Serving images in Flask - Mini Project: Build & Apply

Serving images with Flask

📖 Scenario: You are building a simple web app using Flask that shows a picture on a webpage. The picture file is stored in a folder inside your project.

🎯 Goal: Create a Flask app that serves an image file from a folder called static/images and displays it on the homepage.

📋 What You'll Learn

Create a Flask app instance

Set up a route for the homepage at /

Use the url_for function to link to the image in the template

Store the image file in static/images folder

Render an HTML template that shows the image

💡 Why This Matters

🌍 Real World

Serving images is a common need in web apps for user profiles, product pictures, or blog posts.

💼 Career

Knowing how to serve static files like images is essential for backend web developers working with Flask or similar frameworks.

1

Create the Flask app and homepage route

Write code to import Flask, create an app instance called app, and define a route for / that returns the text 'Hello, Flask!'.

Flask

# Your code here

Hint

Start by importing Flask and creating the app instance. Then use @app.route('/') to make the homepage.

2

Add a configuration variable for the image filename

Create a variable called image_filename and set it to the string 'cat.jpg'. This will be the image file you want to serve.

Flask

from flask import Flask

app = Flask(__name__)

image_filename = ''  # Set the image filename here

@app.route('/')
def home():
    return 'Hello, Flask!'

Hint

Just create a variable named image_filename and assign the exact string 'cat.jpg'.

3

Use url_for to link the image in the homepage route

Import url_for from flask. Change the home function to return an HTML string that includes an <img> tag. Use url_for('static', filename='images/' + image_filename) as the src attribute value.

Flask

from flask import Flask, url_for

app = Flask(__name__)

image_filename = 'cat.jpg'

@app.route('/')
def home():
    # Return HTML with an  tag using url_for
    # Your code here

Hint

Use an f-string to build the HTML string. The src attribute should call url_for('static', filename='images/' + image_filename).

4

Add the static folder and place the image file

Create a folder named static/images in your project directory. Place the image file named cat.jpg inside static/images. This allows Flask to serve the image file when requested.

Flask

# Create the folder static/images and add cat.jpg image file there
# Your code here

Hint

Flask automatically serves files from the static folder. Make sure your image is inside static/images/cat.jpg.

Practice

1. What is the main purpose of using send_from_directory in a Flask app when serving images?

easy

A. To send image files from a specific folder to the user's browser

B. To create new image files dynamically

C. To convert images to text format

D. To upload images from the user to the server

Solution

1. Step 1: Understand the role of send_from_directory

   This function is designed to send files stored in a folder on the server to the client browser.

2. Step 2: Identify its use with images

   When serving images, it helps Flask deliver image files from a folder to display in the browser.

3. Final Answer:

   To send image files from a specific folder to the user's browser -> Option A

4. Quick Check:

   send_from_directory serves files = D [OK]

Hint: Remember: send_from_directory sends files from folders [OK]

Common Mistakes:

• Thinking it creates or modifies images
• Confusing it with file upload functions
• Assuming it converts image formats

2. Which of the following is the correct way to import send_from_directory in a Flask app?

easy

A. from flask import send_file

B. from flask import send_from_directory

C. import send_from_directory from flask

D. from flask.static import send_from_directory

Solution

1. Step 1: Recall Flask import syntax

   Flask functions are imported using from flask import function_name.

2. Step 2: Match the function name

   The correct function name is send_from_directory, so the import is from flask import send_from_directory.

3. Final Answer:

   from flask import send_from_directory -> Option B

4. Quick Check:

   Correct import syntax = B [OK]

Hint: Use 'from flask import function' to import Flask helpers [OK]

Common Mistakes:

• Using 'import send_from_directory from flask' (wrong syntax)
• Importing send_file instead of send_from_directory
• Importing from flask.static which doesn't contain it

3. Given this Flask route code, what will happen when accessing /image/cat.png if the file exists in the 'static/images' folder?

from flask import Flask, send_from_directory
app = Flask(__name__)

@app.route('/image/<filename>')
def image(filename):
    return send_from_directory('static/images', filename)

medium

A. The image 'cat.png' will be sent and displayed in the browser

B. A 404 error because the route is incorrect

C. The server will crash with a syntax error

D. The image will be uploaded to the server

Solution

1. Step 1: Analyze the route and function

   The route captures a filename and uses send_from_directory to send that file from 'static/images'.

2. Step 2: Check the file existence and response

   If 'cat.png' exists in 'static/images', Flask will send it to the browser to display.

3. Final Answer:

   The image 'cat.png' will be sent and displayed in the browser -> Option A

4. Quick Check:

   send_from_directory sends existing files = A [OK]

Hint: If file exists in folder, send_from_directory serves it [OK]

Common Mistakes:

• Assuming route syntax is wrong
• Confusing sending with uploading
• Expecting server crash without syntax errors

4. Identify the error in this Flask code snippet meant to serve images from 'images' folder:

from flask import Flask, send_from_directory
app = Flask(__name__)

@app.route('/img/<filename>')
def img(filename):
    return send_from_directory('image', filename)

medium

A. Route decorator syntax is invalid

B. send_from_directory is not imported correctly

C. Function name 'img' is not allowed

D. The folder name 'image' is likely incorrect; should be 'images'

Solution

1. Step 1: Check folder name consistency

   The route suggests images are in 'images' folder, but code uses 'image' (singular), likely a typo causing file not found.

2. Step 2: Verify other parts

   Imports and route syntax are correct; function name is valid in Python.

3. Final Answer:

   The folder name 'image' is likely incorrect; should be 'images' -> Option D

4. Quick Check:

   Folder name mismatch causes errors = C [OK]

Hint: Check folder names carefully for typos [OK]

Common Mistakes:

• Assuming function names must be special
• Thinking route syntax is wrong
• Ignoring folder name typos

5. You want to serve user-uploaded images stored in a folder outside your Flask app directory called /var/www/uploads. Which is the best way to serve these images securely?

hard

A. Directly link to the folder path in HTML without a Flask route

B. Copy images into the app's static folder and serve from there

C. Use send_from_directory('/var/www/uploads', filename) in a route with filename validation

D. Use send_file without checking filename

Solution

1. Step 1: Understand serving files outside app folder

   Flask's send_from_directory can serve files from any folder if given the full path.

2. Step 2: Importance of filename validation

   To avoid security risks, validate filenames to prevent path traversal attacks.

3. Step 3: Evaluate other options

   Copying files is inefficient; direct linking won't work without web server config; send_file without checks is unsafe.

4. Final Answer:

   Use send_from_directory('/var/www/uploads', filename) in a route with filename validation -> Option C

5. Quick Check:

   Secure serving outside app folder = A [OK]

Hint: Use send_from_directory with validation for external folders [OK]

Common Mistakes:

• Serving files without validating filename
• Linking directly to server paths in HTML
• Copying files unnecessarily