2012-05-07T06:05:40Z

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

This is the first article in a series where I will be documenting my experience writing web applications in Python using the Flask microframework.

NOTE: This article was revised in September 2014 to be in sync with current versions of Python and Flask.

Here is an index of all the articles in the series that have been published to date:

My background

I'm a software engineer with double digit years of experience developing complex applications in several languages. I first learned Python as part of an effort to create bindings for a C++ library at work.

In addition to Python, I've written web apps in PHP, Ruby, Smalltalk and believe it or not, also in C++. Of all these, the Python/Flask combination is the one that I've found to be the most flexible.

UPDATE: I have written a book titled "Flask Web Development", published in 2014 by O'Reilly Media. The book and the tutorial complement each other, the book presents a more updated usage of Flask and is, in general, more advanced than the tutorial, but some topics are only covered in the tutorial. Visit http://flaskbook.com for more information.

The application

The application I'm going to develop as part of this tutorial is a decently featured microblogging server that I decided to call microblog. Pretty creative, I know.

These are some of the topics I will cover as we make progress with this project:

  • User management, including managing logins, sessions, user roles, profiles and user avatars.
  • Database management, including migration handling.
  • Web form support, including field validation.
  • Pagination of long lists of items.
  • Full text search.
  • Email notifications to users.
  • HTML templates.
  • Support for multiple languages.
  • Caching and other performance optimizations.
  • Debugging techniques for development and production servers.
  • Installation on a production server.

So as you see, I'm going pretty much for the whole thing. I hope this application, when finished, will serve as a sort of template for writing other web applications.

Requirements

If you have a computer that runs Python then you are probably good to go. The tutorial application should run just fine on Windows, OS X and Linux. Unless noted, the code presented in these articles has been tested against Python 2.7 and 3.4, though it will likely be okay if you use a newer 3.x release.

The tutorial assumes that you are familiar with the terminal window (command prompt for Windows users) and know the basic command line file management functions of your operating system. If you don't, then I recommend that you learn how to create directories, copy files, etc. using the command line before continuing.

Finally, you should be somewhat comfortable writing Python code. Familiarity with Python modules and packages is also recommended.

Installing Flask

Okay, let's get started!

If you haven't yet, go ahead and install Python.

Now we have to install Flask and several extensions that we will be using. My preferred way to do this is to create a virtual environment where everything gets installed, so that your main Python installation is not affected. As an added benefit, you won't need root access to do the installation in this way.

So, open up a terminal window, choose a location where you want your application to live and create a new folder there to contain it. Let's call the application folder microblog.

If you are using Python 3, then cd into the microblog folder and then create a virtual environment with the following command:

$ python -m venv flask

Note that in some operating systems you may need to use python3 instead of python. The above command creates a private version of your Python interpreter inside a folder named flask.

If you are using any other version of Python older than 3.4, then you need to download and install virtualenv.py before you can create a virtual environment. If you are on Mac OS X, then you can install it with the following command:

$ sudo easy_install virtualenv

On Linux you likely have a package for your distribution. For example, if you use Ubuntu:

$ sudo apt-get install python-virtualenv

Windows users have the most difficulty in installing virtualenv, so if you want to avoid the trouble then install Python 3. If you want to install virtualenv on Windows then the easiest way is by installing pip first, as explaned in this page. Once pip is installed, the following command installsvirtualenv`:

$ pip install virtualenv

We've seen above how to create a virtual environment in Python 3. For older versions of Python that have been expanded with virtualenv, the command that creates a virtual environment is the following:

$ virtualenv flask

Regardless of the method you use to create the virtual environment, you will end up with a folder named flask that contains a complete Python environment ready to be used for this project.

Virtual environments can be activated and deactivated, if desired. An activated environment adds the location of its bin folder to the system path, so that for example, when you type python you get the environment's version and not the system's one. But activating a virtual environment is not necessary, it is equally effective to invoke the interpreter by specifying its pathname.

If you are on Linux, OS X or Cygwin, install flask and extensions by entering the following commands, one after another:

$ flask/bin/pip install flask
$ flask/bin/pip install flask-login
$ flask/bin/pip install flask-openid
$ flask/bin/pip install flask-mail
$ flask/bin/pip install flask-sqlalchemy
$ flask/bin/pip install sqlalchemy-migrate
$ flask/bin/pip install flask-whooshalchemy
$ flask/bin/pip install flask-wtf
$ flask/bin/pip install flask-babel
$ flask/bin/pip install guess_language
$ flask/bin/pip install flipflop
$ flask/bin/pip install coverage

If you are on Windows the commands are slightly different:

$ flask\Scripts\pip install flask
$ flask\Scripts\pip install flask-login
$ flask\Scripts\pip install flask-openid
$ flask\Scripts\pip install flask-mail
$ flask\Scripts\pip install flask-sqlalchemy
$ flask\Scripts\pip install sqlalchemy-migrate
$ flask\Scripts\pip install flask-whooshalchemy
$ flask\Scripts\pip install flask-wtf
$ flask\Scripts\pip install flask-babel
$ flask\Scripts\pip install guess_language
$ flask\Scripts\pip install flipflop
$ flask\Scripts\pip install coverage

These commands will download and install all the packages that we will use for our application.

