How Starlette Shapes UploadFile in FastAPI

[Kfir-G]

Hey FastAPI community!

I wanted to clarify something that often trips up developers using FastAPI: the UploadFile class and its relationship with Starlette.

What's the Confusion?

In FastAPI, the UploadFile class is key for handling file uploads. However, many developers find themselves confused when they check the type of an UploadFile instance. They discover it's actually an instance of Starlette’s UploadFile.

Here’s the Lowdown:

1. Architecture: FastAPI is built on top of Starlette, which is a lightweight ASGI framework. When you use UploadFile in FastAPI, you're actually working with a subclass of Starlette’s UploadFile, which adds some handy features.

2. Type Confusion:
   Here's a quick example:

   from fastapi import FastAPI, UploadFile, File
   
   app = FastAPI()
   
   @app.post("/upload/")
   async def upload_file(file: UploadFile = File(...)):
       return {
           "is_fastapi_instance": isinstance(file, UploadFile),
           "file_type": type(file),
           "filename": file.filename
       }

   In this case, isinstance(file, UploadFile) will return False ⚠️ , when you will check the type with type(file), you’ll see it as starlette.datastructures.UploadFile.

3. Why It Matters: Understanding this inheritance is crucial for debugging and writing logic that relies on type checking. It highlights how FastAPI builds upon Starlette while adding its own capabilities.

Additional Resources

If you're interested in digging deeper, check out these links:

• FastAPI Documentation: Upload File
• FastAPI UploadFile Class
• Starlette Documentation: UploadFile
• Understanding ASGI
• Starlette UploadFile Class
• Understanding FastAPI: How Starlette Works

I hope this helps clear things up.

[yangyushi]

Thanks for the clarification and this seems like a really confusing design choice to me. Would it be fixed at some point so that fastapi.UploadFile would work as one would expected naively at some point?