidden	Insufficient permissions
E_SERVICE_NOT_READY	503 Service Unavailable	Service dependencies not ready
E_SERVICE_TIMEOUT	504 Gateway Timeout	External service timeout
E_INTERNAL_ERROR	500 Internal Server Error	Unexpected server error

Integration Tests

Test Structure

"""Tests for GET /api/items endpoint.

Tests verify:
- Endpoint returns 200 with correct data
- Error handling works correctly
- Response format matches schema
- Request validation works
"""

import pytest
from fastapi.testclient import TestClient

from your_app import app  # Import your FastAPI app

@pytest.fixture
def client() -> TestClient:
    """Create test client."""
    return TestClient(app)

class TestItemsEndpoint:
    """Test GET /api/items endpoint."""

    def test_get_items_returns_200(self, client: TestClient) -> None:
        """Test GET /api/items returns 200 with list of items."""
        response = client.get("/api/items")

        assert response.status_code == 200
        data = response.json()
        assert "items" in data
        assert "count" in data
        assert isinstance(data["items"], list)

    def test_create_item_returns_201(self, client: TestClient) -> None:
        """Test POST /api/items creates item successfully."""
        response = client.post(
            "/api/items",
            json={"name": "Test Item", "price": 10.99}
        )

        assert response.status_code == 201
        data = response.json()
        assert data["name"] == "Test Item"
        assert data["price"] == 10.99
        assert "id" in data

    def test_create_item_validates_request(self, client: TestClient) -> None:
        """Test POST /api/items validates request data."""
        response = client.post(
            "/api/items",
            json={"name": "", "price": -5}  # Invalid data
        )

        assert response.status_code == 422  # Pydantic validation error
        data = response.json()
        assert "detail" in data

    def test_endpoint_handles_errors(self, client: TestClient) -> None:
        """Test endpoint handles business logic errors correctly."""
        # Simulate conflict scenario
        response = client.post(
            "/api/items",
            json={"name": "Existing Item", "price": 10.99}
        )

        assert response.status_code == 409
        data = response.json()
        assert data["status"] == "failure"
        assert data["error_code"] == "E_RESOURCE_CONFLICT"

Health and Ready Endpoints

Health Endpoint Pattern

# Track server state
_server_start_time: float | None = None
_server_shutting_down: bool = False

@app.get("/health")
async def health() -> JSONResponse:
    """Health check endpoint (liveness probe)."""
    if _server_shutting_down:
        return JSONResponse(
            status_code=status.HTTP_503_SERVICE_UNAVAILABLE,
            content={"status": "unhealthy", ...}
        )

    uptime_seconds = round(time.time() - _server_start_time, 2)
    return JSONResponse(
        status_code=status.HTTP_200_OK,
        content={
            "status": "healthy",
            "timestamp": datetime.now(UTC).isoformat().replace("+00:00", "Z"),
            "uptime_seconds": uptime_seconds,
            "version": "0.1.0"
        }
    )

Ready Endpoint Pattern

@app.get("/ready")
async def ready() -> JSONResponse:
    """Readiness probe - checks dependencies and service state."""
    checks = {
        "database": check_database_connection(),
        "cache": check_cache_connection(),
        "external_api": check_external_service(),
        # Add other critical dependencies
    }

    if all(v == "ok" for v in checks.values()):
        return JSONResponse(
            status_code=status.HTTP_200_OK,
            content={
                "status": "ready",
                "checks": checks,
                "timestamp": datetime.now(UTC).isoformat().replace("+00:00", "Z")
            }
        )
    else:
        failed_checks = {k: v for k, v in checks.items() if v != "ok"}
        return JSONResponse(
            status_code=status.HTTP_503_SERVICE_UNAVAILABLE,
            content={
                "status": "not_ready",
                "checks": checks,
                "failed_checks": list(failed_checks.keys()),
                "timestamp": datetime.now(UTC).isoformat().replace("+00:00", "Z")
            }
        )

def check_database_connection() -> str:
    """Check if database is accessible."""
    try:
        # Your database health check logic
        return "ok"
    except Exception:
        return "error"

def check_cache_connection() -> str:
    """Check if cache is accessible."""
    try:
        # Your cache health check logic
        return "ok"
    except Exception:
        return "error"

def check_external_service() -> str:
    """Check if external service is reachable."""
    try:
        # Your external service check logic
        return "ok"
    except Exception:
        return "error"

Resource Management Endpoints

CRUD Pattern Example

import uuid
from fastapi import Path, Query

# GET - List resources
@app.get("/api/items")
async def list_items(
    skip: int = Query(0, ge=0),
    limit: int = Query(100, ge=1, le=1000)
) -> JSONResponse:
    """List items with pagination."""
    items = get_items(skip=skip, limit=limit)
    return JSONResponse(
        status_code=status.HTTP_200_OK,
        content={
            "items": [item.model_dump() for item in items],
            "count": len(items),
            "skip": skip,
            "limit": limit
        }
    )

