Document type annotation principles for FastAPI dependency injection

[Copilot]

The problem statement asks how to properly annotate return types in dependency functions: ChatService() vs ChatService, and requests a principle-based explanation.

Changes

Added comprehensive documentation section "依赖注入的类型注解原理" to FastAPI框架.md covering:

• Core principle: Use ChatService (type) not ChatService() (call expression) in type annotations
• Annotated mechanics: First parameter is the type for type checkers, second is FastAPI metadata (Depends)
• Dependency injection workflow: How FastAPI resolves, calls, validates, and injects dependencies
• Complete examples: ChatService and CanvasService with correct/incorrect patterns
• Summary table: Quick reference for return types, parameter types, and return values

Example

# Dependency function - return type uses the class (type)
chat_service = ChatService()

def get_chat_service() -> ChatService:  # ✅ Type annotation
    return chat_service  # ✅ Returns instance

# Route function - parameter uses Annotated with type and metadata
@router.get("/chat_session/{session_id}")
async def get_chat_session(
    session_id: str,
    chat_service: Annotated[ChatService, Depends(get_chat_service)]
):
    return await chat_service.get_chat_history(session_id=session_id)

Type annotations always use types (classes), never call expressions. The dependency system handles instantiation and injection at runtime.

Original prompt

@router.get("/chat_session/{session_id}")
async def get_chat_session(
session_id: str, chat_service=Annotated[ChatService, Depends(get_chat_service)]
):
return await chat_service.get_chat_history(session_id=session_id)
chat_service = ChatService()

def get_canvas_service() -> CanvasService:
return canvas_service

def get_chat_service() -> ChatService:
return chat_service

返回类实例时 Annotated[ChatService, 注解 如何注解 ChatService() 还是 ChatService 从原理上讲清楚

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