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      How To Use a PostgreSQL Database in a Flask Application


      The author selected the Free and Open Source Fund to receive a donation as part of the Write for DOnations program.

      Introduction

      In web applications, you usually need a database, which is an organized collection of data. You use a database to store and maintain persistent data that can be retrieved and manipulated efficiently. For example, in a social media application, you have a database where user data (personal information, posts, comments, followers) is stored in a way that can be efficiently manipulated. You can add data to a database, retrieve it, modify it, or delete it, depending on different requirements and conditions. In a web application, these requirements might be a user adding a new post, deleting a post, or deleting their account, which might or might not delete their posts. The actions you perform to manipulate data will depend on specific features in your application. For example, you might not want users to add posts with no titles.

      Flask is a lightweight Python web framework that provides useful tools and features for creating web applications in the Python Language. PostgreSQL, or Postgres, is a relational database management system that provides an implementation of the SQL querying language. It’s standards-compliant and has many advanced features such as reliable transactions and concurrency without read locks.

      In this tutorial, you’ll build a small book review web application that demonstrates how to use the psycopg2 library, a PostgreSQL database adapter that allows you to interact with your PostgreSQL database in Python. You’ll use it with Flask to perform basic tasks, such as connecting to a database server, creating tables, inserting data to a table, and retrieving data from a table.

      Prerequisites

      Step 1 — Creating the PostgreSQL Database and User

      In this step, you’ll create a database called flask_db and a database user called sammy for your Flask application.

      During the Postgres installation, an operating system user named postgres was created to correspond to the postgres PostgreSQL administrative user. You need to use this user to perform administrative tasks. You can use sudo and pass in the username with the -iu option.

      Log in to an interactive Postgres session using the following command:

      You will be given a PostgreSQL prompt where you can set up your requirements.

      First, create a database for your project:

      • CREATE DATABASE flask_db;

      Note: Every Postgres statement must end with a semi-colon, so make sure that your command ends with one if you are experiencing issues.

      Next, create a database user for our project. Make sure to select a secure password:

      • CREATE USER sammy WITH PASSWORD "https://www.digitalocean.com/community/tutorials/password';

      Then give this new user access to administer your new database:

      • GRANT ALL PRIVILEGES ON DATABASE flask_db TO sammy;

      To confirm the database was created, get the list of databases by typing the following command:

      You’ll see flask_db in the list of databases.

      When you are finished, exit out of the PostgreSQL prompt by typing:

      Postgres is now set up so that you can connect to and manage its database information via Python using the psycopg2 library. Next, you’ll install this library alongside the Flask package.

      Step 2 — Installing Flask and psycopg2

      In this step, you will install Flask and the psycopg2 library so that you can interact with your database using Python.

      With your virtual environment activated, use pip to install Flask and the psycopg2 library:

      • pip install Flask psycopg2-binary

      Once the installation is successfully finished, you’ll see a line similar to the following at the end of the output:

      Output

      Successfully installed Flask-2.0.2 Jinja2-3.0.3 MarkupSafe-2.0.1 Werkzeug-2.0.2 click-8.0.3 itsdangerous-2.0.1 psycopg2-binary-2.9.2

      You now have the required packages installed on your virtual environment. Next, you’ll connect to and set up your database.

      Step 3 — Setting up a Database

      In this step, you’ll create a Python file in your flask_app project directory to connect to the flask_db database, create a table for storing books, and insert some books with reviews into it.

      First with your programming environment activated, open a new file called init_db.py in your flask_app directory.

      This file will open a connection to the flask_db database, create a table called books, and populate the table using sample data. Add the following code to it:

      flask_app/init_db.py

      import os
      import psycopg2
      
      conn = psycopg2.connect(
              host="localhost",
              database="flask_db",
              user=os.environ['DB_USERNAME'],
              password=os.environ['DB_PASSWORD'])
      
      # Open a cursor to perform database operations
      cur = conn.cursor()
      
      # Execute a command: this creates a new table
      cur.execute('DROP TABLE IF EXISTS books;')
      cur.execute('CREATE TABLE books (id serial PRIMARY KEY,'
                                       'title varchar (150) NOT NULL,'
                                       'author varchar (50) NOT NULL,'
                                       'pages_num integer NOT NULL,'
                                       'review text,'
                                       'date_added date DEFAULT CURRENT_TIMESTAMP);'
                                       )
      
      # Insert data into the table
      
      cur.execute('INSERT INTO books (title, author, pages_num, review)'
                  'VALUES (%s, %s, %s, %s)',
                  ('A Tale of Two Cities',
                   'Charles Dickens',
                   489,
                   'A great classic!')
                  )
      
      
      cur.execute('INSERT INTO books (title, author, pages_num, review)'
                  'VALUES (%s, %s, %s, %s)',
                  ('Anna Karenina',
                   'Leo Tolstoy',
                   864,
                   'Another great classic!')
                  )
      
      conn.commit()
      
      cur.close()
      conn.close()
      

      Save and close the file.

