Step 2: Application Setup Code

Next, we will create the application module, flaskr.py. Just like the schema.sql file you created in the previous step, this file should be placed inside of the flaskr/flaskr folder.

For this tutorial, all the Python code we use will be put into this file (except for one line in __init__.py, and any testing or optional files you decide to create).

The first several lines of code in the application module are the needed import statements. After that there will be a few lines of configuration code.

For small applications like flaskr, it is possible to drop the configuration directly into the module. However, a cleaner solution is to create a separate .py file, load that, and import the values from there.

Here are the import statements (in flaskr.py):

import os
import sqlite3

from flask import (Flask, request, session, g, redirect, url_for, abort,
    render_template, flash)

The next couple lines will create the actual application instance and initialize it with the config from the same file in flaskr.py:

app = Flask(__name__) # create the application instance :)
app.config.from_object(__name__) # load config from this file , flaskr.py

# Load default config and override config from an environment variable
app.config.update(
    DATABASE=os.path.join(app.root_path, 'flaskr.db'),
    SECRET_KEY=b'_5#y2L"F4Q8z\n\xec]/',
    USERNAME='admin',
    PASSWORD='default'
)
app.config.from_envvar('FLASKR_SETTINGS', silent=True)

In the above code, the Config object works similarly to a dictionary, so it can be updated with new values.

Database Path

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 os.path module, 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 configuration setups. from_envvar() can help achieve this.

app.config.from_envvar('FLASKR_SETTINGS', silent=True)

If you want to do this (not required for this tutorial) 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 from_object() 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.

The SECRET_KEY is needed to keep the client-side sessions secure. Choose that key wisely and as hard to guess and complex as possible.

Lastly, add a method that allows for easy connections to the specified database.

def connect_db():
    """Connects to the specific database."""

    rv = sqlite3.connect(app.config['DATABASE'])
    rv.row_factory = sqlite3.Row
    return rv

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. You can create a simple database connection through SQLite and then tell it to use the sqlite3.Row object to represent rows. This allows the rows to be treated as if they were dictionaries instead of tuples.

In the next section you will see how to run the application.

Continue with Step 3: Installing flaskr as a Package.