Interview Questions and Answers

Flask is a Python-based framework that builds web applications. The interface is mostly based on HTTP, REST, GraphQL, or Websockets. This framework is based on the Jinja2 templates engine and WSGI web application library. The creator, Armin Ronacher, developed it for the Pallets project initially.

Yes, Flask is an open-source framework.

Flask framework is based on the Python programming language. It has a quick microframework based on prototyping web and networking applications to execute the code faster. We can use Flask in the below-mentioned cases:

1. When we need to develop an API or ML Model.
2. To control security camera by API abstraction.
3. When we need to work on no-SQL like DynamoDB.
4. When we need to do ElasticSearch.
5. When we need to prepare a microservice adapter to translate SOAP API into JSON.

We can get the Flask development version by using the given below command:

git clone https://github.com/pallets/flask
cd flask && python3 setup.py develop

You can install Flask in a Linux environment using Python package manager, pip.

The default Flask local host is 127.0.0.1 and the default port is 5000.

We can get the string by using the argument's value. This value will be used as the request object in Flask.

For example, we can try this sample code:

from flask import Flask
from flask import request
app=Flask(_name_)
@app.route(“/”)
def index():
  val=request.args.get(“var”)
  return “Hello,World {}”
  if_name_==”_main_”:
    app.run(host=”0.0.0.0”,port=8080)

It has some core features like requests, routing, and blueprints, as well as a few other features like coaching, ORM, and forms. Because of all these features, we call Flask a microframework.

Flask is a better quick development Python framework. It uses cases and perfects prototyping. It is better for lightweight web applications. It is best to develop microservice and server-less applications. In comparison, it has full features that are built-in. Flask is also easy to learn with so many APIs. In Django, there are fewer APIs.

It is the basic idea to complete the response circle in the Flask application. During any request of the CLI command, it maintains all track of application-level data. We can use g object or current _app to access those data. Always Flask forced application context with every request to complete the circle.

There are so many extensions that we can use to create the RESTFul application in Flask. We need to choose them depending upon the requirements.

A few of them are as follows:

1. Flask-RESTFul.
2. Flask-API.
3. Flask-RESTX.
4. Connexion.

Here we also need to use one of the Flask extensions named Flask-Admin. It helps to group all individual views together in classes. Another extension named Flask-Appbuilder can be used. It has a built-in admin interface.

We can debug the Flask application. Every development server has a debugging facility. Flask also comes with one server, so one server is there by default. If we run the method to call the Flask application object, we need to keep the debug mode value true. 

Remember that we need to deactivate the debug mode before deploying; otherwise, a full stack trace will be displayed in a browser. It is not secure as it has confidential details. One extension is also available named Flask-DebugToolbar.

Actually, it can be any length. But there are certain rules which we must follow:

1. The identifier should start with a  character or an underscore, or from A-Z or a-z.
2. A-Z or a-z can be stored as a name in the identifier.
3. Python Flask is case-sensitive.
4. Few keywords are not usable in identifiers like and, false, import, true, del, try, etc.

Basically, we use 5 types of HTTP methods to retrieve data from URLs. They are:

1. GET: It sends the unencrypted data to the server.
2. POST: Post server caches all data except HTML.
3. HEAD: It is similar to GET without any response body.
4. PUT: It can replace current data.
5. DELETE: It can delete the current data requested by any URL.

Yes, it is an MVC( Model View Control) framework. Because it has a feature named session, this helps to remember information from one request to another request. It uses a signed cookie to show the contents of that session to the user. If the user wants to modify. They have to use one secret key named Flask.secret_key in Flask. Flask perfectly behaves like one MVC framework.

We need to run Python files on the shell. The command will be app.debug=True

With Flask, we can create almost all types of web applications. We can create Single Page Application, RESTful API based Applications, SAS applications, Small to medium size websites, static websites, Microservices, and serverless apps.

Flask is so versatile and flexible as it can be integrated with other technologies very quickly to achieve the same. 

For example, Flask can be combined with the NodeJS serverless, AWS lambda, and similar other third party services to build new-age systems.

Yes, we can add an emailing function in the Flask framework. Actually, we need to install the Flask-Mail extension. We can use the given below command:

Pip install Flask-Mail 

Now, after installation, we need to go to the Flask Config API. Under config, we will get Mail-Server, Mail_Port, Mail_username, Mail_Password, etc options. We can use the mail.send() method to compose the message.

from flask_mail import Mail,Message 
from flask import Flaskapp=Flask(_name_)
mail=Mail(app)
@app.route(\"/mail\")
def email():
  Msg=Message(\"Hello Message\",Sender=\"admin@test.com\",recipients=[\"to@test.com\"])
  Mail.send(msg)

Yes, it is built-in with Python as a database. We don’t have to install any extensions. Inside the Python view, we can import SQLite. There we can write SQL queries to interact with a database. Generally, the Python Flask developers use Flask-SQLAlchemy, which makes the SQL queries easier. It has one ORM, which helps to interact with the SQLite database.

