The Flask Mega-Tutorial Part I: Hello, World!

Welcome! You are about to start on a journey to learn how to create web applications with Python and the Flask framework. In this first chapter, you are going to learn how to set up a Flask project. By the end of this chapter you are going to have a simple Flask web application running on your computer!

For your reference, below is a list of the articles in this series.

All the code examples presented in this book are hosted on a GitHub repository. Downloading the code from GitHub can save you a lot of typing, but I strongly recommend that you type the code yourself, at least for the first few chapters. Once you become more familiar with Flask and the example application you can access the code directly from GitHub if the typing becomes too tedious.

At the beginning of each chapter, I'm going to give you three GitHub links that can be useful while you work through the chapter. The Browse link will open the GitHub repository for Microblog at the place where the changes for the chapter you are reading were added, without including any changes introduced in future chapters. The Zip link is a download link for a zip file including the entire application up to and including the changes in the chapter. The Diff link will open a graphical view of all the changes that were made in the chapter you are about to read.

The GitHub links for this chapter are: Browse, Zip, Diff.

Installing Python

If you don't have Python installed on your computer, go ahead and install it now. If your operating system does not provide you with a Python package, you can download an installer from the Python official website. If you are using Microsoft Windows along with WSL or Cygwin, note that you will not be using the Windows native version of Python, but a Unix-friendly version that you need to obtain from Ubuntu (if you are using WSL) or from Cygwin.

To make sure your Python installation is functional, you can open a terminal window and type python3, or if that does not work, just python. Here is what you should expect to see:

$ python3
Python 3.9.6 (default, Jul 10 2021, 16:13:29)
[Clang 12.0.0 (clang-1200.0.32.29)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> _

The Python interpreter is now waiting at an interactive prompt, where you can enter Python statements. In future chapters you will learn what kinds of things this interactive prompt is useful for. But for now, you have confirmed that Python is installed on your system. To exit the interactive prompt, you can type exit() and press Enter. On the Linux and Mac OS X versions of Python you can also exit the interpreter by pressing Ctrl-D. On Windows, the exit shortcut is Ctrl-Z followed by Enter.

Installing Flask

The next step is to install Flask, but before I go into that I want to tell you about the best practices associated with installing Python packages.

In Python, packages such as Flask are available in a public repository, from where anybody can download them and install them. The official Python package repository is called PyPI, which stands for Python Package Index (some people also refer to this repository as the "cheese shop"). Installing a package from PyPI is very simple, because Python comes with a tool called pip that does this work.

To install a package on your machine, you use pip as follows:

$ pip install <package-name>

Interestingly, this method of installing packages will not work in most cases. If your Python interpreter was installed globally for all the users of your computer, chances are your regular user account is not going to have permission to make modifications to it, so the only way to make the command above work is to run it from an administrator account. But even without that complication, consider what happens when you install a package as above. The pip tool is going to download the package from PyPI, and then add it to your Python installation. From that point on, every Python script that you have on your system will have access to this package. Imagine a situation where you have completed a web application using version 1.1 of Flask, which was the most current version of Flask when you started, but now has been superseded by version 2.0. You now want to start a second application, for which you'd like to use the 2.0 version, but if you replace the 1.1 version that you have installed you risk breaking your older application. Do you see the problem? It would be ideal if it was possible to have Flask 1.1 installed and accessible to your old application, while also install Flask 2.0 for your new one.

To address the issue of maintaining different versions of packages for different applications, Python uses the concept of virtual environments. A virtual environment is a complete copy of the Python interpreter. When you install packages in a virtual environment, the system-wide Python interpreter is not affected, only the copy is. So the solution to have complete freedom to install any versions of your packages for each application is to use a different virtual environment for each application. Virtual environments have the added benefit that they are owned by the user who creates them, so they do not require an administrator account.

Let's start by creating a directory where the project will live. I'm going to call this directory microblog, since that is the name of the application:

$ mkdir microblog
$ cd microblog

Support for virtual environments is included in all recent versions of Python, so all you need to do to create one is this:

$ python3 -m venv venv

With this command, I'm asking Python to run the venv package, which creates a virtual environment named venv. The first venv in the command is the name of the Python virtual environment package, and the second is the virtual environment name that I'm going to use for this particular environment. If you find this confusing, you can replace the second venv with a different name that you want to assign to your virtual environment. In general I create my virtual environments with the name venv in the project directory, so whenever I cd into a project I find its corresponding virtual environment.

