Define and Access the Database¶
The application will use a SQLite database to store users and posts.
Python comes with built-in support for SQLite in the sqlite3
module.
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
from datetime import datetime
import click
from flask import current_app, g
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
request.
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
by the 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 g.db
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
in the 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
db.py
file:
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')
def init_db_command():
"""Clear the existing data and create new tables."""
init_db()
click.echo('Initialized the database.')
sqlite3.register_converter(
"timestamp", lambda v: datetime.fromisoformat(v.decode())
)
open_resource()
opens a file relative to
the flaskr
package, which is useful since you won’t necessarily know
where that location is when deploying the application later. get_db
returns a database connection, which is used to execute the commands
read from the file.
click.command()
defines a command line command called init-db
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.
The call to sqlite3.register_converter()
tells Python how to
interpret timestamp values in the database. We convert the value to a
datetime.datetime
.
Register with the Application¶
The close_db
and 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)
app.teardown_appcontext()
tells
Flask to call that function when cleaning up after returning the
response.
app.cli.add_command()
adds a new
command that can be called with the flask
command.
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¶
Now that init-db
has been registered with the app, it can be called
using the flask
command, similar to the run
command from the
previous page.
Note
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.
Run the init-db
command:
$ flask --app flaskr init-db
Initialized the database.
There will now be a flaskr.sqlite
file in the instance
folder in
your project.
Continue to Blueprints and Views.