"Hello, World" in Flask

You now have a flask sub-folder inside your microblog folder that is populated with a Python interpreter and the Flask framework and extensions that we will use for this application. Now it's time to write our first web application!

After you cd to the microblog folder, let's create the basic folder structure for our application:

$ mkdir app
$ mkdir app/static
$ mkdir app/templates
$ mkdir tmp

The app folder will be where we will put our application package. The static sub-folder is where we will store static files like images, javascripts, and cascading style sheets. The templates sub-folder is obviously where our templates will go.

Let's start by creating a simple init script for our app package (file app/__init__.py):

from flask import Flask

app = Flask(__name__)
from app import views

The script above simply creates the application object (of class Flask) and then imports the views module, which we haven't written yet. Do not confuse app the variable (which gets assigned the Flask instance) with app the package (from which we import the views module).

If you are wondering why the import statement is at the end and not at the beginning of the script as it is always done, the reason is to avoid circular references, because you are going to see that the views module needs to import the app variable defined in this script. Putting the import at the end avoids the circular import error.

The views are the handlers that respond to requests from web browsers or other clients. In Flask handlers are written as Python functions. Each view function is mapped to one or more request URLs.

Let's write our first view function (file app/views.py):

from app import app

@app.route('/')
@app.route('/index')
def index():
    return "Hello, World!"

This view is actually pretty simple, it just returns a string, to be displayed on the client's web browser. The two route decorators above the function create the mappings from URLs / and /index to this function.

The final step to have a fully working web application is to create a script that starts up the development web server with our application. Let's call this script run.py, and put it in the root folder:

#!flask/bin/python
from app import app
app.run(debug=True)

The script simply imports the app variable from our app package and invokes its run method to start the server. Remember that the app variable holds the Flask instance that we created it above.

To start the app you just run this script. On OS X, Linux and Cygwin you have to indicate that this is an executable file before you can run it:

$ chmod a+x run.py

Then the script can simply be executed as follows:

./run.py

On Windows the process is a bit different. There is no need to indicate the file is executable. Instead you have to run the script as an argument to the Python interpreter from the virtual environment:

$ flask\Scripts\python run.py

After the server initializes it will listen on port 5000 waiting for connections. Now open up your web browser and enter the following URL in the address field:

http://localhost:5000

Alternatively you can use the following URL:

http://localhost:5000/index

Do you see the route mappings in action? The first URL maps to /, while the second maps to /index. Both routes are associated with our view function, so they produce the same result. If you enter any other URL you will get an error, since only these two have been defined.

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

And with this I conclude this first installment of this tutorial.

For those of you that are lazy typists, you can download the code from this tutorial below:

Download microblog-0.1.zip.

Note that you still need to install Flask as indicated above before you can run the application.

What's next

In the next part of the series we will modify our little application to use HTML templates.

I hope to see you in the next chapter.

Miguel

350 comments

  • #326 Gunther Leenaert said 2015-05-15T20:13:29Z

    Up to date command for creating a venv with python3: virtualenv -p python3 envname

  • #327 Miguel Grinberg said 2015-05-16T00:50:41Z

    @Gunther: you don't need to use virtualenv to create a virtual environment in Python 3. The method that I use only requires Python no additional stuff.

  • #328 Nikov said 2015-05-19T02:29:09Z

    Hi Miguel, Thanks for the great tutorial. I am on MacOS and ./run.py produces the following error: Traceback (most recent call last): File "./run.py", line 2, in <module> from app import app File "/Users/nikovsieber/microblog/app/__init__.py", line 1, in <module> from flask import Flask ImportError: No module named flask I can run it using: python run.py So I can continue with the tutorial, but was just wondering why I'm getting this error when trying to run it the way you suggest. Regards

  • #329 Miguel Grinberg said 2015-05-19T04:26:53Z

    @Nikov: probably because you used a different virtualenv name. Look at the first line in run.py, make sure the path to your python executable is correct. If it is correct, but you can only get things to work using "python run.py", then that means that you accidentally installed Flask in the global interpreter, not in the virtualenv.

  • #330 Nikov said 2015-05-19T22:37:28Z

    Yes I think that's it. When I followed your instructions to install Flask and the extensions the output indicated that everything was already installed. I must have accidentally installed it while attempting a different Flask tutorial. Would it be safer for my system if I unistalled flask and then reinstalled in the virtualenv for this project?