Note that in some operating systems you may need to use python instead of python3 in the command above. Some installations use python for Python 2.x releases and python3 for the 3.x releases, while others map python to the 3.x releases.

After the command completes, you are going to have a directory named venv where the virtual environment files are stored.

Now you have to tell the system that you want to use this virtual environment, and you do that by activating it. To activate your brand new virtual environment you use the following command:

$ source venv/bin/activate
(venv) $ _

If you are using a Microsoft Windows command prompt window, the activation command is slightly different:

$ venv\Scripts\activate
(venv) $ _

When you activate a virtual environment, the configuration of your terminal session is modified so that the Python interpreter stored inside it is the one that is invoked when you type python. Also, the terminal prompt is modified to include the name of the activated virtual environment. The changes made to your terminal session are all temporary and private to that session, so they will not persist when you close the terminal window. If you work with multiple terminal windows open at the same time, it is perfectly fine to have different virtual environments activated on each one.

Now that you have a virtual environment created and activated, you can finally install Flask in it:

(venv) $ pip install flask

If you want to confirm that your virtual environment now has Flask installed, you can start the Python interpreter and import Flask into it:

>>> import flask
>>> _

If this statement does not give you any errors you can congratulate yourself, as Flask is installed and ready to be used.

Note that the above installation commands does not specify which version of Flask you want to install. The default when no version is specified is to install the latest version available in the package repository. This tutorial can be followed with Flask versions 1 and 2. The above command will install the latest 2.x version. If for any reason you prefer to follow this tutorial on a 1.x release of Flask, you can use the following command to install the latest 1.x version:

(venv) % pip install "flask<2"

A "Hello, World" Flask Application

If you go to the Flask website, you are welcomed with a very simple example application that has just five lines of code. Instead of repeating that trivial example, I'm going to show you a slightly more elaborate one that will give you a good base structure for writing larger applications.

The application will exist in a package. In Python, a sub-directory that includes a __init__.py file is considered a package, and can be imported. When you import a package, the __init__.py executes and defines what symbols the package exposes to the outside world.

Let's create a package called app, that will host the application. Make sure you are in the microblog directory and then run the following command:

(venv) $ mkdir app

The __init__.py for the app package is going to contain the following code:

app/__init__.py: Flask application instance

from flask import Flask

app = Flask(__name__)

from app import routes

The script above simply creates the application object as an instance of class Flask imported from the flask package. The __name__ variable passed to the Flask class is a Python predefined variable, which is set to the name of the module in which it is used. Flask uses the location of the module passed here as a starting point when it needs to load associated resources such as template files, which I will cover in Chapter 2. For all practical purposes, passing __name__ is almost always going to configure Flask in the correct way. The application then imports the routes module, which doesn't exist yet.

One aspect that may seem confusing at first is that there are two entities named app. The app package is defined by the app directory and the __init__.py script, and is referenced in the from app import routes statement. The app variable is defined as an instance of class Flask in the __init__.py script, which makes it a member of the app package.

Another peculiarity is that the routes module is imported at the bottom and not at the top of the script as it is always done. The bottom import is a workaround to circular imports, a common problem with Flask applications. You are going to see that the routes module needs to import the app variable defined in this script, so putting one of the reciprocal imports at the bottom avoids the error that results from the mutual references between these two files.

So what goes in the routes module? The routes are the different URLs that the application implements. In Flask, handlers for the application routes are written as Python functions, called view functions. View functions are mapped to one or more route URLs so that Flask knows what logic to execute when a client requests a given URL.

Here is the first view function for this application, which you need to write in a new module named app/routes.py:

app/routes.py: Home page route

from app import app

def index():
    return "Hello, World!"

This view function is actually pretty simple, it just returns a greeting as a string. The two strange @app.route lines above the function are decorators, a unique feature of the Python language. A decorator modifies the function that follows it. A common pattern with decorators is to use them to register functions as callbacks for certain events. In this case, the @app.route decorator creates an association between the URL given as an argument and the function. In this example there are two decorators, which associate the URLs / and /index to this function. This means that when a web browser requests either of these two URLs, Flask is going to invoke this function and pass the return value of it back to the browser as a response. If this does not make complete sense yet, it will in a little bit when you run this application.

