Flask project structure conventions - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when you run this Flask app with this structure?

Consider a Flask app with this structure:

myapp/
  app.py
  templates/
    home.html
  static/
    style.css

The app.py contains:

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

@app.route('/')
def home():
    return render_template('home.html')

What will the app do when you visit the root URL?

Flask

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

@app.route('/')
def home():
    return render_template('home.html')

A Serve the style.css file automatically at the root URL.

B Raise a TemplateNotFound error because templates folder is missing.

C Render the home.html template from the templates folder successfully.

D Return a 404 error because no route is defined.

💡 Hint

Flask looks for templates in the templates folder by default.

Flask automatically finds templates in the templates folder inside the project directory. The route '/' renders home.html correctly.

📝 Syntax

intermediate

Which folder is Flask expecting for static files by default?

In a standard Flask project, where should you place CSS, JavaScript, and image files so Flask can serve them automatically?

A assets

B static

C public

D resources

💡 Hint

Flask uses a default folder name for static files.

Flask serves static files from the static folder by default.

🔧 Debug

advanced

Why does this Flask app fail to find the template?

Given this project structure:

myapp/
  app/
    __init__.py
    routes.py
  templates/
    index.html

And app/__init__.py contains:

from flask import Flask
app = Flask(__name__)

from app import routes

But running the app raises TemplateNotFound when rendering index.html. What is the likely cause?

Flask

from flask import Flask
app = Flask(__name__)

from app import routes

A The __init__.py file is missing a route decorator.

B The templates folder must be renamed to 'views' for Flask to find it.

C Flask cannot find templates because the app variable is not named 'application'.

D The Flask app is created in a subpackage, so the templates folder should be inside the app folder.

💡 Hint

Flask looks for templates relative to the app package location.

When the Flask app is created inside a package (app), Flask expects the templates folder inside that package folder. Having templates outside causes TemplateNotFound.

🧠 Conceptual

advanced

What is the purpose of the __init__.py file in a Flask app package?

In a Flask project structured as a package, why do we include an __init__.py file inside the app folder?

A It marks the folder as a Python package and initializes the Flask app instance.

B It contains all the HTML templates for the app.

C It stores static files like CSS and JavaScript.

D It is required to run the Flask development server.

💡 Hint

Think about Python packages and app initialization.

The __init__.py file makes the folder a Python package and usually contains the Flask app creation code.

❓ state_output

expert

How many routes are registered in this Flask app structure?

Given this Flask project structure:

myapp/
  app/
    __init__.py
    routes.py
  run.py

With app/__init__.py:

from flask import Flask
app = Flask(__name__)

from app import routes

And app/routes.py:

from app import app

@app.route('/')
def home():
    return 'Home'

@app.route('/about')
def about():
    return 'About'

And run.py:

from app import app

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

How many routes will the app have when running?

Flask

from flask import Flask
app = Flask(__name__)

from app import routes

# routes.py
from app import app

@app.route('/')
def home():
    return 'Home'

@app.route('/about')
def about():
    return 'About'

# run.py
from app import app

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

A 2 routes: '/' and '/about'

B 3 routes including a default error handler

C No routes registered because routes.py is not imported

D 1 route: '/' only

💡 Hint

Check where routes are defined and imported.

Both routes '/' and '/about' are defined in routes.py, which is imported in __init__.py, so both are registered.

Practice

1. Which folder is conventionally used to store HTML files in a Flask project?

easy

A. templates

B. static

C. app

D. config

Solution

1. Step 1: Understand Flask folder roles

   Flask uses a folder named templates to hold HTML files for rendering views.

2. Step 2: Identify correct folder for HTML

   Since HTML files are templates, they belong in the templates folder, not static or others.

3. Final Answer:

   templates -> Option A

4. Quick Check:

   HTML files go in templates [OK]

Hint: HTML files live in templates folder by Flask convention [OK]

Common Mistakes:

• Confusing static folder for HTML files
• Putting HTML files directly in app folder
• Using config folder for templates

2. Which file is used to initialize a Flask app package inside the app folder?

easy

A. run.py

B. config.py

C. __init__.py

D. app.py

Solution

1. Step 1: Recognize Python package initialization

   Python packages use __init__.py to initialize modules and setup code.

