Flask Python API library

Eager to get started? This page gives a good introduction to Flask. It assumes you already have Flask installed.

Requirements

If you have a computer that runs Python then you are probably good to go. The tutorial application should run just fine on  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.

You should use pip to install and deploy Flask. To install Flask generally on your system, give this command on the command prompt:

       pip install flask

Note that you can also specify the Flask version, should you need (you won't need this one):

       pip install flask=0.10

This should install Flask and everything it depends on. Don't be surprised if you find it has installed several Python modules as a result.

Flask should now be installed! Now, time to create our first app.

A Minimal Application

A minimal Flask application looks something like this:

from flask import Flask
app = Flask(__name__)

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

So what did that code do?

1. First we imported the Flask class. An instance of this class will be our WSGI application.
2. Next we create an instance of this class. The first argument is the name of the application’s module or package. If you are using a single module (as in this example), you should use __name__ because depending on if it’s started as application or imported as module the name will be different ('__main__' versus the actual import name). This is needed so that Flask knows where to look for templates, static files, and so on. For more information have a look at the Flask documentation.
3. We then use the route() decorator to tell Flask what URL should trigger our function.
4. The function is given a name which is also used to generate URLs for that particular function, and returns the message we want to display in the user’s browser.

Just save it as hello.py or something similar. Make sure to not call your application flask.py because this would conflict with Flask itself.
To run the application you can either use the flask command or python’s -m switch with Flask. Before you can do that you need to tell your terminal the application to work with by exporting the FLASK_APP environment variable:

$ export FLASK_APP=hello.py
$ flask run
 * Running on http://127.0.0.1:5000/

If you are on Windows you need to use set instead of export.
Alternatively you can use python -m flask:

$ export FLASK_APP=hello.py
$ python -m flask run
 * Running on http://127.0.0.1:5000/

This launches a very simple builtin server, which is good enough for testing but probably not what you want to use in production.
Now head over to http://127.0.0.1:5000/, and you should see your hello world greeting.

Externally Visible Server

If you run the server you will notice that the server is only accessible from your own computer, not from any other in the network. This is the default because in debugging mode a user of the application can execute arbitrary Python code on your computer.
If you have the debugger disabled or trust the users on your network, you can make the server publicly available simply by adding --host=0.0.0.0 to the command line:

flask run --host=0.0.0.0

This tells your operating system to listen on all public IPs.

What to do if the Server does not Start

In case the python -m flask fails or flask does not exist, there are multiple reasons this might be the case. First of all you need to look at the error message.

Old Version of Flask

Versions of Flask older than 0.11 use to have different ways to start the application. In short, the flask command did not exist, and neither did python -m flask.

Invalid Import Name

The FLASK_APP environment variable is the name of the module to import at flask run. In case that module is incorrectly named you will get an import error upon start (or if debug is enabled when you navigate to the application). It will tell you what it tried to import and why it failed.
The most common reason is a typo or because you did not actually create an app object.

Debug Mode

(Want to just log errors and stack traces? )
The flask script is nice to start a local development server, but you would have to restart it manually after each change to your code. That is not very nice and Flask can do better. If you enable debug support the server will reload itself on code changes, and it will also provide you with a helpful debugger if things go wrong.
To enable debug mode you can export the FLASK_DEBUG environment variable before running the server:

$ export FLASK_DEBUG=1
$ flask run

(On Windows you need to use set instead of export).
This does the following things:

1. it activates the debugger
2. it activates the automatic reloader
3. it enables the debug mode on the Flask application.

Attention

Even though the interactive debugger does not work in forking environments (which makes it nearly impossible to use on production servers), it still allows the execution of arbitrary code. This makes it a major security risk and therefore it must never be used on production machines.