Here is an index of all the articles in the series that have been published to date:
> You can learn more about Python: https://www.pcwdld.com/python-cheat-sheet
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.
The app 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 the app:
So as you see, I'm going pretty much for the whole thing. I hope this app, when finished, will serve as a sort of template for writing other web applications.
If you have a computer that runs Python 2.6 or 2.7 then you are probably good to go. The tutorial application should run just fine on Windows, OS X and Linux.
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.
Okay, let's get started!
If you haven't yet, go ahead and install Python 2.7.
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 foldermicroblog.
Next, download virtualenv.py and put it inside the new folder.
To create the virtual environment enter the following command:
python virtualenv.py flask
The above command creates a complete Python environment inside theflaskfolder.
Virtual environments can be activated and deactivated, if desired. An activated environment adds the location of itsbinfolder to the system path, so that for example, when you typepythonyou get the environment's version and not the system's one. I personally do not like this feature, so I never activate any of my environments and instead just invoke the interpreter I want by typing 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 flup
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-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 flup
These commands will download and install all the packages that we will use for our application.
Note about SQLAlchemy: version 0.8 of SQLAlchemy is not backwards compatible with previous releases. In particular, the sqlalchemy-migrate module does not work with it. For that reason we need to force the install of release 0.7.9, with the following commands:
flask/bin/pip uninstall sqlalchemy flask/bin/pip install sqlalchemy==0.7.9
Once sqlalchemy-migrate is updated to support 0.8 we should be able to work with the latest and greatest.
Windows users have one more step. If you have good observation skills you may have noticed thatflask-mailwas not included in the Windows installation command list. This extension does not install cleanly on Windows, so we have to get it installed via a workaround:
flask\Scripts\pip install --no-deps lamson chardet flask-mail
I won't go into details regarding this, if you want more information please refer to the flask-mail documentation.
If the installation of all the packages was successful you can deletevirtualenv.py, since we won't need it anymore.
You now have aflasksub-folder inside yourmicroblogfolder 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 youcdto themicroblogfolder, let's create the basic folder structure for our app:
mkdir app mkdir app/static mkdir app/templates mkdir tmp
Let's start by creating a simple init script for ourapppackage (fileapp/__init__.py):
from flask import Flask app = Flask(__name__) from app import views
The script above simply creates the application object (of classFlask) and then imports the views module, which we haven't written yet.
The views are the handlers that respond to requests from web browsers. In Flask views are written as Python functions. Each view function is mapped to one or more request URLs.
Let's write our first view function (fileapp/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 tworoutedecorators above the function create the mappings from urls/and/indexto this function.
The final step to have a fully working web app is to create a script that starts up the development web server with our application. Let's call this scriptrun.py.
#!flask/bin/python from app import app app.run(debug = True)
The script simply imports theappvariable from our app package and invokes itsrunmethod to start the server. Remember that theappvariable holds theFlaskinstance, we created it above.