      In this file, you first import the os module you’ll use to access environment variables where you’ll store your database username and password so that they are not visible in your source code.

      You import the psycopg2 library. Then you open a connection to the flask_db database using the psycopg2.connect() function. You specify the host, which is the localhost in this case. You pass the database name to the database parameter.

      You provide your username and password via the os.environ object, which gives you access to environment variables you set in your programming environment. You will store the database username in an environment variable called DB_USERNAME and the password in an environment variable called DB_PASSWORD. This allows you to store your username and password outside your source code, so that your sensitive information is not leaked when the source code is saved in source control or uploaded to a server on the internet. Even if an attacker gains access to your source code, they will not gain access to the database.

      You create a cursor called cur using the connection.cursor() method, which allows Python code to execute PostgreSQL commands in a database session.

      You use the cursor’s execute() method to delete the books table if it already exists. This avoids the possibility of another table named books existing, which might result in confusing behavior (for example, if it has different columns). This isn’t the case here, because you haven’t created the table yet, so the SQL command won’t be executed. Note that this will delete all of the existing data whenever you execute this init_db.py file. For our purposes, you will only execute this file once to initiate the database, but you might want to execute it again to delete whatever data you inserted and start with the initial sample data again.

      Then you use CREATE TABLE books to create a table named books with the following columns:

      • id: An ID of the serial type, which is an autoincrementing integer. This column represents a primary key you specify using the PRIMARY KEY keywords. The database will assign a unique value to this key for each entry.
      • title: The book’s title of the varchar type, which is a character type of variable length with a limit. varchar (150) means that the title can be up to 150 characters long. NOT NULL signifies that this column can’t be empty.
      • author: The book’s author, with a limit of 50 characters. NOT NULL signifies that this column can’t be empty.
      • pages_num: An integer representing the number of pages the book has. NOT NULL signifies that this column can’t be empty.
      • review: The book review. The text type signifies that the review can be text of any length.
      • date_added: The date the book was added to the table. DEFAULT sets the default value of the column to CURRENT_TIMESTAMP, which is the time at which the book was added to the database. Just like id, you don’t need to specify a value for this column, as it will be automatically filled in.

      After creating the table, you use the cursor’s execute() method to insert two books into the table, A Tale of Two Cities by Charles Dickens, and Anna Karenina by Leo Tolstoy. You use the %s placeholder to pass the values to the SQL statement. psycopg2 handles the insertion in the background in a way that prevents SQL Injection attacks.

      Once you finish inserting book data into your table, you use the connection.commit() method to commit the transaction and apply the changes to the database. Then you clean things up by closing the cursor with cur.close(), and the connection with conn.close().

      For the database connection to be established, set the DB_USERNAME and DB_PASSWORD environment variables by running the following commands. Remember to use your own username and password:

      • export DB_USERNAME="https://www.digitalocean.com/community/tutorials/sammy"
      • export DB_PASSWORD="https://www.digitalocean.com/community/tutorials/password"

      Now, run your init_db.py file in the terminal using the python command:

      Once the file finishes execution with no errors, a new books table will be added to your flask_db database.

      Log in to an interactive Postgres session to check out the new books table.

      Connect to the flask_db database using the c command:

      Then use a SELECT statement to get the titles and authors of books from the books table:

      • SELECT title, author FROM books;

      You’ll see an output like the following:

              title         |      author
      ----------------------+------------------
       A Tale of Two Cities | Charles Dickens
       Anna Karenina        | Leo Tolstoy
      

      Quit the interactive session with q.

      Next, you’ll create a small Flask application, connect to the database, retrieve the two book reviews you inserted into the database, and display them on the index page.

      Step 4 — Displaying Books

      In this step, you’ll create a Flask application with an index page that retrieves the books that are in the database, and display them.

