A Flask application is an instance of the
Everything about the application, such as configuration and URLs, will
be registered with this class.
The most straightforward way to create a Flask application is to create
Flask instance directly at the top of your code, like
how the “Hello, World!” example did on the previous page. While this is
simple and useful in some cases, it can cause some tricky issues as the
Instead of creating a
Flask instance globally, you will create
it inside a function. This function is known as the application
factory. Any configuration, registration, and other setup the
application needs will happen inside the function, then the application
will be returned.
The Application Factory¶
It’s time to start coding! Create the
flaskr directory and add the
__init__.py file. The
__init__.py serves double duty: it will
contain the application factory, and it tells Python that the
directory should be treated as a package.
$ mkdir flaskr
from flask import Flask
# create and configure the app
app = Flask(__name__, instance_relative_config=True)
if test_config is None:
# load the instance config, if it exists, when not testing
# load the test config if passed in
# ensure the instance folder exists
# a simple page that says hello
return 'Hello, World!'
create_app is the application factory function. You’ll add to it
later in the tutorial, but it already does a lot.
app = Flask(__name__, instance_relative_config=True)creates the
__name__is the name of the current Python module. The app needs to know where it’s located to set up some paths, and
__name__is a convenient way to tell it that.
instance_relative_config=Truetells the app that configuration files are relative to the instance folder. The instance folder is located outside the
flaskrpackage and can hold local data that shouldn’t be committed to version control, such as configuration secrets and the database file.
app.config.from_mapping()sets some default configuration that the app will use:
SECRET_KEYis used by Flask and extensions to keep data safe. It’s set to
'dev'to provide a convenient value during development, but it should be overridden with a random value when deploying.
DATABASEis the path where the SQLite database file will be saved. It’s under
app.instance_path, which is the path that Flask has chosen for the instance folder. You’ll learn more about the database in the next section.
app.config.from_pyfile()overrides the default configuration with values taken from the
config.pyfile in the instance folder if it exists. For example, when deploying, this can be used to set a real
test_configcan also be passed to the factory, and will be used instead of the instance configuration. This is so the tests you’ll write later in the tutorial can be configured independently of any development values you have configured.
app.instance_pathexists. Flask doesn’t create the instance folder automatically, but it needs to be created because your project will create the SQLite database file there.
@app.route()creates a simple route so you can see the application working before getting into the rest of the tutorial. It creates a connection between the URL
/helloand a function that returns a response, the string
'Hello, World!'in this case.
Run The Application¶
Now you can run your application using the
flask command. From the
terminal, tell Flask where to find your application, then run it in
debug mode. Remember, you should still be in the top-level
flask-tutorial directory, not the
Debug mode shows an interactive debugger whenever a page raises an exception, and restarts the server whenever you make changes to the code. You can leave it running and just reload the browser page as you follow the tutorial.
$ flask --app flaskr run --debug
You’ll see output similar to this:
* Serving Flask app "flaskr"
* Debug mode: on
* Running on http://127.0.0.1:5000/ (Press CTRL+C to quit)
* Restarting with stat
* Debugger is active!
* Debugger PIN: nnn-nnn-nnn
Visit http://127.0.0.1:5000/hello in a browser and you should see the “Hello, World!” message. Congratulations, you’re now running your Flask web application!
If another program is already using port 5000, you’ll see
OSError: [Errno 98] or
OSError: [WinError 10013] when the
server tries to start. See Address already in use for how to
Continue to Define and Access the Database.