Downloading ZIP Files with aiohttp in Python

When building web applications and APIs in Python, you may need to allow users to download ZIP archive files. The aiohttp library provides an easy way to handle ZIP file downloads asynchronously.

What is aiohttp?

aiohttp is a popular Python library for asynchronous HTTP clients and servers. It allows you to make requests and handle responses without blocking your application. This is useful for building highly scalable systems.

Here's a quick example of making a GET request with aiohttp:

import aiohttp

async with aiohttp.ClientSession() as session:
    async with session.get('http://example.com') as response:
        print(response.status)

Streaming ZIP File Downloads

To download a ZIP file, we can use the response.content attribute. However, for potentially large archives, we should stream the content instead of loading the entire file into memory.

Here is an example route handler for starting a ZIP file download:

from aiohttp import web
import zipfile

async def handle_download(request):
    zf = zipfile.ZipFile('files.zip')
    response = web.StreamResponse()
    response.content_type = 'application/zip'
    response.content_disposition = 'attachment; filename="files.zip"'
    
    await response.prepare(request)

    for name in zf.namelist():
        data = zf.read(name)
        await response.write(data)

    return response

Let's break this down:

By streaming the ZIP file instead of loading it all into memory, we can efficiently handle large downloads even with limited resources.

Practical Challenges

There are a few challenges to consider when implementing ZIP downloads:

Here is an example with some error handling:

@routes.get('/download')
async def download(request):
    try: 
        zf = zipfile.ZipFile('files.zip')
    except FileNotFoundError:
        raise web.HTTPNotFound()

    try:
        return await handle_download(request)
    except ConnectionResetError:
        pass # client disconnected
    finally:
        zf.close()

This helps make the downloader more robust.

Next Steps

Some ideas for building on top of basic ZIP downloading:

The aiohttp library handles the complexity of asynchronous streaming for us out of the box. We just have to put it together with the right handlers to enable rich file downloading behaviors in our web apps.