Python Flask Interview Questions

Python Flask is a lightweight WSGI web application framework. It is designed to make getting started quick and easy, with the ability to scale up to complex applications. It began as a simple wrapper around Werkzeug and Jinja and has become one of the most popular Python web application frameworks.

Flask supports Python 3.5 and newer, Python 2.7, and PyPy. You can install flask through pip using below command.

pip install flask

g is an object provided by Flask. It is a global namespace for holding any data you want during a single app context. For example, a before_request handler could set g.user, which will be accessible to the route and other functions.

from flask import g
@app.before_request
def load_user():
    user = User.query.get(request.session.get("user_id"))
    g.user = user

@app.route("/admin")
def admin():
    if g.user is None or not g.user.is_admin:
        return redirect(url_for("index"))

In Flask, an app context lasts for one request / response cycle, g is not appropriate for storing data across requests.

In Flask, flask.request.headers is used to get http headers.

Example

app = flask.Flask(__name__)
@app.route("/")
def index():
    headers = flask.request.headers
    return "Request headers:\n" + str(headers)

app.run(host="127.1.0.1", port=8080)

You can run the below command to check the installed version of the python flask

import flask
flask.__version__

Standard Python logging is used in flask framework for logging. You can log messages about your Flask Application using app.logger, which takes the same name as app.name.

Below is an example to log your own messages in Python Flask

def login():
    user = get_user(request.form['username'])

    if user.check_password(request.form['password']):
        login_user(user)
        app.logger.info('%s logged in successfully', user.username)
        return redirect(url_for('index'))
    else:
        app.logger.info('%s failed to log in', user.username)
        abort(401)

You can use redirect() method to redirect a url in Python Flask. Here is an example code

import os
from flask import Flask,redirect

app = Flask(__name__)

@app.route('/')
def hello():
    return redirect("http://www.abc.com", code=301)

A macro in Flask is a tool to build modular user interface on web pages. Macro in flask contains

• Html file with the macro definition.
• Styles in the homonymous css file.
• Scripts in the homonymous js file with the class for manage of ui element.

WTF in Flask stands for WT Forms which is intended to provide the interactive user interface for the user. The WTF is a built-in module of the flask which provides an alternative way of designing forms in the flask web applications.

jsonify() in Flask is a function in flask. json module. It serializes data to JavaScript Object Notation (JSON) format, wraps it in a Response object with the application/json mimetype. Note that jsonify is sometimes imported directly from the flask module instead of from flask.

You can use request.remote_addr to get visitor IP address in Flask.

Example Code

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-Sijax is an extension for the Flask microframework, to simplify Sijax (PyPi, GitHub) setup and usage for Flask users. Sijax is a Python/jQuery library that makes AJAX easy to use in web applications.

You can use current_user.get_id() to get logged in user id in flask.

Example Code

from flask import g
if current_user.is_authenticated():
        g.user = current_user.get_id()

Below are few important extentions in Flask, that makes development easier and quick

• flask-debugtoolbar
• Flask-WTF & WTForms-Components
• Flask-Login
• Flask-Limiter
• Flask-Philo-SQLAlchemy
• flask-pwa

A WSGI middleware component is a Python callable that is itself a WSGI application, but may handle requests by delegating to other WSGI applications. These applications can themselves be WSGI middleware components.

A middleware component can perform such functions as:

• Routing a request to different application objects based on the target URL, after changing the environment variables accordingly.
• Allowing multiple applications or frameworks to run side-by-side in the same process
• Load balancing and remote processing, by forwarding requests and responses over a network
• Performing content post-processing, such as applying XSLT stylesheets