To complete the application, you need to have a Python script at the top-level that defines the Flask application instance. Let's call this script microblog.py, and define it as a single line that imports the application instance:

microblog.py: Main application module

from app import app

Remember the two app entities? Here you can see both together in the same sentence. The Flask application instance is called app and is a member of the app package. The from app import app statement imports the app variable that is a member of the app package. If you find this confusing, you can rename either the package or the variable to something else.

Just to make sure that you are doing everything correctly, below you can see a diagram of the project structure so far:


Believe it or not, this first version of the application is now complete! Before running it, though, Flask needs to be told how to import it, by setting the FLASK_APP environment variable:

(venv) $ export FLASK_APP=microblog.py

If you are using the Microsoft Windows command prompt, use set instead of export in the command above.

Are you ready to be blown away? You can run your first web application, with the following command:

(venv) $ flask run
 * Serving Flask app 'microblog.py' (lazy loading)
 * Environment: production
   WARNING: This is a development server. Do not use it in a production deployment.
   Use a production WSGI server instead.
 * Debug mode: off
 * Running on (Press CTRL+C to quit)

After the server initializes it will wait for client connections. The output from flask run indicates that the server is running on IP address, which is always the address of your own computer. This address is so common that is also has a simpler name that you may have seen before: localhost. Network servers listen for connections on a specific port number. Applications deployed on production web servers typically listen on port 443, or sometimes 80 if they do not implement encryption, but access to these ports requireis administration rights. Since this application is running in a development environment, Flask uses the freely available port 5000. Now open up your web browser and enter the following URL in the address field:


Alternatively you can use this other URL:


Do you see the application route mappings in action? The first URL maps to /, while the second maps to /index. Both routes are associated with the only view function in the application, so they produce the same output, which is the string that the function returns. If you enter any other URL you will get an error, since only these two URLs are recognized by the application.

Hello, World!

When you are done playing with the server you can just press Ctrl-C to stop it.

Congratulations, you have completed the first big step to become a web developer!

Before I end this chapter, I will do one more thing. Since environment variables aren't remembered across terminal sessions, you may find tedious to always have to set the FLASK_APP environment variable when you open a new terminal window. Starting with version 1.0, Flask allows you to register environment variables that you want to be automatically imported when you run the flask command. To use this option you have to install the python-dotenv package:

(venv) $ pip install python-dotenv

Then you can just write the environment variable name and value in a file named .flaskenv located in the top-level directory of the project:

.flaskenv: Environment variables for flask command



  • #501 Miguel Grinberg said 2021-03-28T18:27:52Z

    @app: this is a very common pattern. You should learn it and understand the different roles "app" plays in here. And if you still find it confusing, you are welcome to use your renamed application instance.

  • #502 tech psycho said 2021-04-17T10:30:15Z

    Awesome tutorial

  • #503 Vamsi Ramineedi said 2021-05-23T09:49:30Z

    Hi Miguel,

    Thanks for making these tutorials freely available. I am starting to learn flask now. Are these tutorials still valid, since it has been 3.5 years from the date these were made (or) would you recommend any updated tutorials/books?

    • Vamsi
  • #504 Miguel Grinberg said 2021-05-23T10:00:38Z

    @Vamsi: The tutorial is updated regularly to keep up with new releases. I'm currently working on some minor updates after the Flask 2.0 release, but overall everything is up to date and continues to work.

  • #505 fusion1275 said 2021-06-02T13:58:57Z

    Hi, I would just like to thank you your work on this tutorial. Personally, I am learning from scratch the basics of Flask and how I can implement it into a few ideas in my head. You have provided me the information needed and because you are keeping this up to date your site is always my go to.

    I know life gets busy but I can hopefully speak for anyone that comes across this site is that please please do stay focused on this and keep it up to date as you are helping a lot of people. Specially me :o)

    Many many thanks.

  • #506 L.Wilson said 2021-06-11T16:38:13Z

    Not sure how to go about making/saving a .env file. What kind of extension does it get saved with? Where should it be saved? Does Python automatically recognize and run this type of file?

    I appreciate the tutorial! I have gotten your code to work, while the Visual Studio tutorial code fails with no good explanation or what's going on or how to fix it. Thank you.