2. Step 2: Apply to Flask app folder

   In Flask, the app folder is a package, so __init__.py initializes the Flask app instance.

3. Final Answer:

   __init__.py -> Option C

4. Quick Check:

   __init__.py initializes app package [OK]

Hint: Use __init__.py to initialize app package in Flask [OK]

Common Mistakes:

• Using run.py to initialize app package
• Confusing config.py with initialization
• Naming app.py as initializer

3. Given this Flask project structure:

myproject/
 ├── app/
 │    ├── __init__.py
 │    ├── routes.py
 │    ├── templates/
 │    │    └── home.html
 │    └── static/
 │         └── style.css
 ├── config.py
 └── run.py

Which file is typically used to start the Flask application?

medium

A. config.py

B. app/__init__.py

C. app/routes.py

D. run.py

Solution

1. Step 1: Identify the purpose of run.py

   In Flask projects, run.py is commonly used as the entry point to start the app server.

2. Step 2: Differentiate from other files

   config.py holds settings, __init__.py initializes app, and routes.py defines routes, but none start the server directly.

3. Final Answer:

   run.py -> Option D

4. Quick Check:

   run.py starts Flask app [OK]

Hint: run.py usually starts the Flask app server [OK]

Common Mistakes:

• Trying to run config.py to start app
• Running __init__.py directly
• Running routes.py as main file

4. You have this run.py file:

from app import app

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

But running python run.py gives an ImportError: cannot import name 'app'. What is the likely cause?

medium

A. run.py is missing the Flask import statement

B. The 'app' variable is not defined in app/__init__.py

C. The templates folder is missing

D. Debug mode cannot be set in run.py

Solution

1. Step 1: Understand the import error

   The error means app is not found inside the app package's __init__.py.

2. Step 2: Check app/__init__.py for app variable

   Usually, app = Flask(__name__) is defined in __init__.py. If missing, import fails.

3. Final Answer:

   The 'app' variable is not defined in app/__init__.py -> Option B

4. Quick Check:

   app must be defined in __init__.py [OK]

Hint: Ensure app variable is defined in __init__.py before importing [OK]

Common Mistakes:

• Assuming missing Flask import causes this error
• Thinking templates folder affects imports
• Believing debug mode setting causes import error

5. You want to organize a Flask project with multiple blueprints for different features. Which structure best supports this while following Flask conventions?

hard

A. myproject/
├── app/
│ ├── __init__.py
│ ├── auth/
│ │ ├── __init__.py
│ │ └── routes.py
│ ├── blog/
│ │ ├── __init__.py
│ │ └── routes.py
│ ├── templates/
│ └── static/
├── config.py
└── run.py

B. myproject/
├── auth/
│ ├── __init__.py
│ └── routes.py
├── blog/
│ ├── __init__.py
│ └── routes.py
├── templates/
├── static/
├── config.py
└── run.py

C. myproject/
├── app.py
├── auth_routes.py
├── blog_routes.py
├── templates/
├── static/
├── config.py
└── run.py

D. myproject/
├── app/
│ ├── routes.py
│ ├── templates/
│ └── static/
├── auth.py
├── blog.py
├── config.py
└── run.py

Solution

1. Step 1: Understand blueprint organization

   Blueprints are best organized as sub-packages inside the main app package, each with its own __init__.py and routes.

2. Step 2: Evaluate options for Flask conventions

   myproject/
    ├── app/
    │    ├── __init__.py
    │    ├── auth/
    │    │    ├── __init__.py
    │    │    └── routes.py
    │    ├── blog/
    │    │    ├── __init__.py
    │    │    └── routes.py
    │    ├── templates/
    │    └── static/
    ├── config.py
    └── run.py

   places blueprints inside app/ folder as sub-packages, keeping templates and static centralized, which is recommended.

3. Final Answer:

   Option A structure best supports multiple blueprints with Flask conventions -> Option A

4. Quick Check:

   Blueprints as sub-packages inside app folder [OK]

Hint: Put blueprints as subfolders inside app with __init__.py [OK]

Common Mistakes:

• Placing blueprints outside app folder
• Using single file for all routes
• Not including __init__.py in blueprint folders