# Python Fastapi Patterns

> Skill for Python FastAPI backend development in this project. Use when creating API endpoints, working with Pydantic models, handling streaming responses, or implementing Azure authentication patterns. Use when this capability is needed.

- Skill: `tomevault-io/python-fastapi-patterns-2` (Agent Skill, multi-file: 2 files)
- Install (CLI): `npx skillmds@latest add tomevault-io/python-fastapi-patterns-2`
- Raw SKILL.md: https://api.skillmd.com/api/skills/tomevault-io/python-fastapi-patterns-2/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: DevOps & Infra
- Author: tomevault-io (https://skillmd.com/u/tomevault-io)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/tomevault-io/python-fastapi-patterns-2

---


# Python FastAPI Development Skill

This skill provides patterns and guidance for developing the Python FastAPI backend in the Foundry Agent Accelerator.

## Project Structure

```
src/api/
├── main.py          # App entry point, agent initialization
├── routes.py        # API endpoint definitions
├── util.py          # Shared utilities (logger, Pydantic models)
├── prompts/         # System prompt text files
└── templates/       # Jinja2 HTML templates
```

## Required Documentation Format

Every Python file MUST start with this header:

```python
"""
=============================================================================
MODULE NAME - Brief Description
=============================================================================

WHAT THIS FILE DOES:
--------------------
1. First key responsibility
2. Second key responsibility

KEY CONCEPTS:
-------------
Explanation of important concepts for beginners.

=============================================================================
"""
```

## Code Section Headers

Use visual separators:

```python
# =============================================================================
# IMPORTS
# =============================================================================

import os
import logging

# =============================================================================
# HELPER FUNCTIONS
# =============================================================================
```

## Type Hints (Required)

Always use type hints:

```python
from typing import Optional, Dict, List, Any

def get_agent_config(name: str, version: Optional[int] = None) -> Dict[str, Any]:
    """Get the agent configuration."""
    pass
```

## Pydantic Models

Use Pydantic for request/response validation:

```python
import pydantic

class FileAttachment(pydantic.BaseModel):
    """Represents a file attached to a message."""
    name: str
    type: str
    data: str  # Base64-encoded

class Message(pydantic.BaseModel):
    """Represents a chat message."""
    content: str
    role: str = "user"
    attachments: list[FileAttachment] = []

class ChatRequest(pydantic.BaseModel):
    """Request body for the /chat endpoint."""
    messages: list[Message]
```

## Route Patterns

### Basic GET Endpoint

```python
@router.get("/", response_class=HTMLResponse)
async def serve_homepage(request: Request):
    """
    Serve the main chat interface.
    
    This endpoint renders the HTML template that loads the React frontend.
    """
    return templates.TemplateResponse("index.html", {"request": request})
```

### Streaming POST Endpoint

```python
@router.post("/chat")
async def chat_endpoint(
    request: Request,
    chat_request: ChatRequest
) -> StreamingResponse:
    """
    Handle chat messages and stream AI responses.
    
    This endpoint:
    1. Receives chat messages from the frontend
    2. Sends them to the Foundry Agent
    3. Streams the response back using SSE
    """
    async def generate():
        async for chunk in agent_response:
            yield f"data: {json.dumps({'type': 'message', 'content': chunk})}\n\n"
        yield f"data: {json.dumps({'type': 'stream_end'})}\n\n"
    
    return StreamingResponse(generate(), media_type="text/event-stream")
```

## Logging Pattern

Use the centralized logger:

```python
from .util import get_logger
import logging
import os

logger = get_logger(
    name="foundry-agent-module-name",
    log_level=logging.INFO,
    log_file_name=os.getenv("APP_LOG_FILE"),
    log_to_console=True
)

# Usage
logger.info("Starting operation...")
logger.error(f"Failed to process: {error}")
```

## Authentication Pattern

```python
from fastapi import Depends, HTTPException
from fastapi.security import HTTPBasic, HTTPBasicCredentials
import secrets

security = HTTPBasic()
username = os.getenv("WEB_APP_USERNAME")
password = os.getenv("WEB_APP_PASSWORD")
basic_auth_enabled = username and password

def authenticate(credentials: HTTPBasicCredentials = Depends(security)) -> None:
    """Verify credentials if basic auth is enabled."""
    if not basic_auth_enabled:
        return
    
    if not (secrets.compare_digest(credentials.username, username) and
            secrets.compare_digest(credentials.password, password)):
        raise HTTPException(status_code=401, detail="Invalid credentials")
```

## Error Handling

```python
from fastapi import HTTPException, status

try:
    result = await process_request(data)
except ValidationError as e:
    logger.warning(f"Validation error: {e}")
    raise HTTPException(
        status_code=status.HTTP_400_BAD_REQUEST,
        detail=str(e)
    )
except AzureError as e:
    logger.error(f"Azure error: {e}")
    raise HTTPException(
        status_code=status.HTTP_503_SERVICE_UNAVAILABLE,
        detail="Azure service temporarily unavailable"
    )
```

## Environment Variables

```python
from dotenv import load_dotenv
import os

load_dotenv()

# Required variables
project_endpoint = os.getenv("AZURE_EXISTING_AIPROJECT_ENDPOINT")
model_name = os.getenv("AZURE_AI_CHAT_DEPLOYMENT_NAME")
agent_name = os.getenv("AZURE_AI_AGENT_NAME")

# Optional with defaults
config_source = os.getenv("AGENT_CONFIG_SOURCE", "local")
```

## Common Imports

```python
# Standard library
import json
import logging
import os
from typing import Optional, Dict, List, Any
from pathlib import Path

# Third-party
import yaml
import pydantic
from dotenv import load_dotenv

# FastAPI
import fastapi
from fastapi import Request, Depends, HTTPException, status
from fastapi.responses import StreamingResponse, JSONResponse
from fastapi.templating import Jinja2Templates

# Azure
from azure.ai.projects import AIProjectClient
from azure.identity import DefaultAzureCredential
```

---
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<!-- tomevault:4.0:skill_md:2026-04-13 -->