1. Handling Form Fields#

FastAPI provides the Form class to define attributes that should be received as form fields rather than JSON.

from fastapi import FastAPI, Form

app = FastAPI()

@app.post("/login/")
async def login(username: str = Form(...), password: str = Form(...)):
    return {"username": username}

2. Efficient File Uploads with UploadFile#

Using UploadFile has several advantages over bytes:

• Memory Efficient: It doesn’t load the whole file into RAM immediately.
• Rich Attributes: You get easy access to file.filename and file.content_type.
• File-like Object: You can use await file.read() or await file.write() just like a standard Python file object.

from fastapi import FastAPI, File, UploadFile

@app.post("/upload-profile-pic/")
async def upload_image(file: UploadFile = File(...)):
    contents = await file.read() # Read the file content
    return {
        "filename": file.filename,
        "content_type": file.content_type,
        "size": len(contents)
    }

3. Mixing Forms and Files#

You can even combine them! For example, a user might upload an image and provide a description in the same request. FastAPI handles the parsing of this complex “multipart” request automatically.

🛠️ Implementation Checklist#

• Installed python-multipart using uv.
• Created a POST endpoint using Form.
• Created a POST endpoint using UploadFile.
• Verified the “File Picker” works in Swagger UI (/docs).

📚 Resources#

1. Official Docs: FastAPI Form Data
2. Official Docs: FastAPI Request Files
3. Book: FastAPI: Modern Python Web Development (Chapter 4: Form Data & Files).