Step 5: The View Functions

Now that the database connections are working, we can start writing the view functions. We will need four of them:

Show Entries

This view shows all the entries stored in the database. It listens on the root of the application and will select title and text from the database. The one with the highest id (the newest entry) will be on top. The rows returned from the cursor look a bit like dictionaries because we are using the sqlite3.Row row factory.

The view function will pass the entries to the show_entries.html template and return the rendered one:

@app.route('/')
def show_entries():
    db = get_db()
    cur = db.execute('select title, text from entries order by id desc')
    entries = cur.fetchall()
    return render_template('show_entries.html', entries=entries)

Add New Entry

This view lets the user add new entries if they are logged in. This only responds to POST requests; the actual form is shown on the show_entries page. If everything worked out well, we will flash() an information message to the next request and redirect back to the show_entries page:

@app.route('/add', methods=['POST'])
def add_entry():
    if not session.get('logged_in'):
        abort(401)
    db = get_db()
    db.execute('insert into entries (title, text) values (?, ?)',
                 [request.form['title'], request.form['text']])
    db.commit()
    flash('New entry was successfully posted')
    return redirect(url_for('show_entries'))

Note that we check that the user is logged in here (the logged_in key is present in the session and True).

Security Note

Be sure to use question marks when building SQL statements, as done in the example above. Otherwise, your app will be vulnerable to SQL injection when you use string formatting to build SQL statements. See Using SQLite 3 with Flask for more.

Login and Logout

These functions are used to sign the user in and out. Login checks the username and password against the ones from the configuration and sets the logged_in key for the session. If the user logged in successfully, that key is set to True, and the user is redirected back to the show_entries page. In addition, a message is flashed that informs the user that he or she was logged in successfully. If an error occurred, the template is notified about that, and the user is asked again:

@app.route('/login', methods=['GET', 'POST'])
def login():
    error = None
    if request.method == 'POST':
        if request.form['username'] != app.config['USERNAME']:
            error = 'Invalid username'
        elif request.form['password'] != app.config['PASSWORD']:
            error = 'Invalid password'
        else:
            session['logged_in'] = True
            flash('You were logged in')
            return redirect(url_for('show_entries'))
    return render_template('login.html', error=error)

The logout function, on the other hand, removes that key from the session again. We use a neat trick here: if you use the pop() method of the dict and pass a second parameter to it (the default), the method will delete the key from the dictionary if present or do nothing when that key is not in there. This is helpful because now we don’t have to check if the user was logged in.

@app.route('/logout')
def logout():
    session.pop('logged_in', None)
    flash('You were logged out')
    return redirect(url_for('show_entries'))

Note that it is not a good idea to store passwords in plain text. You want to protect login credentials if someone happens to have access to your database. One way to do this is to use Security Helpers from Werkzeug to hash the password. However, the emphasis of this tutorial is to demonstrate the basics of Flask and plain text passwords are used for simplicity.

Continue with Step 6: The Templates.