File Uploads in FastAPI

Key Concepts

Handling file uploads in FastAPI involves understanding the following key concepts:

• File Uploads: The process of receiving and processing files sent by clients.
• File Storage: Where and how uploaded files are stored on the server.
• File Validation: Ensuring that uploaded files meet certain criteria (e.g., file type, size).

Explaining Each Concept

1. File Uploads

File uploads in FastAPI are handled using the File and UploadFile classes from the fastapi module. The File class is used to define the expected file in the request body, while UploadFile provides a higher-level interface for handling uploaded files.

2. File Storage

Once a file is uploaded, it needs to be stored somewhere on the server. Common storage options include saving files to the local filesystem, uploading them to cloud storage services, or storing them in a database. The choice of storage depends on the application's requirements and scalability needs.

3. File Validation

File validation ensures that only files meeting certain criteria are accepted. This can include checking the file type (e.g., image, document), file size, and other attributes. FastAPI allows you to define validation rules using Pydantic models or custom functions.

Examples

Example 1: Basic File Upload

Here is a simple example of handling a file upload in FastAPI:

from fastapi import FastAPI, File, UploadFile

app = FastAPI()

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

Example 2: Saving Uploaded File to Local Storage

In this example, we save the uploaded file to the local filesystem:

from fastapi import FastAPI, File, UploadFile
import shutil

app = FastAPI()

@app.post("/uploadfile/")
async def upload_file(file: UploadFile = File(...)):
    with open(f"uploads/{file.filename}", "wb") as buffer:
        shutil.copyfileobj(file.file, buffer)
    return {"filename": file.filename}
    

Example 3: File Validation

Here, we validate the uploaded file to ensure it is an image and does not exceed a certain size:

from fastapi import FastAPI, File, UploadFile, HTTPException
from fastapi.responses import JSONResponse

app = FastAPI()

@app.post("/uploadfile/")
async def upload_file(file: UploadFile = File(...)):
    if not file.content_type.startswith("image/"):
        raise HTTPException(status_code=400, detail="File is not an image")
    if file.size > 5 * 1024 * 1024:
        raise HTTPException(status_code=400, detail="File size exceeds 5MB")
    return {"filename": file.filename}
    

Analogies

Think of file uploads as sending a package to a friend. The package (file) needs to be properly wrapped (uploaded) and sent to the correct address (server). The friend (server) then checks the package to ensure it is not damaged (validated) and stores it in a safe place (storage).

Another analogy is a library where users submit books. The librarian (server) checks the book (file) to ensure it is in good condition (validated) and places it on the correct shelf (storage).