text: Introduction
  Flask is one of the most popular web application frameworks written in Python. It is a microframework designed for an easy and quick start. Extending with tools and libraries adds more functionality to Flask for more complex projects.
  This article explains how to install Flask in a virtual testing environment and create a simple Flask application.
  Prerequisites
  - Installed Python 2.7 or Python 3.5 and newer
  - CLI with administrator privileges
  Note: Python 2 has reached the end-of-life maintenance status. It officially no longer has support as of 2020. Follow one of our guides on installing Python 3: How to install Python 3 on CentOS 7, How to install Python 3 on CentOS 8, How to install Python 3 on Ubuntu, How to install Python on Windows.
  Step 1: Install Virtual Environment
  Install Flask in a virtual environment to avoid problems with conflicting libraries. Check Python version before starting:
  - Python 3 comes with a virtual environment module called venv preinstalled. If you have Python 3 installed, skip to Step 2.
  - Python 2 users must install the virtualenv module. If you have Python 2, follow the instructions outlined in Step 1.
  Install virtualenv on Linux
  The package managers on Linux provides virtualenv.
  - For Debian/Ubuntu:
  1. Start by opening the Linux terminal.
  2. Use
  apt to install virtualenv on Debian, Ubuntu and other related distributions:
  sudo apt install python-virtualenv
  - For CentOS/Fedora/Red Hat:
  1. Open the Linux terminal.
  2. Use
  yum to install virtualenv on CentOS, Red Hat, Fedora and related distributions:
  sudo yum install python-virtualenv
  Install virtualenv on MacOS
  1. Open the terminal.
  2. Install virtualenv on Mac using
  pip:
  sudo python2 -m pip install virtualenv
  Install virtualenv on Windows
  1. Open the command line with administrator privileges.
  2. Use
  pip to install virtualenv on Windows:
  py -2 -m pip install virtualenv
  Note: To install pip on Windows, follow our How to install pip on Windows guide.
  Step 2: Create an Environment
  1. Make a separate directory for your project:
  mkdir <project name>
  2. Move into the directory:
  cd <project name>
  3. Within the directory, create the virtual environment for Flask. When you create the environment, a new folder appears in your project directory with the environment’s name.
  Create an Environment in Linux and MacOS
  - For Python 3:
  To create a virtual environment for Python 3, use the venv module and give it a name:
  python3 -m venv <name of environment>
  - For Python 2:
  For Python 2, use the virtualenv module to create a virtual environment and name it:
  python -m virtualenv <name of environment>
  Listing the directory structure with the ls command shows the newly created environment:
  Create an Environment in Windows
  - For Python 3:
  Create and name a virtual environment in Python 3 with:
  py -3 -m venv <name of environment>
  - For Python 2:
  For Python 2, create the virtual environment with the virtualenv module:
  py -2 -m virtualenv <name of environment>
  List the folder structure using the
  dir command:
  dir *<project name>*
  The project directory shows the newly created environment:
  Step 3: Activate the Environment
  Activate the virtual environment before installing Flask. The name of the activated environment shows up in the CLI after activation.
  Activate the Environment on Linux and MacOS
  Activate the virtual environment in Linux and MacOS with:
  . <name of environment>/bin/activate
  Activate the Environment on Windows
  For Windows, activate the virtual environment with:
  <name of environment>Scriptsactivate
  Step 4: Install Flask
  Install Flask within the activated environment using
  pip:
  pip install Flask
  Flask is installed automatically with all the dependencies.
  Note:
  pip is a Python package manager. To install
  pip follow one of our guides: How to install pip on CentOS 7, How to install pip on CentOS 8, How to install pip on Debian, or How to install pip on Ubuntu.
  Step 5: Test the Development Environment
  1. Create a simple Flask application to test the newly created development environment.
  2. Make a file in the Flask project folder called hello.py.
  3. Edit the file using a text editor and add the following code to make an application that prints “Hello world!”:
  from flask import Flask app = Flask(__name__) @app.route('/') def hello_world(): return 'Hello world!'
  Note: Pick any name for the project except flask.py. The Flask library is in a flask.py file.
  4. Save the file and close.
  5. Using the console, navigate to the project folder using the
  cd command.
  6. Set the FLASK_APP environment variable.
  - For Linux and Mac:
  export FLASK_APP=hello.py
  - For Windows:
  setx FLASK_APP “hello.py”
  Note: Windows users must restart the console to set the environment variable. Learn more about setting environment variables by reading one of our guides: How to set environmet variables in Linux, How to set environment variables in MacOS, How to set environment variables in Windows.
  7. Run the Flask application with:
  flask run
  The output prints out a confirmation message and the address.
  8. Copy and paste the address into the browser to see the project running:
  Conclusion
  Flask web applications are easy to configure and run. It is one of the most popular web application frameworks for Python.
  Read about the best Python IDEs and code editors to choose the best environment for further web development with Flask.