File download responses in Flask

Introduction

File download responses let your web app send files to users so they can save them on their devices.

When you want users to download reports or documents your app creates.

When providing images, PDFs, or other files for users to save.

When letting users download backups or exported data.

When sharing software or updates as downloadable files.

When sending any file that should not just display in the browser but be saved.

Syntax

Flask

from flask import send_file

@app.route('/download')
def download():
    return send_file('path/to/file', as_attachment=True, download_name='filename.ext')

send_file is the main function to send files in Flask.

Use as_attachment=True to force the browser to download instead of display.

Examples

Sends a PDF file named 'report.pdf' for download.

Flask

return send_file('files/report.pdf', as_attachment=True, download_name='report.pdf')

Sends an image file that the browser may display instead of download.

Flask

return send_file('images/photo.jpg', mimetype='image/jpeg')

Sends a CSV file for download using the original filename.

Flask

return send_file('files/data.csv', as_attachment=True)

Sample Program

This Flask app has a home page with a link. Clicking it downloads 'sample.txt' as 'example.txt'.

Flask

from flask import Flask, send_file

app = Flask(__name__)

@app.route('/')
def home():
    return '<a href="/download">Download File</a>'

@app.route('/download')
def download():
    # Sends a text file for download with a custom name
    return send_file('sample.txt', as_attachment=True, download_name='example.txt')

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

Important Notes

Make sure the file path is correct and accessible by your Flask app.

Use download_name to set the filename users see when saving.

For large files, consider using send_from_directory or streaming responses.

Summary

Use send_file to send files from Flask to users.

Set as_attachment=True to force download instead of display.

Customize the saved filename with download_name.

Practice

1. What does the as_attachment=True parameter do when used with Flask's send_file function?

easy

A. Encrypts the file before sending it.

B. Changes the file's content type to plain text.

C. Forces the browser to download the file instead of displaying it.

D. Prevents the file from being downloaded.

Solution

1. Step 1: Understand send_file usage

   The send_file function sends files from the server to the client in Flask.

2. Step 2: Role of as_attachment=True

   Setting as_attachment=True tells the browser to download the file instead of trying to open or display it.

3. Final Answer:

   Forces the browser to download the file instead of displaying it. -> Option C

4. Quick Check:

   as_attachment=True means download file [OK]

Hint: Remember: as_attachment=True means download, not display [OK]

Common Mistakes:

• Thinking it changes file content type
• Assuming it encrypts the file
• Believing it blocks download

2. Which of the following is the correct syntax to send a file named 'report.pdf' as a download in Flask?

easy

A. return send_file('report.pdf', as_attachment=True)

B. return send_file('report.pdf', download=True)

C. return send_file('report.pdf', attachment=True)

D. return send_file('report.pdf', force_download=True)

Solution

1. Step 1: Recall correct parameter for download

   The correct parameter to force download is as_attachment=True.

2. Step 2: Check syntax correctness

   return send_file('report.pdf', as_attachment=True) uses send_file('report.pdf', as_attachment=True), which is the correct syntax.

3. Final Answer:

   return send_file('report.pdf', as_attachment=True) -> Option A

4. Quick Check:

   Correct parameter is as_attachment=True [OK]

Hint: Use as_attachment=True to force download in send_file [OK]

Common Mistakes:

• Using incorrect parameter names like download or attachment
• Forgetting to set as_attachment=True
• Using non-existent parameters like force_download

3. What will be the filename suggested to the user when downloading a file with this Flask code?

return send_file('data.csv', as_attachment=True, download_name='user_data.csv')

medium

A. data.csv

B. user_data.csv

C. download_name.csv

D. No filename suggested

Solution

1. Step 1: Understand download_name parameter

   The download_name parameter sets the filename the browser suggests when saving the file.

2. Step 2: Analyze given code

   The code sets download_name='user_data.csv', so the browser will suggest this name.

3. Final Answer:

   user_data.csv -> Option B

4. Quick Check:

   download_name sets saved filename [OK]

Hint: download_name sets the saved filename for downloads [OK]

Common Mistakes:

• Assuming original filename is always used
• Confusing download_name with as_attachment
• Thinking no filename is suggested

4. Identify the error in this Flask code snippet intended to send a file for download:

return send_file('image.png', attachment=True)

medium

A. The parameter should be as_attachment=True, not attachment=True.

B. The filename must be absolute path, not relative.

C. send_file cannot send image files.

D. Missing download_name parameter.

Solution

1. Step 1: Check parameter correctness

   The correct parameter to force download is as_attachment=True, not attachment=True.

2. Step 2: Validate other options

   Relative paths are allowed, images can be sent, and download_name is optional.

3. Final Answer:

   The parameter should be as_attachment=True, not attachment=True. -> Option A

4. Quick Check:

   Correct parameter is as_attachment=True [OK]

Hint: Use as_attachment=True, not attachment=True [OK]

Common Mistakes:

• Using attachment=True instead of as_attachment=True
• Thinking download_name is mandatory
• Believing relative paths are invalid

5. You want to send a PDF file named 'manual.pdf' for download but want the user to save it as 'User_Manual_2024.pdf'. Which Flask code snippet correctly achieves this?

hard

A. return send_file('manual.pdf', as_attachment=True, filename='User_Manual_2024.pdf')

B. return send_file('manual.pdf', as_attachment=False, download_name='User_Manual_2024.pdf')

C. return send_file('manual.pdf', download_name='User_Manual_2024.pdf')

D. return send_file('manual.pdf', as_attachment=True, download_name='User_Manual_2024.pdf')

Solution

1. Step 1: Ensure file is sent as download

   Set as_attachment=True to force download instead of display.

2. Step 2: Set the saved filename

   Use download_name='User_Manual_2024.pdf' to specify the filename suggested to the user.

3. Step 3: Check other options

   return send_file('manual.pdf', as_attachment=False, download_name='User_Manual_2024.pdf') disables download, C misses as_attachment, D uses wrong parameter filename.

4. Final Answer:

   return send_file('manual.pdf', as_attachment=True, download_name='User_Manual_2024.pdf') -> Option D

5. Quick Check:

   Use as_attachment=True and download_name for filename [OK]

Hint: Use as_attachment=True and download_name to rename download [OK]

Common Mistakes:

• Forgetting as_attachment=True disables download
• Using filename instead of download_name
• Setting as_attachment=False causes inline display