Installation

Flask depends on two external libraries, Werkzeug and Jinja2. Werkzeug is a toolkit for WSGI, the standard Python interface between web applications and a variety of servers for both development and deployment. Jinja2 renders templates.

So how do you get all that on your computer quickly? There are many ways which this section will explain, but the most kick-ass method is virtualenv, so let’s look at that first.

Either way, you will need Python 2.5 or higher to get started, so be sure to have an up to date Python 2.x installation. At the time of writing, the WSGI specification is not yet finalized for Python 3, so Flask cannot support the 3.x series of Python.

virtualenv

Virtualenv is probably what you want to use during development, and in production too if you have shell access there.

What problem does virtualenv solve? If you like Python as I do, chances are you want to use it for other projects besides Flask-based web applications. But the more projects you have, the more likely it is that you will be working with different versions of Python itself, or at least different versions of Python libraries. Let’s face it; quite often libraries break backwards compatibility, and it’s unlikely that any serious application will have zero dependencies. So what do you do if two or more of your projects have conflicting dependencies?

Virtualenv to the rescue! It basically enables multiple side-by-side installations of Python, one for each project. It doesn’t actually install separate copies of Python, but it does provide a clever way to keep different project environments isolated.

So let’s see how virtualenv works!

If you are on Mac OS X or Linux, chances are that one of the following two commands will work for you:

$ sudo easy_install virtualenv

or even better:

$ sudo pip install virtualenv

One of these will probably install virtualenv on your system. Maybe it’s even in your package manager. If you use Ubuntu, try:

$ sudo apt-get install python-virtualenv

If you are on Windows and don’t have the easy_install command, you must install it first. Check the easy_install on Windows section for more information about how to do that. Once you have it installed, run the same commands as above, but without the sudo prefix.

Once you have virtualenv installed, just fire up a shell and create your own environment. I usually create a project folder and an env folder within:

$ mkdir myproject
$ cd myproject
$ virtualenv env
New python executable in env/bin/python
Installing setuptools............done.

Now, whenever you want to work on a project, you only have to activate the corresponding environment. On OS X and Linux, do the following:

$ . env/bin/activate

(Note the space between the dot and the script name. The dot means that this script should run in the context of the current shell. If this command does not work in your shell, try replacing the dot with source)

If you are a Windows user, the following command is for you:

$ env\scripts\activate

Either way, you should now be using your virtualenv (see how the prompt of your shell has changed to show the virtualenv).

Now you can just enter the following command to get Flask activated in your virtualenv:

$ easy_install Flask

A few seconds later you are good to go.

System Wide Installation

This is possible as well, but I do not recommend it. Just run easy_install with root rights:

$ sudo easy_install Flask

(Run it in an Admin shell on Windows systems and without sudo).

Living on the Edge

If you want to work with the latest version of Flask, there are two ways: you can either let easy_install pull in the development version, or tell it to operate on a git checkout. Either way, virtualenv is recommended.

Get the git checkout in a new virtualenv and run in development mode:

$ git clone http://github.com/mitsuhiko/flask.git
Initialized empty Git repository in ~/dev/flask/.git/
$ cd flask
$ virtualenv env
$ . env/bin/activate
New python executable in env/bin/python
Installing setuptools............done.
$ python setup.py develop
...
Finished processing dependencies for Flask

This will pull in the dependencies and activate the git head as the current version inside the virtualenv. Then you just have to git pull origin to get the latest version.

To just get the development version without git, do this instead:

$ mkdir flask
$ cd flask
$ virtualenv env
$ . env/bin/activate
New python executable in env/bin/python
Installing setuptools............done.
$ easy_install Flask==dev
...
Finished processing dependencies for Flask==dev

easy_install on Windows

On Windows, installation of easy_install is a little bit trickier because slightly different rules apply on Windows than on Unix-like systems, but it’s not difficult. The easiest way to do it is to download the ez_setup.py file and run it. The easiest way to run the file is to open your downloads folder and double-click on the file.

Next, add the easy_install command and other Python scripts to the command search path, by adding your Python installation’s Scripts folder to the PATH environment variable. To do that, right-click on the “Computer” icon on the Desktop or in the Start menu, and choose “Properties”. Then, on Windows Vista and Windows 7 click on “Advanced System settings”; on Windows XP, click on the “Advanced” tab instead. Then click on the “Environment variables” button and double click on the “Path” variable in the “System variables” section. There append the path of your Python interpreter’s Scripts folder; make sure you delimit it from existing values with a semicolon. Assuming you are using Python 2.6 on the default path, add the following value:

;C:\Python26\Scripts

Then you are done. To check that it worked, open the Command Prompt and execute easy_install. If you have User Account Control enabled on Windows Vista or Windows 7, it should prompt you for admin privileges.