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Ishaan Sehgal

FastAPI Dependency Injection

2 min read

In FastAPI you can do dependency injection like so

async def k8s_get_documentation(
	request: K8sDocRequest, # ← From request body
	api_key_data: Dict[str, Any] = Depends(validate_api_key) # ← FastAPI magic!
):

The client doesn’t send api_key_data! Here’s what actually happens:

1. Client Sends Simple HTTP Request

const response = await fetch(`${BASE_URL}/api/kubernetes/documentation`, {
	method: "POST",
	headers: {
		"Content-Type": "application/json",
		"x-api-key": "gd_abc123..." // ← Only this!
	},
	body: JSON.stringify({ query: "...", version: "..." })
});

2. FastAPI Receives Raw HTTP Request

FastAPI gets:

  • Headers: { “x-api-key”: “gd_abc123…”, “Content-Type”: ”…” }
  • Body: { “query”: ”…”, “version”: ”…” }
  • URL: /api/kubernetes/documentation

3. FastAPI Sees Depends(validate_api_key)

Before calling your endpoint, FastAPI automatically:

  1. Creates request object from the raw HTTP request
  2. Calls validate_api_key(request, credentials)
  3. Takes the return value and assigns it to api_key_data

How validate_api_key Parameters Get Populated

async def validate_api_key(
    request: Request, # FastAPI provides this
    credentials: Optional[HTTPAuthorizationCredentials] = Depends(security) # HTTPBearer provides this
) -> Dict[str, Any]:
  • request: FastAPI automatically creates this from the incoming HTTP request
  • credentials: The HTTPBearer() security scheme extracts this from the Authorization header (if present)
📱 Client sends:
   POST /api/kubernetes/documentation
   Headers: { "x-api-key": "gd_abc123..." }
   Body: { "query": "..." }
 
⬇️
 
🔧 FastAPI automatically:
   1. Creates Request object from HTTP request
   2. HTTPBearer extracts credentials (if any)
   3. Calls: validate_api_key(request, credentials)
 
⬇️
 
🔍 validate_api_key function:
   1. Gets API key: request.headers.get("x-api-key")
   2. Validates against database
   3. Returns: { "user_id": "...", "usage_count": 5, ... }
 
⬇️
 
🎯 FastAPI assigns return value to api_key_data:
   api_key_data = { "user_id": "...", "usage_count": 5, ... }
 
⬇️
 
🚀 Your endpoint runs with populated parameters:
   k8s_get_documentation(request=..., api_key_data=...)

Dependency Injection Pattern

This is FastAPI’s dependency injection:

# This:
api_key_data: Dict[str, Any] = Depends(validate_api_key)
 
# Is equivalent to:
# api_key_data = await validate_api_key(request, credentials)
# (but FastAPI handles it automatically)

The client never knows about api_key_data- it’s purely a server-side construct created by the dependency injection system! 🎯

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