Thread local Flask objects are mostly available in a valid request context. Because of this, we don’t need to pass objects from one method to another. Because in Flask thread safety is declared out of the box. We can access the objects by a command like the current _app.

We can use the request object, see the given below code:

From flask import Flask
From flask import request
app=Flask(_name_)
@app.route(“/”)
Def index():
  val=request.args.get(“var”)
  user_agent=request.headers.get(‘User-Agent’)
 response = """
     <p>
    Hello, World! {}
    <br/>
    You are accessing this app with {}
    </p>""".format(val, user_agent)
    return response

Hello, World!{}

you are accessing this app with {}

if_name_==”_main_”;            
app.run(host=”0.0.0.0”,port=8080)

We need to call the view function with parameters and give them values to generate URLs. Mainly Flask url_for the function we use here. It can also be used in Flask templates.

The session is mainly used to store data in requests. We can store and get data from the session object in Flask. As shown below code we can try to do this:

From flask import Flask
sessionapp=Flask(_name_)
@app.route(‘/use_session’)
Def use_session():                 
  If ’song’ not in session;                 
    session[;songs’]={‘title’;’Tapestry’,’Bruno Major’}
  return session.get(‘songs’)

@app.route(‘/delete_session’)
def delete_session():                   
  session.pop(‘song’,None)                   
  return “removed song from session”

We can do it in three ways, they are:

1. after_request(): It helps make a request and passes the response, which will be sent to the client.
2. before-request(): It is called before the request without any argument passing.
3. teardown_request(): In case we get an exception, then this connection will be used and response is not guaranteed.

The full form of WSGI is Web Server Gateway Interface. This is actually a Python standard in PEP 3333. It provides the protocol for web servers to communicate with a web application. 

It mainly plays a role at the time of project deployment.

It provides an easy integration process with WTForms.

A few features are as follows:

1. Integration with WTForms.
2. CSRF token provides security globally.
3. Provide Recaptcha.
4. File upload facility.

We can integrate it easily with the help of the Flask extension named Flask-Social. It gives multiple access to users for other social platforms also. One thing we need to remember is that we have to use the Flask-Security extension of Flask for security purposes. For this, we need to install all social API libraries in Python. We have to register from an external API service provider.

The Flask template engine allows developers to create HTML templates with placeholders for dynamic data. Actually, a template is a file that contains two types of data, one is static and another is dynamic. Mostly this dynamic template is popular because the data is in run time. Flask allows the Jinja2 template engine to be used mostly as a template engine. Flask’s render_template method needs parameters and their values. 

We have to use the Flask- extension to implement forms in Flask. It’s called WTForms. It’s a Python-based rendering and validation library. It does data validation, CSRF protection, and internationalization. Once we use Flask-uploads, ReCaptcha from Flask-WTF helps to upload files. We can also manage JavaScript requests and customization of error responses.

If we want to hold any data during the application context, the Flask g object is used as a global namespace for keeping any data. It’s not suitable for storing data within requests. Actually, this letter g stands for global. Suppose we need to store a global variable in an application context, then g object is best in place of creating global variables. Here the g object will work as a request in a separate Flask g object. It always saves self-defined global variables.

It can be created in two easy steps. They are:

1. It can be created on its own when the application receives a request from the system.
2. We can do it manually by calling app.test_request_context

It’s an inbuilt library in Python, making it easy for Ajax to work in web applications. Sijax uses JSON while passing data to a server called by the browser.

We need to follow the steps below to structure a huge big flask application:

1. We need to move the functions to different files until the applications get started.
2. We need to use the blueprint to view the categories like auth and profile.
3. We need to register all functions on a central URL map using the Werkzeug URL.

This is one of the functions under the flask.json module. It can convert data to JSON and store it in the response object. It provides a response object with an application where json.dumps() only returns a JSON data string.

Memory management handles memory allocation in Python. Flask has a built-in garbage collector. It collects all wastage data and makes the space free. It manages memory by private heap space. This is not accessible by developers.

Few of them are accessed by users by using a few API tools. All data allocation is done by Flask memory management.

A Validator can take the input to check if it meets some criteria like string limit with returns. In case of failure, a validation error will come. This is a straightforward method. In Flask there are a few validator classes of WTForms. They are:

1. DataRequired: Checks the input field.
2. Email: It checks the email id conventions.
3. IP Address: It checks the IP address.
4. Length: It validates the string length with the given range.
5. NumberRange: It validates the number in the input field with the given range.
6. URL: Checks the URL input field.

Yes, we can do it. Request.remoote_addr is useful to get the IP address in Flask. An example is given below:

From flask import request
From flask import jsonify

@app.route(\"/get_user_ip\",method=[\"GET\"]
def get_user_ip():
  return jsonify({\'ip\': request.remote_addr}), 200

Generally, an HTTP error code will be returned when one error comes. Suppose the error code is within 400 to 499; then it is sure that a mistake is happening in the client-side request. Otherwise, if the error code is within 500 to 599, the error comes from a server-side request.

HTTP error code can show custom error pages to the user. This HTTPS error code is not set to the error handler’s code. While returning a message from the error handler, we have to include it.