2017-12-05T17:15:48Z

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 courses.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

@app.route('/')
@app.route('/index')
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:

microblog/
  venv/
  app/
    __init__.py
    routes.py
  microblog.py

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 http://127.0.0.1:5000/ (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 127.0.0.1, 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:

    http://localhost:5000/

Alternatively you can use this other URL:

    http://localhost:5000/index

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 want to mention 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 .flaskenv file in the top-level directory of the project:

.flaskenv: Environment variables for flask command

FLASK_APP=microblog.py

Doing this is optional. If you prefer to set the environment variable manually, that is perfectly fine, as long as you always remember to do it.

291 comments

  • #51 Alexandre Franco said 2018-01-17T16:37:59Z

    Miguel, your tutorial is really great and I look forward using your eBook for a reference for a particular project I'm starting. I do have a dumb question: it seems to me you are using a Linux distribution for coding and running examples. Any particular distribution you recommend as most friendly for Python/Flask developers?

  • #52 Miguel Grinberg said 2018-01-17T18:16:24Z

    @Alexandre: I have actually developed this tutorial and the code that comes with it so that it works on the three major platforms. If you want a recommendation for Linux, then Ubuntu 16.04 is a good distribution to learn as there is a lot of information about it out there. But any platform with a Python interpreter would work for this tutorial.

  • #53 David Collins said 2018-01-18T10:36:48Z

    Great Tutorial! Thanks for making these. I got part way through the one on databases without a virtual environment. I couldn't import db, so I'm back here reproducing your directory structure. Are you aware of any complications associated with installing Python & Flask, or creating a virtual environment, through an Anaconda shell? Many thanks!

  • #54 VS said 2018-01-19T00:16:00Z

    Thank you Miguel, Is this book/content is different from your book in Oreilley Safari(labeled as "Early Release")? Flask Web Development, 2nd Edition

  • #55 John said 2018-01-19T05:21:13Z

    Thanks for the effort in making these. I found this page via Google because a Udemy class I'm taking on Python had a module on flask - and none of his example stuff works. Sadly, I can't make the stuff from this page work, either. And, I really can't even begin to figure out what's wrong: --------------------------------------------- (venv) john@TPP51:~/PYTHONS/microblog$ mkdir app (venv) john@TPP51:~/PYTHONS/microblog$ export FLASK_APP=microblog.py (venv) john@TPP51:~/PYTHONS/microblog$ flask run Usage: flask run [OPTIONS] Error: The file/path provided (microblog.py) does not appear to exist. Please verify the path is correct. If app is not on PYTHONPATH, ensure the extension is .py (venv) john@TPP51:~/PYTHONS/microblog$ ---------------------------------------------- Can someone please direct me to a tutorial for Flask that's REALLY for actual beginners? Everything I find is like some advanced computer science course - and with "examples" that don't actually work. Thanks!

  • #56 Miguel Grinberg said 2018-01-19T06:26:17Z

    @John: sorry, but aren't you skipping some steps? Have you create the application files, as indicated in this article? There is some more work you need to do after you run "mkdir app" and before you can run the application. Follow "all" the instructions in this article and I think you will be fine.

  • #57 Miguel Grinberg said 2018-01-19T06:27:46Z

    @VS: there is some overlap, but they are not the same. This tutorial covers several topics that are not strictly related to Flask, but more to web development in general. The book is more focused on Flask and goes deeper than the tutorial on its coverage of the framework.

  • #58 Miguel Grinberg said 2018-01-19T06:29:48Z

    @David: I do not use Anaconda, but my understanding is that once you create a virtualenv the rest should work the same.

  • #59 Sergio Batista said 2018-01-19T17:09:01Z

    Great work Miguel! I am actually doing the tutorial on Raspberry pi 2 b.It is amazing the power of these two things combined, Flask and the Raspberry!!! I have a small contribution for this section. If you want to make your app available across the computers on your network you only need to launch the server with an additional option. flask run --host=0.0.0.0

  • #60 Brian said 2018-01-19T18:29:55Z

    Miguel, thank you for taking the time to put this tutorial together. I'm not a novice, but I picked up a few things via your post here and I wanted to express gratitude for your hard work.

  • #61 Miguel Grinberg said 2018-01-19T20:20:07Z

    @Sergio: Thanks! I actually do not want to promote the use of the Flask server in this way, because this is not a production-ready server. There are three chapters in this series that deal with different deployment methodologies, including the Raspberry Pi.

  • #62 Jeb Stone said 2018-01-22T02:48:18Z

    The tutorial instructs that microblog.py is a single line as follows: from app import app However, that's an unused import; nothing is called to run when microblog.py runs. Shouldn't microblog.py look more like the following? from app import app if __name__ == '__main__': app.run()

  • #63 Miguel Grinberg said 2018-01-22T05:17:09Z

    @Jeb: No, the code is correct. Recent versions of Flask have an alternative method to start the application, you can use app.run() if you like, but you do not need to. The import is not really unused, since Flask will import this module and reference "app" when it loads the application.

