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Installation

Flask is a microframework and yet it depends on external libraries. There are various ways how you can install that library and this explains each way and why there are multiple ways.

Flask depends on two external libraries: Werkzeug and Jinja2. The first one is responsible for interfacing WSGI the latter for rendering templates. Now you are maybe asking, what is WSGI? WSGI is a standard in Python that is basically responsible for ensuring that your application is behaving in a specific way so that you can run it on different environments (for example on a local development server, on an Apache2, on lighttpd, on Google's App Engine or whatever you have in mind).

So how do you get all that on your computer in no time? The most kick-ass method is virtualenv, so let's look at that first.

virtualenv

Virtualenv is what you want to use during development and in production if you have shell access. So first: what does virtualenv do? If you are like me and you like Python, chances are you want to use it for another project as well. Now the more projects you have, the more likely it is that you will be working with different versions of Python itself or at least an individual library. Because let's face it: quite often libraries break backwards compatibility and it's unlikely that your application will not have any dependencies, that just won't happen. So virtualenv to the rescue!

It basically makes it possible to have multiple side-by-side "installations" of Python, each for your own project. It's not actually an installation but a clever way to keep things separated.

So let's see how that works!

If you are on 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

Chances are you have virtualenv installed on your system then. Maybe it's even in your package manager (on ubuntu try sudo apt-get install python-virtualenv).

If you are on Windows and missing the easy_install command you have to install it first. Check the :ref:`windows-easy-install` section for more information about how to do that. Once you have it installed, run the same commands as above, but without the sudo part.

So now that you have virtualenv running just fire up a shell and create your own environment. I usually create a folder and a env folder within:

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

Now you only have to activate it, whenever you work with it. On OS X and Linux do the following:

$ . env/bin/activate

(Note the whitespace between the dot and the script name. This means execute this file in context of the shell. If the dot does not work for whatever reason in your shell, try substituting it 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 would 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 the sudo).

Living on the Edge

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 it's recommended to do that in a virtualenv.

Get the git checkout in a new virtualenv and run in develop 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 current version. 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 tricker because on Windows slightly different rules apply, but it's not a biggy. The easiest way to accomplish that is downloading the ez_setup.py file and running it. (Double clicking should do the trick)

Once you have done that it's important to add the easy_install command and other Python scripts to the path. To do that you have to add the Python installation's Script folder to the PATH variable.

To do that, right-click on your "Computer" desktop icon and click "Properties". On Windows Vista and Windows 7 then 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 Script folder to the end of the last (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 cmd and execute "easy_install". If you have UAC enabled it should prompt you for admin privileges.

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