  • #507 Miguel Grinberg said 2021-06-11T22:51:50Z

    @L: The file is named .env. It is an empty name, a dot, and a env file extension. It is a text file, so you can write it in your text editor or IDE. It has to be saved in the root directory of your project, as indicated in this article. Python does not recognize this file, that is why I'm asking you to install the python-dotenv package.

  • #508 Edik Mkoyan said 2021-08-02T15:38:54Z

    Finally I've found someone who can explain things in simple words. A sign of true intelligence. Thanks a lot.

  • #509 muro said 2021-09-09T12:57:32Z

    Thanks a lot for a great tutorial!!!

  • #510 Mike said 2021-09-13T17:20:42Z

    @ Miguel Grinberg

    I have used this tutorial off and on for several years, but I am running into an error that I have not encountered before. Below is a copy from the command line. Working in VSCode WSL if that matters.

    (venv) mike@DESKTOP-N0GKBM8:~/microblog$ export FLASK_APP=microblog.py (venv) mike@DESKTOP-N0GKBM8:~/microblog$ flask run * Serving Flask app 'microblog.py' (lazy loading) * Environment: production WARNING: This is a development server. Do not use it in a production deployment. Use a production WSGI server instead. * Debug mode: off Usage: flask run [OPTIONS] Try 'flask run --help' for help.

    Error: Failed to find Flask application or factory in module 'microblog'. Use 'FLASK_APP=microblog:name' to specify one. (venv) mike@DESKTOP-N0GKBM8:~/microblog$

    Any help is greatly appreciated.

  • #511 Miguel Grinberg said 2021-09-13T22:30:47Z

    @Mike: the error says that Flask could not find an application instance in microblog.py. What do you have there? Does the error occur if you use my code from GitHub?

  • #512 Mike said 2021-09-14T23:24:57Z

    VS Code is giving me following error in the init.py file.

    Import "flask" could not be resolved from source PyLance(reportMissingModuleSource)

    Could that be the underlying issue? Below is what I have in the init.py file

    from flask import Flask
    app = Flask(__name__)
    from app import routes
  • #513 Miguel Grinberg said 2021-09-15T07:46:17Z

    @Mike: you haven't answered any of my questions, and instead are looking elsewhere for a problem that should be easy to fix. As I said above, the code on GitHub is 100% working. There is a download link in the introduction section of this article. Have you tried that?

  • #514 Evgeniy said 2021-09-19T16:48:37Z

    Hi Miguel,

    Thank you for your tutorial! Maybe can you tell, if you do: from app import routes - it works. But if I have 2 files with exactly same code (main.py and routes.py) and inside main.py I have following:

    from flask import Flask app = Flask(name) import routes if name == 'main': app.run(debug=True)

    Why routes do not work at all in this case? it seems that in routes file it cannot work with app variable, even if I import it.

  • #515 Miguel Grinberg said 2021-09-20T06:50:55Z

    @Evgeniy: in routes.py you have to use from __main__ import app. The main module in a Python app is always called __main__.

  • #516 Mike said 2021-09-23T15:09:19Z

    @Miguel Grinberg. Your solution did work. Using your code from Github worked. ( So thank you for that.) I am still trying to figure out where the error is in my on code, since I'm working on trying to gain an understanding of building projects from the ground up. Thanks again for this tutorial, it absolutely one of the best resources for learning flask that I have found.

  • #517 Towfiq said 2021-10-06T03:25:50Z

    Hi, Hope you are doing well. I am a beginner, I was told to follow the pep8 convention, and importing at the bottom of the file is considered wrong. Is there any exception to this rule? Circular dependencies are also considered a flaw in the structure and not following the separation of concern. Could you please elaborate on the structure decision?

  • #518 Miguel Grinberg said 2021-10-07T23:02:21Z

    @Towfiq: this is really up to each developer. Circular dependencies are difficult to manage, and because of the heavy use of decorators they are very hard to avoid in Flask applications. So you have two options: either complicate the structure more to avoid those imports at the bottom, or tolerate the imports with the end goal of having a simpler file structure. I prefer the simpler structure.

  • #519 Hamid Allaoui said 2021-10-20T06:15:08Z

    Thank you Miguel for this great work.

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