      With your programming environment activated and Flask installed, open a file called app.py for editing inside your flask_app directory:

      This file will set up your database connection and create a single Flask route to use that connection. Add the following code to the file:

      flask_app/app.py

      import os
      import psycopg2
      from flask import Flask, render_template
      
      app = Flask(__name__)
      
      def get_db_connection():
          conn = psycopg2.connect(host="localhost",
                                  database="https://www.digitalocean.com/community/tutorials/flask_db",
                                  user=os.environ['DB_USERNAME'],
                                  password=os.environ['DB_PASSWORD'])
          return conn
      
      
      @app.route('/')
      def index():
          conn = get_db_connection()
          cur = conn.cursor()
          cur.execute('SELECT * FROM books;')
          books = cur.fetchall()
          cur.close()
          conn.close()
          return render_template('index.html', books=books)
      

      Save and close the file.

      Here, you import the os module, the psycopg2 library, and the Flask class and the render_template() from the flask package. You make a Flask application instance called app.

      You define a function called get_db_connection(), which opens a connection to the flask_db database using the user and password you store in your DB_USERNAME and DB_PASSWORD environment variables. The function returns the conn connection object you’ll be using to access the database.

      Then you create a main / route and an index() view function using the app.route() decorator. In the index() view function, you open a database connection using the get_db_connection() function, you create a cursor, and execute the SELECT * FROM books; SQL statement to get all the books that are in the database. You use the fetchall() method to save the data in a variable called books. Then you close the cursor and the connection. Lastly, you return a call to the render_template() function to render a template file called index.html passing it the list of books you fetched from the database in the books variable.

      To display the books you have in your database on the index page, you will first create a base template, which will have all the basic HTML code other templates will also use to avoid code repetition. Then you’ll create the index.html template file you rendered in your index() function. To learn more about templates, see How to Use Templates in a Flask Application.

      Create a templates directory, then open a new template called base.html:

      • mkdir templates
      • nano templates/base.html

      Add the following code inside the base.html file:

      flask_app/templates/base.html

      <!DOCTYPE html>
      <html lang="en">
      <head>
          <meta charset="UTF-8">
          <title>{% block title %} {% endblock %}- FlaskApp</title>
          <style>
              nav a {
                  color: #d64161;
                  font-size: 3em;
                  margin-left: 50px;
                  text-decoration: none;
              }
      
              .book {
                  padding: 20px;
                  margin: 10px;
                  background-color: #f7f4f4;
              }
      
              .review {
                      margin-left: 50px;
                      font-size: 20px;
              }
      
          </style>
      </head>
      <body>
          <nav>
              <a href="https://www.digitalocean.com/community/tutorials/{{ url_for('index') }}">FlaskApp</a>
              <a href="#">About</a>
          </nav>
          <hr>
          <div class="content">
              {% block content %} {% endblock %}
          </div>
      </body>
      </html>
      

      Save and close the file.

      This base template has all the HTML boilerplate you’ll need to reuse in your other templates. The title block will be replaced to set a title for each page, and the content block will be replaced with the content of each page. The navigation bar has two links, one for the index page where you use the url_for() helper function to link to the index() view function, and the other for an About page if you choose to include one in your application.

      Next, open a template called index.html. This is the template you referenced in the app.py file:

      • nano templates/index.html

      Add the following code to it:

      flask_app/templates/index.html

      
      {% extends 'base.html' %}
      
      {% block content %}
          <h1>{% block title %} Books {% endblock %}</h1>
          {% for book in books %}
              <div class="book">
                  <h3>#{{ book[0] }} - {{ book[1] }} BY {{ book[2] }}</h3>
                  <i><p>({{ book[3] }} pages)</p></i>
                  <p class="review">{{ book[4] }}</p>
                  <i><p>Added {{ book[5] }}</p></i>
              </div>
          {% endfor %}
      {% endblock %}
      

      Save and close the file.

      In this file, you extend the base template, and replace the contents of the content block. You use an <h1> heading that also serves as a title.

      You use a Jinja for loop in the line {% for book in books %} to go through each book in the books list. You display the book ID, which is the first item using book[0]. You then display the book title, author, number of pages, review, and the date the book was added.

      While in your flask_app directory with your virtual environment activated, tell Flask about the application (app.py in this case) using the FLASK_APP environment variable. Then set the FLASK_ENV environment variable to development to run the application in development mode and get access to the debugger. For more information about the Flask debugger, see How To Handle Errors in a Flask Application. Use the following commands to do this:

      • export FLASK_APP=app
      • export FLASK_ENV=development

      Make sure you set the DB_USERNAME and DB_PASSWORD environment variables if you haven’t already:

      • export DB_USERNAME="https://www.digitalocean.com/community/tutorials/sammy"
      • export DB_PASSWORD="https://www.digitalocean.com/community/tutorials/password"

      Next, run the application:

      With the development server running, visit the following URL using your browser:

      http://127.0.0.1:5000/
      

      You’ll see the books you added to the database on the first initiation.

