Step 2: Application Setup Code¶
Now that we have the schema in place, we can create the application module.
Let’s call it
flaskr.py. We will place this file inside the
folder. We will begin by adding the imports we need and by adding the config
section. For small applications, it is possible to drop the configuration
directly into the module, and this is what we will be doing here. However,
a cleaner solution would be to create a separate
load that, and import the values from there.
First, we add the imports in
# all the imports import os import sqlite3 from flask import Flask, request, session, g, redirect, url_for, abort, \ render_template, flash
Next, we can create our actual application and initialize it with the
config from the same file in
# create our little application :) app = Flask(__name__) app.config.from_object(__name__) # Load default config and override config from an environment variable app.config.update(dict( DATABASE=os.path.join(app.root_path, 'flaskr.db'), SECRET_KEY='development key', USERNAME='admin', PASSWORD='default' )) app.config.from_envvar('FLASKR_SETTINGS', silent=True)
Config object works similarly to a dictionary so we
can update it with new values.
Operating systems know the concept of a current working directory for each process. Unfortunately, you cannot depend on this in web applications because you might have more than one application in the same process.
For this reason the
app.root_path attribute can be used to
get the path to the application. Together with the
files can then easily be found. In this example, we place the
database right next to it.
For a real-world application, it’s recommended to use Instance Folders instead.
Usually, it is a good idea to load a separate, environment-specific
configuration file. Flask allows you to import multiple configurations and it
will use the setting defined in the last import. This enables robust
from_envvar() can help achieve this.
Simply define the environment variable
FLASKR_SETTINGS that points to
a config file to be loaded. The silent switch just tells Flask to not complain
if no such environment key is set.
In addition to that, you can use the
method on the config object and provide it with an import name of a
module. Flask will then initialize the variable from that module. Note
that in all cases, only variable names that are uppercase are considered.
SECRET_KEY is needed to keep the client-side sessions secure.
Choose that key wisely and as hard to guess and complex as possible.
We will also add a method that allows for easy connections to the
specified database. This can be used to open a connection on request and
also from the interactive Python shell or a script. This will come in
handy later. We create a simple database connection through SQLite and
then tell it to use the
sqlite3.Row object to represent rows.
This allows us to treat the rows as if they were dictionaries instead of
def connect_db(): """Connects to the specific database.""" rv = sqlite3.connect(app.config['DATABASE']) rv.row_factory = sqlite3.Row return rv
With that out of the way, you should be able to start up the application without problems. Do this with the following commands:
export FLASK_APP=flaskr export FLASK_DEBUG=1 flask run
(In case you are on Windows you need to use set instead of export).
FLASK_DEBUG flag enables or disables the interactive debugger.
Never leave debug mode activated in a production system, because it will
allow users to execute code on the server!
You will see a message telling you that server has started along with the address at which you can access it.
When you head over to the server in your browser, you will get a 404 error because we don’t have any views yet. We will focus on that a little later, but first, we should get the database working.
Externally Visible Server
Want your server to be publicly available? Check out the externally visible server section for more information.
Continue with Step 3: Database Connections.