File download responses in Flask - Mini Project: Build & Apply

File Download Responses with Flask

📖 Scenario: You are building a simple web app using Flask. The app should allow users to download a text file when they visit a specific URL.

🎯 Goal: Create a Flask app that sends a file download response with a text file named example.txt containing some content.

📋 What You'll Learn

Create a Flask app instance named app

Create a route /download that triggers the file download

Use Flask's Response object to send the file content

Set the correct headers to make the browser download the file as example.txt

💡 Why This Matters

🌍 Real World

Web apps often need to let users download files like reports, images, or documents. This project shows how to send such files correctly.

💼 Career

Backend developers frequently implement file download endpoints. Knowing how to set response headers properly is essential for user-friendly downloads.

1

Set up Flask app and file content

Import Flask and create a Flask app instance called app. Then create a variable file_content with the string value 'Hello, this is a sample file for download.'.

Flask

# Your code here

Hint

Start by importing Flask and creating the app instance. Then assign the exact string to file_content.

2

Create download route and import Response

Import Response from flask. Then create a route /download using @app.route('/download') and define a function download_file().

Flask

from flask import Flask
from flask import Response

app = Flask(__name__)

file_content = 'Hello, this is a sample file for download.'

# Create route /download and function download_file
# Your code here

Hint

Remember to import Response and use the exact route and function names.

3

Create Response with file content and headers

Inside download_file(), create a Response object named response with file_content as data and mimetype='text/plain'. Then set the header Content-Disposition to 'attachment; filename=example.txt' on response.headers. Finally, return response.

Flask

from flask import Flask
from flask import Response

app = Flask(__name__)

file_content = 'Hello, this is a sample file for download.'

@app.route('/download')
def download_file():
    # Create Response with file_content and mimetype
    # Set Content-Disposition header
    # Return response
    # Your code here

Hint

Use the Response constructor with the file content and set the header exactly as shown.

4

Add app run block for local testing

Add the standard Flask app run block: if __name__ == '__main__': and inside it call app.run(debug=true).

Flask

from flask import Flask
from flask import Response

app = Flask(__name__)

file_content = 'Hello, this is a sample file for download.'

@app.route('/download')
def download_file():
    response = Response(file_content, mimetype='text/plain')
    response.headers['Content-Disposition'] = 'attachment; filename=example.txt'
    return response

# Add app run block below
# Your code here

Hint

This block allows you to run the Flask app locally for testing.

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