How to Send File in Flask: Simple Guide with Example

In Flask, you can send a file to the client using the send_file function from flask. This function takes the file path or file-like object and returns it as a response, allowing users to download or view the file.

Syntax

The send_file function is used to send files from your Flask server to the client. It requires the file path or a file-like object as the first argument. You can also specify options like as_attachment to force download, download_name to set the filename, and mimetype to specify the file type.

• file: Path or file object to send
• as_attachment: Boolean to force file download (default is False)
• download_name: Name for the downloaded file
• mimetype: MIME type of the file (e.g., 'image/png')

python

from flask import send_file

send_file(file, as_attachment=False, download_name=None, mimetype=None)

Example

This example shows a simple Flask app that sends a file named example.txt located in the same folder. When you visit /download, the file is sent as an attachment for download with a custom filename.

python

from flask import Flask, send_file

app = Flask(__name__)

@app.route('/download')
def download_file():
    # Send example.txt as a downloadable file named 'downloaded_example.txt'
    return send_file('example.txt', as_attachment=True, download_name='downloaded_example.txt')

if __name__ == '__main__':
    app.run(debug=True)

Output

When visiting http://localhost:5000/download, the browser prompts to download 'downloaded_example.txt' containing the contents of example.txt.

Common Pitfalls

• Not setting as_attachment=True when you want the file to download instead of display in the browser.
• Using incorrect file paths causing FileNotFoundError.
• Not specifying download_name if you want a custom filename for the download.
• Serving files without proper security checks can expose sensitive files.

python

from flask import send_file

# Wrong: file will open in browser if supported
send_file('example.txt')

# Right: forces download
send_file('example.txt', as_attachment=True)

# Wrong: wrong path causes error
send_file('wrong_path.txt', as_attachment=True)

# Right: correct path and filename
send_file('example.txt', as_attachment=True, download_name='myfile.txt')

Quick Reference

Use this quick reference to remember key send_file options:

Option	Description	Default
file	Path or file-like object to send	Required
as_attachment	Force file download instead of inline display	False
download_name	Filename for the downloaded file	Original filename
mimetype	MIME type of the file	Auto-detected

Key Takeaways

Use Flask's send_file function to send files to clients easily.

Set as_attachment=True to force file download instead of inline display.

Always verify file paths to avoid errors and security risks.

Use download_name to customize the filename clients see when downloading.

Check MIME types if you want to control how files are handled by browsers.