Serving images in Flask - Cheat Sheet & Quick Revision

Recall & Review

beginner

How do you serve a static image file in Flask?

Place the image in the 'static' folder and use the URL '/static/filename'. Flask automatically serves files from this folder.

intermediate

What Flask function helps to send a file from a folder dynamically?

The function send_from_directory() sends a file from a specified folder, useful for serving images outside the static folder.

beginner

Why should images be served from the 'static' folder in Flask?

Because Flask is configured to serve files from 'static' automatically, making it simple and efficient without extra code.

intermediate

How can you serve an image stored outside the 'static' folder in Flask?

Use send_from_directory() with the folder path and filename to serve the image dynamically.

intermediate

What HTTP header is important when serving images to browsers?

The Content-Type header tells the browser the file type, like 'image/png' or 'image/jpeg', so it displays correctly.

Where should you place images to serve them automatically in Flask?

A In the 'static' folder

B In the 'templates' folder

C In the root folder

D In the 'images' folder

Flask serves files automatically from the 'static' folder.

Which Flask function sends a file from a specific folder?

A render_template()

B send_file()

C send_from_directory()

D url_for()

send_from_directory() sends files from a folder dynamically.

What URL path is used to access a static image named 'cat.png'?

A /files/cat.png

B /images/cat.png

C /cat.png

D /static/cat.png

Static files are accessed under '/static/filename'.

What content type should be set for a JPEG image?

A image/jpeg

B text/html

C application/json

D image/png

JPEG images use the 'image/jpeg' content type.

If an image is outside the 'static' folder, how do you serve it?

A Use render_template()

B Use send_from_directory()

C Move it to 'templates'

D Use url_for() only

Use send_from_directory() to serve files outside 'static'.

Explain how Flask serves images placed in the 'static' folder and how you would reference them in HTML.

Think about the folder Flask uses for static files and how URLs map to it.

Describe how to serve an image file located outside the 'static' folder using Flask code.

Look for a Flask function that sends files from any folder.

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