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. The video above will give you an overview of the contents of this tutorial. 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.

Note 1: If you are looking for the legacy version of this tutorial, it's here.

Note 2: If you would like to support my work on this blog, or just don't have patience to wait for weekly articles, I am offering the complete version of this tutorial packaged as an ebook or a set of videos. For more information, visit learn.miguelgrinberg.com.

All the code examples presented in this tutorial 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 3.5.2 (default, Nov 17 2016, 17:05:23)
[GCC 5.4.0 20160609] on linux
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 (in Python 2.7 pip does not come bundled with Python and needs to be installed separately).

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 0.11 of Flask, which was the most current version of Flask when you started, but now has been superseeded by version 0.12. You now want to start a second application, for which you'd like to use the 0.12 version, but if you replace the 0.11 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 install Flask 0.11 to be used by your old application, and also install Flask 0.12 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

If you are using a Python 3 version, virtual environment support is included in it, 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.

If you are using any version of Python older than 3.4 (and that includes the 2.7 release), virtual environments are not supported natively. For those versions of Python, you need to download and install a third-party tool called virtualenv before you can create virtual environments. Once virtualenv is installed, you can create a virtual environment with the following command:

$ virtualenv venv

Regardless of the method you used to create it, you should have your virtual environment created. Now you have to tell the system that you want to use it, 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.

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 your first view function, which you need to write in the 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 Microsoft Windows, 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"
 * 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 require 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!


  • #101 Miguel Grinberg said 2018-03-16T00:01:03Z

    @Gavin: No, this is something that is not related to the virtualenv. Unfortunately the current release of Flask does not provide a way to save this variable, so you need to use facilities offered by your operating system to do it.

  • #102 Niraj Chavan said 2018-03-18T05:31:32Z

    Hi Miguel, Nice Blog!!! My first python app working fine on my windows 10 machine. But while installing and creating virtual environment on windows machine, it has been little bit confusing and difficult by following this blog. So I would like to suggest please add steps for creating virtual environment on windows 10. It help new people and who are working on windows environment. If anyone found difficulties while installing flask and creating virtual environment on windows, please refer this link - http://timmyreilly.azurewebsites.net/python-pip-virtualenv-installation-on-windows/ Thank you!! Nirajc

  • #103 Miguel Grinberg said 2018-03-18T06:12:18Z

    @Niraj: the instructions to create a virtual environment in the link that you posted are outdated. In current versions of Python 3, you can use the same command to create a virtual environment on any platform, including Windows.

  • #104 Tommy Herbert said 2018-03-23T15:03:01Z

    Should you add 'pip install wheel' before 'pip install flask'?

  • #105 Dodzi said 2018-03-23T15:12:54Z

    Does this work for Mac as well?

  • #106 Miguel Grinberg said 2018-03-23T17:38:47Z

    @Tommy: installing "wheel" is not necessary to install Flask.

  • #107 Miguel Grinberg said 2018-03-23T17:39:10Z

    @Dodzi: Absolutely. I developed this tutorial mostly on a Mac myself.

  • #108 Paul said 2018-03-24T11:07:48Z

    Great tutorial - thanks so much for sharing your knowledge with the world!

  • #109 Ryan Cross said 2018-03-27T19:13:03Z

    Miguel: Thanks for this tutorial. I'm learning flask and have taken a few courses, this one is the gold standard. The book, the accompanying video and the github really give you many resources. I'm following along using pycharm and when it was time to run the first application I had a hell of a time. Why did we use the command line interface instead of the run() function of app?

  • #110 Miguel Grinberg said 2018-03-27T21:53:03Z

    @Ryan: the app.run() method of running the application has been replaced with the CLI. This is not something I fully agree with, I think it was more convenient to have a single script that you run, but that had some issues that the CLI resolves. The Flask documentation includes some info on setting up PyCharm: http://flask.pocoo.org/docs/dev/cli/#pycharm-integration.

  • #111 Joel Tang said 2018-03-29T22:59:13Z

    I have noticed you have said: 'putting one of the reciprocal imports at the bottom avoids the error that results from the mutual references between these two files'. In this case is __init__.py from import routes, models, errors and routes.py from app import app. Can you elaborate why putting imports at end will avoid errors? It is still a circular import for me.

  • #112 Peter Waksman said 2018-03-30T15:03:25Z

    Really great writing. I had trouble connecting the instructions to the Python Tools for Visual Studio ("PTVS") environment because setting up venv is a bit different and because setting FLASK_APP is in an unfamiliar place, and because I am having to "attach to process" explicitly to hit breakpoints. Learning pains.

  • #113 Miguel Grinberg said 2018-04-03T05:38:11Z

    @Joel: It is still circular, but the error is avoided, because the circular dependency is delayed until the end of __init__.py, so at that point when those other modules try to import back from __init__.py the import succeeds.

  • #114 Miguel Grinberg said 2018-04-03T05:47:58Z

    @Peter: Unfortunately the way Flask is started is not friendly to IDEs. If it is any easier, you can still the old "app.run()" format, which should work well in PTVS or any other Python IDE.

  • #115 Vlad said 2018-04-04T19:29:41Z

    Hello, Miguel! Thank you for all the efforts put into educating us newcomers ;) Just a few short ones: 1) any reason you do not install the app into the virtual environment like they do in the official tutorial? 2) any reason you prefer an absolute import 'from app import routes' to a relative import 'from . import routes'? 3) wouldn't it be clearer to create app.py in the package with app=Flask(__name__) and only use __init__.py for imports (no worries about circular dependencies then) or would it cause any issues later? Thanks

  • #116 Miguel Grinberg said 2018-04-04T21:58:58Z

    @Vlad: 1) this is a practice that a few of the other Flask core developers like. I personally don't find any benefit in installing the app, but if you find it useful, then you should do it. 2) No reason. I'm fine either way. 3) I don't understand. Please create a sample project that demonstrates your solution and I'll review it.

  • #117 Prashant said 2018-04-07T01:56:42Z

    Thanks for the nice explanation/presentation. I am starting today... lets see where the tutorial takes me( or vice versa :-) )

  • #118 Lee said 2018-04-09T13:46:33Z

    Hi Miguel, thanks for your amazing tutorial. Just a quick question about your book. Does it cover any API development with Flask?

  • #119 Miguel Grinberg said 2018-04-09T16:04:41Z

    @Lee: Yes, the last chapter of the tutorial covers API development in detail.

  • #120 michael R said 2018-04-10T00:05:00Z

    Thank you for the tutorial! I'm having an issue with and import error: from app import app ImportError: cannot import name 'app' I copied the files exactly as shown and even changed the variable/module name but get the same error each time. what am I doing wrong? thanks!

  • #121 Miguel Grinberg said 2018-04-10T01:46:07Z

    @michael: I believe you are wrong when you say that you copied everything exactly, you must have missed something. Please compare your files against mine as they appear on the GitHub link shown at the top of the article. That's the project as you should have it.

  • #122 Tat said 2018-04-14T20:41:53Z

    Hi Miguel, I see output "Hello world" locally but I could not employ it to Heroku - error messages... I've tried to see your app. on Heroku by cloning it from your Git repository on my local machine(it created more files than discussed on Chapter1) but there are still no view as such. Is it possible to deploy this app. on Heroku or any other online server ? thanks.

  • #123 Miguel Grinberg said 2018-04-14T23:12:43Z

    @Tat: deploying to Heroku is covered later in this tutorial. I recommend that you follow the chapters in order, deploying such a simple application to Heroku or another cloud provider makes little sense, you need an application that does something useful before you attempt a deployment.

Leave a Comment