Python Flask Interview Questions

Flask is a light-weight, and Popular web application Framework. It is written in Python Programming Language. It was developed by Armin Ronacher in 2010. Flask is based on WSGI (Web Server Gateway Interface) toolkit and Jinja2 template engine. It provides you with tools, libraries and technologies that allow you to build a web application.

WSGI stands for Web server Gateway Interface.  It is a specification that describes how a web server communicates with web applications, and how web applications can be chained together to process one request.

Python flask is used for creating API’s, blog or wiki-like platform.

Flask g name stands for “global” which is referring to data being global within a context. The data saved in g is lost after the context ends, and it is not an appropriate place to store data between requests.

• Flask comes with built-in development server as well as fast debugger
• integrated support for unit testing
• Restful request dispatching
• Comes with Jinja2 templating technique
• It supports secure cookies i.e. client-side sessions
• Also has the WSGI 1.0 compliant feature.
• It is based on Unicode.
• Google App Engine compatibility
• Python Flask is extensively documented.

Werkzeug is a utility library meant for usage with the Python language. Mostly, it is a Web Server Gateway Interface or WSGI app that can create software items for request, response, or utility functions.

Jinja is a template engine for Python programming purposes, and it resembles the Django web frameworks templates.

Flask-WTF offers simple integration with WTForms.

Features include for Flask WTF are

• Integration with wtforms
• Secure form with csrf token
• Global csrf protection
• Internationalization integration
• Recaptcha supporting
• File upload that works with Flask Uploads

In Flask, a session allows us to remember information from one request to another. It uses a signed cookie so the user can look at the session contents. We can access session by using the secret key Flask.secret_key in the Flask framework.

Flask provides three ways to establish database connection. These are given below.

before_request() : It is called before a request and requires no arguments.

after_request() : It is called after a request and pass the response that will be sent to the client

teardown_request(): It is used when exception is raised and response are not guaranteed. It is called after the response and not allowed to modify the request or their values.

Flask Sijax is Python/jQuery library to perform Ajax function in Flask.

You can install Flask-Sijax by using below command

easy_install flask-sijax

pip install flask-sijax

HTTPs methods are used to retrieve data from an URL:

GET: The GET is the method that sends data to the server unencrypted.

HEAD:  HEAD is similar to GET, but that it has no response body.

POST: The POST server does not cache the HTML form data that it sendsPUT: It is the method in which the uploaded content replaces current data representations of the target resources.

DELETE: This method removes the current representations of the target resource that is suggested by a URL.

Flask can be stated as a micro framework, which is solely built for a small application, which has simpler requirements. In flask, the users have to use external libraries. Flask is always ready to use.

Pyramid is built for larger application as it provides flexibility and allows the developer use the right features for their project. The developer can choose the database, templating style URL structure and more. Pyramid is therefore heavy configurable.

Redirect() function is used to display the login page again when a login attempt fails.

{% … %}: It is used for Statements

{{ … }}: It is used for Expressions to print to the template output

{# … #}: It is used for Comments not included in the template output

# … ## : It is used for Line Statements

• HTTP_500_INTERNAL_SERVER_ERROR
• HTTP_501_NOT_IMPLEMENTED
• HTTP_502_BAD_GATEWAY
• HTTP_503_SERVICE_UNAVAILABLE
• HTTP_504_GATEWAY_TIMEOUT
• HTTP_505_HTTP_VERSION_NOT_SUPPORTED
• HTTP_511_NETWORK_AUTHENTICATION_REQUIRED

current_user.get_id() is used to get logged in user id in flask.

Here is an example:

from flask import g

if current_user.is_authenticated():

        g.user = current_user.get_id()

request.remote_addr is used to get visitor IP address in Flask. Here is an example:

from flask import request

from flask import jsonify

@app.route(“/get_user_ip”, methods=[“GET”])

def get_user_ip():

    return jsonify({‘ip’: request.remote_addr}), 200

Flask Python is a collection of private heap spaces, which holds all objects and data structures together. Programmers cannot access it. It is the task of the interpreter to manage it. But in the core API, users can access some of the tools. The Flask Python memory manager controls its allocation. Also, an inbuilt garbage collector is present which recycles all unused memory so it is made available for the heap space.

We can get a query string from the flask by using following function.

@app.route(‘/data’)

    def data ( ) :

user = request.arg.get (‘user’)

To make a portable and serialized representations of Python objects, we have the module known as pickle which accepts a Python object (basically everything in Python is an object) and then converts it into a string type, and after that uses the dump () function to dump it into a file. We term this as pickling. On the contrary, retrieving objects from the stored string forms is called as unpickling.

There are two ways by which users can enable debugging in Flask. They are as follows:

By setting the flag on the applications object

By passing the flag as a parameter to run. If the user is enabling to debug support, the server will reload it when the code will change and the user doesn’t have to restart after each change made in the code.

In Flask, the validators class of WTForm are listed in below table:

DataRequired:  To check whether input field is empty

Email: To check whether text in the field follows email ID conventions.

IPAddress: To validate IP address in input field

Length: To verify if length of string in input field is in given range

NumberRange: To validates a number in input field within given range

URL: To validates URL entered in input field

5000-Default port

App routing in Flask is used to map a specific URL with the associated function that is intended to perform some task.

• HTTP_400_BAD_REQUEST
• HTTP_401_UNAUTHORIZED
• HTTP_402_PAYMENT_REQUIRED
• HTTP_403_FORBIDDEN
• HTTP_404_NOT_FOUND
• HTTP_405_METHOD_NOT_ALLOWED
• HTTP_406_NOT_ACCEPTABLE
• HTTP_407_PROXY_AUTHENTICATION_REQUIRED
• HTTP_408_REQUEST_TIMEOUT
• HTTP_409_CONFLICT
• HTTP_410_GONE
• HTTP_411_LENGTH_REQUIRED
• HTTP_412_PRECONDITION_FAILED
• HTTP_413_REQUEST_ENTITY_TOO_LARGE
• HTTP_414_REQUEST_URI_TOO_LONG
• HTTP_415_UNSUPPORTED_MEDIA_TYPE
• HTTP_416_REQUESTED_RANGE_NOT_SATISFIABLE
• HTTP_417_EXPECTATION_FAILED
• HTTP_428_PRECONDITION_REQUIRED
• HTTP_429_TOO_MANY_REQUESTS
• HTTP_431_REQUEST_HEADER_FIELDS_TOO_LARGE