  • #331 Nikov said 2015-05-19T23:28:20Z

    I went ahead and uninstalled flask from my system. Now I'm starting over and I remember how flask ended up in the global interpreter. After I create a virtualenv it won't allow me to install flask without using sudo. That is I get the following error: error: could not create '/Library/Python/2.7/site-packages/flask': Permission denied I have thoroughly read your instructions and am following to the letter as far as I know. I'm on MacOS X.

  • #332 Nikov said 2015-05-20T00:03:46Z

    Hi Miguel, I was able to install on OS X after activating the virtualenv with . flask/bin/activate Thanks

  • #333 sadfrunk said 2015-05-20T10:17:27Z

    app = Flask(__name__) - which gives specific __name__?

  • #334 depeche said 2015-05-20T10:42:25Z

    Why you just not tried to use other variable then "app" to avoid confusion with package name? I mean like this from flask import Flask application = Flask(__name__) from app import views

  • #335 Miguel Grinberg said 2015-05-21T06:26:49Z

    @Nikov: Sure. You can run "pip freeze" to find out what packages got installed globally, then uninstall them.

  • #336 Miguel Grinberg said 2015-05-21T06:38:14Z

    @depeche: yes, a few others suggested the same. Sorry, back when I wrote this article it seemed logical to me to use the same name, clearly I was wrong.

  • #337 Miguel Grinberg said 2015-05-21T06:42:02Z

    @sadfrunk: __name__ is a Python variable, it is the name of the module. See https://docs.python.org/2/tutorial/modules.html?highlight=__name__

  • #338 Abraham said 2015-05-28T11:00:19Z

    Hi Miguel, first I want to say thanks for the tutorial. I have been looking for something like this to help me understand flask. As for my question, I try to download flask environment on python 3.4 with "python3 -m venv flask" command but got this error: Traceback (most recent call last): File "<frozen importlib._bootstrap>", line 2237, in _find_and_load File "<frozen importlib._bootstrap>", line 2222, in _find_and_load_unlocked File "<frozen importlib._bootstrap>", line 2164, in _find_spec File "<frozen importlib._bootstrap>", line 1940, in find_spec File "<frozen importlib._bootstrap>", line 1911, in _get_spec File "<frozen importlib._bootstrap>", line 1879, in _path_importer_cache FileNotFoundError: [Errno 2] No such file or directory I don't know how to interpret this error. Thanks again

  • #339 Miguel Grinberg said 2015-05-29T21:40:33Z

    @Abraham: the closest report I can find to your error is this Python bug: https://bugs.python.org/issue22834. Not sure if this matches your environment.

  • #340 amarjeet said 2015-05-31T12:20:17Z

    Nice article.can you suggest some IDE which is good for development using flask.

  • #341 Miguel Grinberg said 2015-06-01T14:26:02Z

    @amarjeet: you can use any text editor, or if you prefer something a bit more complex PyCharm. If you are on Windows, the Python extension for Visual Studio is very good as well.

  • #342 Sid said 2015-06-14T11:33:52Z

    Miguel, why can't i invoke the program from another machine by replacing localhost by the machine ip. Thanks.

  • #343 Miguel Grinberg said 2015-06-15T17:04:13Z

    @Sid: Set the host to "0.0.0.0" when you want to access from other machines.

  • #344 Scott Lorimor said 2015-06-16T03:01:57Z

    Hi Miguel, Great tutorial. I am running into this importing issues on my mac and didn't see the fix for it. Traceback (most recent call last): File "./run.py", line 2, in <module> from app import app File "/Users/scottlorimor/microblog/app/__init__.py", line 17, in <module> from flask import Flask ImportError: No module named flask

  • #345 Miguel Grinberg said 2015-06-17T00:19:15Z

    @Scott: did you follow the instructions to install and use a virtualenv?

  • #346 Rishi said 2015-06-18T09:41:59Z

    Hi, I have installed the virtualenv as you explained , My directory structure is also correct , as my app folder a, flask folder and run.py are at same level, still I am facing this issue . Traceback (most recent call last): File "run.py", line 2, in <module> from app import app ImportError: cannot import name app

  • #347 Carlos Lomeli said 2015-06-20T05:48:03Z

    Miguel, How I can delete a venv created in the way you did here. I need to download venvwapper or something? Or just deleting the files directories it would be ok?

  • #348 Miguel Grinberg said 2015-06-22T22:22:06Z

    @Rishi: compare against my files on Github. Maybe your app package constructor (file app/__init__.py) is incorrect.

  • #349 Miguel Grinberg said 2015-06-22T22:24:29Z

    @Carlos: just delete the "venv" folder.

  • #350 Eduardo said 2015-06-27T20:47:22Z

    Hola Miguel, felicitaciones desde Argentina por el tutorial!