File validation (size, type) in FastAPI - Mini Project: Build & Apply

File validation (size, type) with FastAPI

📖 Scenario: You are building a simple web API that accepts file uploads. To keep your server safe and efficient, you want to check that uploaded files are not too large and are of allowed types.

🎯 Goal: Create a FastAPI app that accepts a file upload and validates the file size and type before accepting it.

📋 What You'll Learn

Create a FastAPI app instance named app

Create an endpoint /upload that accepts a file upload using UploadFile

Add a configuration variable MAX_FILE_SIZE set to 1_000_000 bytes (1 MB)

Check the uploaded file's content type is either image/jpeg or image/png

Check the uploaded file's size does not exceed MAX_FILE_SIZE

Return a JSON response with a success message if validations pass

💡 Why This Matters

🌍 Real World

File upload validation is essential for web apps that accept user files to prevent server overload and security risks.

💼 Career

Backend developers often implement file validation to ensure safe and efficient file handling in APIs.

1

Create FastAPI app and upload endpoint

Import FastAPI and UploadFile from fastapi. Create a FastAPI app instance called app. Define a POST endpoint /upload that accepts a parameter file of type UploadFile.

FastAPI

from fastapi import FastAPI, UploadFile

app = FastAPI()

@app.post("/upload")
async def upload(file: UploadFile):
    # Your code here

Need a hint?

Start by importing FastAPI and UploadFile. Then create the app and define the upload function with the correct decorator and parameter.

2

Add max file size configuration

Add a variable called MAX_FILE_SIZE and set it to 1_000_000 (1 million bytes).

FastAPI

from fastapi import FastAPI, UploadFile

app = FastAPI()

MAX_FILE_SIZE = 1_000_000

@app.post("/upload")
async def upload(file: UploadFile):
    # Your code here

Need a hint?

Define a constant variable for the maximum allowed file size in bytes.

3

Check file type and size

Inside the upload function, check if file.content_type is either image/jpeg or image/png. Then read the file content using await file.read() and check if its length is less than or equal to MAX_FILE_SIZE.

FastAPI

from fastapi import FastAPI, UploadFile

app = FastAPI()

MAX_FILE_SIZE = 1_000_000

@app.post("/upload")
async def upload(file: UploadFile):
    # Your code here

Need a hint?

Use an if statement to check the content type. Use await file.read() to get the file bytes and check its length.

4

Return success message

If the file passes both checks, return a dictionary with message key and value "File uploaded successfully".

FastAPI

from fastapi import FastAPI, UploadFile

app = FastAPI()

MAX_FILE_SIZE = 1_000_000

@app.post("/upload")
async def upload(file: UploadFile):
    if file.content_type not in ("image/jpeg", "image/png"):
        return {"error": "Invalid file type"}
    content = await file.read()
    if len(content) > MAX_FILE_SIZE:
        return {"error": "File too large"}
    # Your code here

Need a hint?

Return a success message dictionary if all validations pass.