Define and Access the Database¶
SQLite is convenient because it doesn’t require setting up a separate database server and is built-in to Python. However, if concurrent requests try to write to the database at the same time, they will slow down as each write happens sequentially. Small applications won’t notice this. Once you become big, you may want to switch to a different database.
The tutorial doesn’t go into detail about SQL. If you are not familiar with it, the SQLite docs describe the language.
Connect to the Database¶
The first thing to do when working with a SQLite database (and most other Python database libraries) is to create a connection to it. Any queries and operations are performed using the connection, which is closed after the work is finished.
In web applications this connection is typically tied to the request. It is created at some point when handling a request, and closed before the response is sent.
import sqlite3 import click from flask import current_app, g from flask.cli import with_appcontext def get_db(): if 'db' not in g: g.db = sqlite3.connect( current_app.config['DATABASE'], detect_types=sqlite3.PARSE_DECLTYPES ) g.db.row_factory = sqlite3.Row return g.db def close_db(e=None): db = g.pop('db', None) if db is not None: db.close()
g is a special object that is unique for each request. It is
used to store data that might be accessed by multiple functions during
the request. The connection is stored and reused instead of creating a
new connection if
get_db is called a second time in the same
current_app is another special object that points to the Flask
application handling the request. Since you used an application factory,
there is no application object when writing the rest of your code.
get_db will be called when the application has been created and is
handling a request, so
current_app can be used.
sqlite3.connect() establishes a connection to the file pointed at
DATABASE configuration key. This file doesn’t have to exist
yet, and won’t until you initialize the database later.
sqlite3.Row tells the connection to return rows that behave
like dicts. This allows accessing the columns by name.
close_db checks if a connection was created by checking if
was set. If the connection exists, it is closed. Further down you will
tell your application about the
close_db function in the application
factory so that it is called after each request.
Create the Tables¶
In SQLite, data is stored in tables and columns. These need to be
created before you can store and retrieve data. Flaskr will store users
user table, and posts in the
post table. Create a file
with the SQL commands needed to create empty tables:
DROP TABLE IF EXISTS user; DROP TABLE IF EXISTS post; CREATE TABLE user ( id INTEGER PRIMARY KEY AUTOINCREMENT, username TEXT UNIQUE NOT NULL, password TEXT NOT NULL ); CREATE TABLE post ( id INTEGER PRIMARY KEY AUTOINCREMENT, author_id INTEGER NOT NULL, created TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP, title TEXT NOT NULL, body TEXT NOT NULL, FOREIGN KEY (author_id) REFERENCES user (id) );
Add the Python functions that will run these SQL commands to the
def init_db(): db = get_db() with current_app.open_resource('schema.sql') as f: db.executescript(f.read().decode('utf8')) @click.command('init-db') @with_appcontext def init_db_command(): """Clear the existing data and create new tables.""" init_db() click.echo('Initialized the database.')
open_resource() opens a file relative to
flaskr package, which is useful since you won’t necessarily know
where that location is when deploying the application later.
returns a database connection, which is used to execute the commands
read from the file.
click.command() defines a command line command called
that calls the
init_db function and shows a success message to the
user. You can read Command Line Interface to learn more about writing commands.
Register with the Application¶
init_db_command functions need to be registered
with the application instance; otherwise, they won’t be used by the
application. However, since you’re using a factory function, that
instance isn’t available when writing the functions. Instead, write a
function that takes an application and does the registration.
def init_app(app): app.teardown_appcontext(close_db) app.cli.add_command(init_db_command)
Flask to call that function when cleaning up after returning the
app.cli.add_command() adds a new
command that can be called with the
Import and call this function from the factory. Place the new code at the end of the factory function before returning the app.
def create_app(): app = ... # existing code omitted from . import db db.init_app(app) return app
Initialize the Database File¶
init-db has been registered with the app, it can be called
flask command, similar to the
run command from the
If you’re still running the server from the previous page, you can
either stop the server, or run this command in a new terminal. If
you use a new terminal, remember to change to your project directory
and activate the env as described in Installation. You’ll
also need to set
FLASK_ENV as shown on the
$ flask init-db Initialized the database.
There will now be a
flaskr.sqlite file in the
instance folder in
Continue to Blueprints and Views.