# GET - Get single resource
@app.get("/api/items/{item_id}")
async def get_item(item_id: str = Path(...)) -> JSONResponse:
    """Get item by ID."""
    item = get_item_by_id(item_id)
    if not item:
        return JSONResponse(
            status_code=status.HTTP_404_NOT_FOUND,
            content={
                "status": "failure",
                "error_code": "E_RESOURCE_NOT_FOUND",
                "message": f"Item with ID '{item_id}' not found",
                "timestamp": datetime.now(UTC).isoformat().replace("+00:00", "Z")
            }
        )
    return JSONResponse(status_code=200, content=item.model_dump())

# POST - Create resource
@app.post("/api/items")
async def create_item(request: CreateItemRequest) -> JSONResponse:
    """Create new item."""
    item = create_item_logic(request)
    return JSONResponse(
        status_code=status.HTTP_201_CREATED,
        content=item.model_dump()
    )

# PUT - Update resource
@app.put("/api/items/{item_id}")
async def update_item(
    item_id: str = Path(...),
    request: UpdateItemRequest
) -> JSONResponse:
    """Update existing item."""
    item = update_item_logic(item_id, request)
    if not item:
        return JSONResponse(status_code=404, content={"error": "Not found"})
    return JSONResponse(status_code=200, content=item.model_dump())

# DELETE - Delete resource
@app.delete("/api/items/{item_id}")
async def delete_item(item_id: str = Path(...)) -> JSONResponse:
    """Delete item."""
    success = delete_item_logic(item_id)
    if not success:
        return JSONResponse(status_code=404, content={"error": "Not found"})
    return JSONResponse(status_code=204)  # No Content

Testing Requirements

Minimum Test Coverage

Each endpoint should have tests for:

1. Success case: Returns 200 with correct data
2. Error case: Returns appropriate error status
3. Request validation: Invalid requests are rejected
4. Response format: Matches defined schema
5. Edge cases: Boundary conditions and error scenarios

Test File Organization

Organize tests by feature or endpoint:

• Foundation tests: tests/integration/api/test_foundation.py
• Feature-specific tests: tests/integration/api/test_items.py
• Model validation tests: tests/integration/api/test_models.py
• Error handling tests: tests/integration/api/test_errors.py

Or organize by endpoint:

• tests/integration/api/test_health.py
• tests/integration/api/test_ready.py
• tests/integration/api/test_items.py

Best Practices

1. Use type hints: All functions should have type annotations
2. Document endpoints: Include docstrings describing behavior
3. Validate inputs: Use Pydantic models for validation
4. Handle errors: Return appropriate error codes and status codes
5. Test thoroughly: Write tests for success and error cases
6. Follow conventions: Use established patterns from existing endpoints
7. Return consistent format: Use standard response models

Common Patterns

Stateful Endpoints

For endpoints that need to track state or sessions:

# Use in-memory storage, database, or cache
_active_sessions: dict[str, SessionData] = {}

@app.post("/api/sessions/{session_id}/action")
async def perform_action(
    session_id: str = Path(...),
    request: ActionRequest
) -> JSONResponse:
    """Perform action on session."""
    if session_id not in _active_sessions:
        return JSONResponse(
            status_code=status.HTTP_404_NOT_FOUND,
            content={
                "status": "failure",
                "error_code": "E_RESOURCE_NOT_FOUND",
                "message": f"Session '{session_id}' not found"
            }
        )

    session = _active_sessions[session_id]
    result = process_action(session, request)
    return JSONResponse(status_code=200, content=result)

Async Operations

For endpoints that trigger async or long-running operations:

import asyncio
from typing import Awaitable

@app.post("/api/items/{item_id}/process")
async def process_item(item_id: str = Path(...)) -> JSONResponse:
    """Trigger async processing for item."""
    # Validate item exists
    item = get_item_by_id(item_id)
    if not item:
        return JSONResponse(status_code=404, content={"error": "Not found"})

    # Trigger async processing
    task_id = str(uuid.uuid4())
    asyncio.create_task(process_item_async(item_id, task_id))

    return JSONResponse(
        status_code=status.HTTP_202_ACCEPTED,
        content={
            "task_id": task_id,
            "status": "processing",
            "message": "Item processing started"
        }
    )

async def process_item_async(item_id: str, task_id: str) -> None:
    """Background task to process item."""
    # Long-running or async operation
    pass

Dependency Injection

For endpoints that need shared dependencies:

from fastapi import Depends

def get_database():
    """Dependency to get database connection."""
    db = get_db_connection()
    try:
        yield db
    finally:
        db.close()

@app.get("/api/items")
async def list_items(db = Depends(get_database)) -> JSONResponse:
    """List items using database dependency."""
    items = db.query_items()
    return JSONResponse(status_code=200, content={"items": items})