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



  • #526 Miguel Grinberg said 2022-01-03T00:23:23Z

    @Glenford: production deployments are covered in the later chapters of the tutorial. Linux, Heroku and Docker are all discussed.

  • #527 Matt said 2022-01-06T17:38:28Z

    You can't create a file simple named .flaskenv in windows, top directory in our microblog example. We would need to do something like hello.faskenv to allow the file creation. Please clarify how this should look. Not a silly question.

  • #528 Miguel Grinberg said 2022-01-06T23:13:43Z

    @Matt: I don't use Windows often, but I believe it is possible to create files that start with a dot using tricks, like for example, naming the file something such as "hello.flaskenv" and then renaming the file to ".flaskenv" in the command prompt.

  • #529 said 2022-01-28T19:36:57Z

    Dear Miguel, as far as I see, you consequently use TableObject.query... when you work with a database. Could you tell me why? Tutorials (most of I've seen) suggest using db.session.execute(select(TableObject)...) instead of it. I'm a little bit confused... Is it just because that form needs less characters? :)

  • #530 Miguel Grinberg said 2022-01-29T18:31:36Z

    @Zé: There's three reasons. First, my tutorial uses the SQLAlchemy ORM, so what you call a "table object" is really a "model". You may have seen other tutorials that use the SQLAlchemy core, not the ORM. Second, this tutorial uses Flask-SQLAlchemy, not SQLAlchemy directly. The Flask-SQLAlchemy extension introduces some additional things that allow for more convenient queries. The final reason is that you may have seen recent SQLAlchemy tutorials, based on the 2.x query interface. Flask-SQLAlchemy uses the 1.x query interface of SQLAlchemy at this time. The 2.x interface uses db.session.execute() and select() even for the ORM.

  • #531 said 2022-01-29T20:05:37Z

    Sorry, it was my fault, originally I wanted to ask about the db.session.query(...). I found it (among others) in the SQLAlchemy's ORM tutorial (https://docs.sqlalchemy.org/en/14/orm/tutorial.html) - it looks if it was orm feature. Do I misunderstand something? I've asked it, because I'd like to see, what is the difference between the two query methods. For example I have a table, named log. If I want to get all data from log, with core module, I can use something like this: conn=db.engine.connect() res=conn.execute(text('select * from log'))

    If I want to get it using orm, I can define a class Log(db.Model) and then...


    <h1>--- or ---</h1>


    What is the difference between these methods? Both of them returns a flask_sqlalchemy.BaseQuery object. Both of them usable in native sqlalchemy too... Is it just syntactical difference? Or is there any advantages to one of them if I want to run a very complex query?

    Ps: unfortunately, English is not my native-language, so a little bit difficult to say exactly what I want to ask :(

  • #532 Miguel Grinberg said 2022-01-31T10:34:48Z

    @Zé: out of the three reasons I gave you, the one that mostly applies to your case is #2. Flask-SQLAlchemy adds a slightly shorter and more convenient way to start a query. Instead of db.query(Model) in Flask-SQLAlchemy you can use Model.query. Besides that, the queries are in the same way.

  • #533 Ednei said 2022-02-02T23:41:40Z

    hello miguel, I am very grateful for the tutorial, it is teaching me a lot, I was looking for something about flask, and finally I found yours. I'm from Brazil thank you study data science

  • #534 Franzkekko said 2022-02-27T14:08:55Z

    Nice tutorial, anyway on PyCharm IDE it's not running on webserver unless into microblog.py you add this code:

    if name == "main": app.run(debug=True)

    where debug=True is just a personal setting I added to see what happens in console

  • #535 Miguel Grinberg said 2022-02-27T14:21:10Z

    @Franzkekko: your solution is okay, but not great. With this you have deviated from the tutorial and you will have problems later on, when the flask command is used for other purposes beyond running the application. I suggest you start the application from the terminal, or else that you configure PyCharm properly so that it runs the flask run command.

  • #536 Pedro Baesse said 2022-03-17T22:21:59Z

    Hej Miguel,

    I just wanted to let you know that your mega tutorial has been helping many of my students in many interesting research projects ranging from open data applications to mapping ONG looking for help. And not only that, I also use with my students that a learn web development.

    One of them is live and already as a community of 200 people on telegram :D Dados Livres!

    Thanks a lot, mate!

  • #537 Michal said 2022-04-08T19:54:51Z

    When I try to create app following your instructions step by step I get the following error:

    ImportError: cannot import name 'routes' from 'app' (unknown location)

    I found no info anywhere, two people had the same problem with your tutorial on StackOverflow and none of them got any useful answer. So now I'm stuck and I have no idea how to progress.

  • #538 Miguel Grinberg said 2022-04-10T22:09:23Z

    @Michal: Have you considered looking at the source code on GitHub to determine what you did wrong? There is a link in the introduction section. That should allow you to find the mistake.

  • #539 Sheetal Shah said 2022-04-11T06:53:11Z

    Thank you for sharing the blog. Prier I tried many times from different websites but not able to understand the work flow but, now I understand it.

  • #540 Vamsi said 2022-05-16T16:17:02Z

    Hi Miguel,

    Thanks for this tutorial. I am starting a new project at my work using Flask and am wondering if the libraries/techniques explained in the tutorial are up to date?

    Thanks, Vamsi

  • #541 Miguel Grinberg said 2022-05-16T22:27:09Z

    @Vamsi: Yes, the tutorial is up to date.

  • #542 Matias said 2022-06-21T17:52:39Z

    Hello Miguel, excellent tutorial, one question, do you know of any way to integrate payment with cryptocurrencies? Thanks a lot

  • #543 Miguel Grinberg said 2022-06-21T20:20:24Z

    @Matias: I don't have any recommendations to make, as I'm not interested in crypto.

  • #544 zanhua said 2022-06-23T16:32:28Z

    thank you so much .this blog teach me a lot .

  • #545 Ms said 2022-07-16T19:41:50Z


    I was wondering. What is the hype behind FastApi?

    Is it any better, does it really make it async and faster in any meaningful way?

  • #546 Miguel Grinberg said 2022-07-17T07:28:42Z

    @Ms: Some people like Flask better than FastAPI, others like FastAPI. Both are good frameworks. If you really want to know what's best for you, you'll have to test them both and decide for yourself.

  • #547 Jenna said 2022-08-21T17:35:26Z

    Can anyone tell me where exactly can I find video tutorial for thr entire blog project??

  • #548 Miguel Grinberg said 2022-08-21T22:29:01Z

    @Jenna: The video course is available here: https://courses.miguelgrinberg.com/p/flask-mega-tutorial

  • #549 Joko said 2022-09-10T08:20:06Z

    Any plan to update this tutorial?

  • #550 Miguel Grinberg said 2022-09-10T10:03:20Z

    @Joko: the tutorial is constantly being revised and updated. I've made a major revision last year, which I announced on my blog: https://blog.miguelgrinberg.com/post/flask-mega-tutorial-update-flask-2-0-and-more

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