Step 6: The View Functions — Flask 0.12.2 documentation

# Step 6: The View Functions

Now that the database connections are working, you can start writing the view functions. You 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, it 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 this view checks that the user is logged in (that is, if 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. There is 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 it is not necessary 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'))

Security Note

Passwords should never be stored in plain text in a production system. This tutorial uses plain text passwords for simplicity. If you plan to release a project based off this tutorial out into the world, passwords should be both hashed and salted before being stored in a database or file.

Fortunately, there are Flask extensions for the purpose of hashing passwords and verifying passwords against hashes, so adding this functionality is fairly straight forward. There are also many general python libraries that can be used for hashing.

You can find a list of recommended Flask extensions here

Continue with Step 7: The Templates.