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Sending Files

The send_file utility in Lilya makes it easy to return files from your endpoints, similar to Flask’s send_file.

It can return files from the filesystem or from in-memory file-like objects, and supports options such as forcing downloads, setting custom filenames, and controlling cache headers.

When to Use send_file

Use send_file when your API or application needs to:

  • Serve static files on demand (e.g., reports, exports, PDFs).
  • Allow users to download dynamically generated files.
  • Stream file-like objects such as BytesIO.

For static assets (images, CSS, JS) you should use proper static file serving, but for API endpoints that return a file, send_file is the right tool.

Basic Example

Returning a text file directly:

from lilya.apps import Lilya
from lilya.routing import Path
from lilya.contrib.responses.files import send_file

async def get_report(request):
    return send_file("reports/summary.txt")

app = Lilya(routes=[
    Path("/report", get_report)
])

Request:

curl http://localhost:8000/report

Response body will contain the content of summary.txt.

Force Download (Attachment)

To force the browser to download the file instead of displaying it inline:

async def download_invoice(request):
    return send_file("invoices/invoice.pdf", as_attachment=True)

This adds a Content-Disposition: attachment header, prompting the browser to download the file.

Custom Filename

You can customize the filename shown to the user:

async def export_csv(request):
    return send_file(
        "exports/data.csv",
        as_attachment=True,
        attachment_filename="report.csv"
    )

The downloaded file will appear as report.csv instead of data.csv.

Sending a File-Like Object

You can also send an in-memory file, such as a BytesIO:

import io

async def stream_file(request):
    file_like = io.BytesIO(b"some dynamic content")
    return send_file(file_like, mimetype="text/plain")

This is useful for dynamically generated content that you don’t want to store on disk.

Controlling Cache

Set Cache-Control headers with max_age:

async def cached_pdf(request):
    return send_file("docs/manual.pdf", max_age=3600)

Response will include:

Cache-Control: public, max-age=3600

API Reference

send_file(
    filename_or_fp: Union[str, Path, IO[bytes]],
    mimetype: Optional[str] = None,
    as_attachment: bool = False,
    attachment_filename: Optional[str] = None,
    max_age: Optional[int] = None,
)

Parameters

  • filename_or_fp – Path to the file on disk (str or Path) or a file-like object (IO[bytes]).
  • mimetype – Optional MIME type (e.g., "application/pdf"). If not provided, Lilya attempts to infer.
  • as_attachment – If True, adds Content-Disposition: attachment to force download.
  • attachment_filename – Override the filename presented to the user.
  • max_age – Sets Cache-Control: public, max-age=<seconds> for caching.

With send_file, Lilya makes file downloads and streaming as simple and flexible as Flask, but fully async-native.