🎯 Goal:

You want to understand what happens when a file is uploaded from a frontend (like React, HTML form, etc.) to your FastAPI backend, and how it is saved to your computer/server.

🧩 Step-by-Step (Layman’s Understanding)

🧍‍♂️ Step 1: User Uploads File

📤
👉 On the frontend, the user selects a file (e.g., PDF, Image, DOC) and clicks “Upload.”

💡 Example:

<input type="file" name="myfile" />

The file is sent to your backend through an API endpoint using the HTTP method POST.

⚙️ Step 2: FastAPI Receives the File

📥
In your FastAPI code, you define an endpoint to accept file uploads:

from fastapi import FastAPI, File, UploadFile

app = FastAPI()

@app.post("/upload")
async def upload_file(file: UploadFile = File(...)):
    return {"filename": file.filename}

🧠
Here’s what’s happening:

• UploadFile → a special FastAPI object that represents your uploaded file.

• file.filename → gives the name of the uploaded file.

• FastAPI temporarily stores this file in memory (for small files) or in a temporary location.

📂 Step 3: Define Where You Want to Store It

📍
Now, decide a permanent folder where you want to keep all uploaded files.

Example:

UPLOAD_FOLDER = "uploaded_files/"

(You can create that folder in your project directory.)

💾 Step 4: Open a File in Write Binary Mode (wb)

🧠
This step is important!

When you upload a file, you need to read the content from the uploaded file and write it to a file on your disk.

with open(f"{UPLOAD_FOLDER}{file.filename}", "wb") as buffer:
    buffer.write(await file.read())

🧩 What happens here:

• open(..., "wb") → opens a new file to write in binary mode (since it can be images, videos, etc.)

• await file.read() → reads all the bytes from the uploaded file.

• buffer.write(...) → saves those bytes into the new file in your chosen folder.

🪄 Step 5: Done! File Stored Successfully

🎉
Now your file exists in your project directory:

uploaded_files/
 └── my_uploaded_document.pdf

You can later access, move, or process it however you want!

🌈 Full Example Code 

from fastapi import FastAPI, File, UploadFile
import os

app = FastAPI()

UPLOAD_DIR = "uploaded_files"

# 🧱 Create folder if not exists
if not os.path.exists(UPLOAD_DIR):
    os.makedirs(UPLOAD_DIR)

@app.post("/upload")
async def upload_file(file: UploadFile = File(...)):
    # 📌 1️⃣ Get file name
    filename = file.filename

    # 📌 2️⃣ Create full path
    file_path = os.path.join(UPLOAD_DIR, filename)

    # 📌 3️⃣ Read + Write file in binary mode
    with open(file_path, "wb") as f:
        f.write(await file.read())

    # 📌 4️⃣ Return success message
    return {"status": "✅ File uploaded successfully!", "path": file_path}

🧭 Visual Summary

🎨 FRONTEND
   ↓
📤 User uploads → 
   ↓
📬 FASTAPI receives UploadFile
   ↓
🧠 Reads file (in bytes)
   ↓
💾 Opens destination in write-binary mode
   ↓
📂 Writes + Saves file in chosen folder
   ↓
✅ Success message sent back