  • #64 Valdinia said 2018-01-23T10:01:31Z

    I'm following your tutorial on PythonAnywhere. My application is in the mysite folder As long as I have the following lines in __init__.py the page shows correctly: from flask import Flask app = Flask(__name__) @app.route('/') def hello_world(): return 'Hello from Flask!' As loing as I replace the route with from mysite import routes the page displays: 404 Not Found message (I have no error on log) routes.py contains: from mysite import app @app.route('/') @app.route('/index') def index(): return "Hello, World from ROUTES!" The microblog.py file contains: from mysite import app I set the FLASK_APP environment variable: export FLASK_APP=microblog.py What am I doing wrong? There are hours and hours already since I'm trying to solve the problem. Thank you for your help Valdinia

  • #65 Miguel Grinberg said 2018-01-24T00:58:15Z

    @Valdinia: not familiar with the pythonanywhere platform to comment on this. I suggest you compare your code against mine on GitHub and look for differences.

  • #66 Matt Evans said 2018-01-31T18:13:47Z

    Hi Miguel, Very thankful that you are keeping this project up to date as it's a great source of knowledge for me. Something I've always wondered about: is there any reason why we don't rename the 'app' variable we create in __init__.py to have an uppercase 'A' to avoid confusion between the variable and the app package? For instance, instead of app = Flask(__name__) it would be App = Flask(__name__). Looking forward to the next lesson!

  • #67 Miguel Grinberg said 2018-01-31T23:40:00Z

    @Matt: I actually covered this in the video version of this chapter. Some people find it confusing that the app package and the app instance are both called "app". If this is you, then you should rename one of them. For example, change the app package to "myapp" or the actual name of your application. But really using the same name does not cause any problem.

  • #68 Bogdan said 2018-02-03T19:33:59Z

    Probably a novice question, but couldn't you just name the folder something different like "microblog_app" to avoid the "two app" confusion?

  • #69 Miguel Grinberg said 2018-02-03T22:45:15Z

    @Bogdan: If you prefer to use a different name for the package, that's absolutely fine. I honestly prefer both to be called "app'.

  • #70 idiotzavant said 2018-02-10T07:52:46Z

    I am a great fan of your tutorials. I remember when I first tried to learn Django and I just couldn't. The Django abstraction at that time was just too much to take in for me. Until I saw Flask, and I like it because the Flask way of doing things seems to connect to me smoothly. For those having issues related to unicode, the following two commands fixed for me: export LC_ALL=C.UTF-8 export LANG=C.UTF-8 After doing that, ran again as documented by: flask run

  • #71 barka said 2018-02-11T10:34:58Z

    Hi Miquel, Thanks so much for this tutorial. I am a C# programmer learning flask and found this to be a really excellent tutorial. The absolute best one I have found on the web. I ran into a problem when I tried to activate the virtual environment on Windows. How do you execute $ venv\Scripts\activate? Is this supposed to be from the command prompt or Powershell? I have used Visual Studio as my IDE. It creates for you a VS solution that has a basic flask app to start with. In the process of creating the app it asks you to create a virtual environment. It creates that virtual environment in a directory similar to the one you show in the tutorial. \venv\Scripts exits but it does not have a file or executable called activate. I got all the way to data migration section but here I need to run the (venv) $ flask db I am at a loss on how to get into the virtual environment to run this.

  • #72 Miguel Grinberg said 2018-02-11T22:28:05Z

    @barka: all the examples for Windows that I provide are for the command prompt. I do not use PowerShell. I recommend that you create your virtual environment following the instructions on this tutorial, instead of relying on the IDE's own idea of how you should structure your application. Once you have the project and the virtualenv created, you can import it into your IDE and configure location of the Python interpreter to be the one in your virtualenv.

  • #73 Mike FitzGibbon said 2018-02-12T00:15:21Z

    Miguel, No questions for you, I just felt the need to let you know how much i enjoyed your tutorial. I learnt more in the 4 hrs I spent going through it yesterday than I have in the last 4 months I've been using Flask. Thanks for putting your time and effort in to this. Going through how to use WTF, Login, SQLAlchemy etc is within flask in the tutorial is gold. I nearly blew a gasket when I used Migrate. So powerful. Cheers

  • #74 Mark said 2018-02-15T03:22:09Z

    Hi, I like your tutorial, i have been looking to create a web application similar to the one demonstrated. I am wondering if in the "background jobs" chapter you will be talking about running scheduled jobs? for instance the web application i want to write web scrapes a couple of websites to populate some DB tables but i only want this running weekly at most. Is there a chapter on how this can be done? or can you point me in the direction of a tutorial on this?

  • #75 Miguel Grinberg said 2018-02-15T03:48:29Z

    @Mark: the chapter on background jobs discusses jobs that are triggered by an action from the user. For scheduled jobs you don't really need anything in your web application. You can just set up a cron job that starts your task and updates the database with the results of its scraping, independently of the web app. You can use the same database models, and you can even define your scheduled task as an extension to the "flask" command, so for example, "flask scrape" can be the task, and that is the command that you would put in your cron file.

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