Flask send_file: Efficiently Send Files to Clients

Flask's send_file() function is a powerful utility that allows you to send files directly to clients in your web applications. Let's explore how to use it effectively and securely.

Before working with files, ensure you understand how Flask handles static files. You might want to check our guide on Flask Static Folder for better context.

Basic Usage of send_file()

Here's a simple example of how to send a file using send_file():

from flask import Flask, send_file

app = Flask(__name__)

@app.route('/download')
def download_file():
    path = "files/document.pdf"
    return send_file(path, as_attachment=True)

Sending Files with Custom Names

You can specify a custom filename when the client downloads the file:

@app.route('/download/custom')
def download_custom():
    path = "files/document.pdf"
    return send_file(
        path,
        as_attachment=True,
        download_name='custom_name.pdf'
    )

Streaming Large Files

For large files, it's recommended to use streaming to prevent memory issues. This is particularly important when dealing with media files or large documents.

@app.route('/stream')
def stream_file():
    path = "files/large_video.mp4"
    return send_file(
        path,
        as_attachment=False,
        mimetype='video/mp4'
    )

Sending In-Memory Files

You can also send files that are generated in memory using BytesIO:

from io import BytesIO

@app.route('/dynamic')
def send_dynamic_file():
    data = BytesIO()
    data.write(b'Some dynamic content')
    data.seek(0)
    return send_file(
        data,
        mimetype='text/plain',
        as_attachment=True,
        download_name='dynamic.txt'
    )

Error Handling

It's important to handle file-related errors gracefully. You might want to integrate this with Flask Error Handler for better error management.

from flask import abort
import os

@app.route('/download/')
def download(filename):
    try:
        path = f"files/{filename}"
        if not os.path.exists(path):
            abort(404)
        return send_file(path, as_attachment=True)
    except Exception as e:
        abort(500)

Security Considerations

Always validate file paths and ensure users can't access files outside your intended directory. Use path sanitization to prevent directory traversal attacks.

Consider using Flask before_request to implement security checks before serving files.

Conclusion

The send_file() function is essential for handling file downloads in Flask applications. Remember to consider security, performance, and user experience when implementing file serving functionality.