      Index Page

      You’ve displayed the books in your database on the index page. You now need to allow users to add new books. You’ll add a new route for adding books in the next step.

      Step 5 — Adding New Books

      In this step, you’ll create a new route for adding new books and reviews to the database.

      You’ll add a page with a web form where users enter the book title, book author, the number of pages, and the book review.

      Leave the development server running and open a new terminal window.

      First, open your app.py file:

      For handling the web form, you’ll need to import a few things from the flask package:

      • The global request object to access submitted data.
      • The url_for() function to generate URLs.
      • The redirect() function to redirect users to the index page after adding a book to the database.

      Add these imports to the first line in the file:

      flask_app/app.py

      
      from flask import Flask, render_template, request, url_for, redirect
      
      # ...
      

      Then add the following route at the end of the app.py file:

      flask_app/app.py

      
      # ...
      
      
      @app.route('/create/', methods=('GET', 'POST'))
      def create():
          return render_template('create.html')
      

      Save and close the file.

      In this route, you pass the tuple ('GET', 'POST') to the methods parameter to allow both GET and POST requests. GET requests are used to retrieve data from the server. POST requests are used to post data to a specific route. By default, only GET requests are allowed. When the user first requests the /create route using a GET request, a template file called create.html will be rendered. You will later edit this route to handle POST requests for when users fill and submit the web form for adding new books.

      Open the new create.html template:

      • nano templates/create.html

      Add the following code to it:

      flask_app/templates/create.html

      {% extends 'base.html' %}
      
      {% block content %}
          <h1>{% block title %} Add a New Book {% endblock %}</h1>
          <form method="post">
              <p>
                  <label for="title">Title</label>
                  <input type="text" name="title"
                         placeholder="Book title">
                  </input>
              </p>
      
              <p>
                  <label for="author">Author</label>
                  <input type="text" name="author"
                         placeholder="Book author">
                  </input>
              </p>
      
              <p>
                  <label for="pages_num">Number of pages</label>
                  <input type="number" name="pages_num"
                         placeholder="Number of pages">
                  </input>
              </p>
              <p>
              <label for="review">Review</label>
              <br>
              <textarea name="review"
                        placeholder="Review"
                        rows="15"
                        cols="60"
                        ></textarea>
              </p>
              <p>
                  <button type="submit">Submit</button>
              </p>
          </form>
      {% endblock %}
      

      Save and close the file.

      You extend the base template, set a heading as a title, and use a <form> tag with the attribute method set to post to indicate that the form will submit a POST request.

      You have a text field with the name title, which you’ll use to access the title data in your /create route.

      You have a text field for the author, a number field for the number of pages, and a text area for the book review.

      Last, you have a Submit button at the end of the form.

      Now, with the development server running, use your browser to navigate to the /create route:

      http://127.0.0.1:5000/create
      

      You will see an Add a New Book page with an input field for a book title, one for its author, and one for the number of pages the book has, a text area for the book’s review, and a Submit button.

      Add a New Book

      If you fill in the form and submit it, sending a POST request to the server, nothing happens because you did not handle POST requests on the /create route.

      Open app.py to handle the POST request the user submits:

      Edit the /create route to look as follows:

      flask_app/app.py

      
      # ...
      
      @app.route('/create/', methods=('GET', 'POST'))
      def create():
          if request.method == 'POST':
              title = request.form['title']
              author = request.form['author']
              pages_num = int(request.form['pages_num'])
              review = request.form['review']
      
              conn = get_db_connection()
              cur = conn.cursor()
              cur.execute('INSERT INTO books (title, author, pages_num, review)'
                          'VALUES (%s, %s, %s, %s)',
                          (title, author, pages_num, review))
              conn.commit()
              cur.close()
              conn.close()
              return redirect(url_for('index'))
      
          return render_template('create.html')
      

      Save and close the file.

      You handle POST requests inside the if request.method == 'POST' condition. You extract the title, author, number of pages, and the review the user submits from the request.form object.

      You open a database using the get_db_connection() function, and create a cursor. Then you execute an INSERT INTO SQL statement to insert the title, author, number of pages, and review the user submitted into the books table.

      You commit the transaction and close the cursor and connection.

      Lastly, you redirect the user to the index page where they can see the newly added book below the existing books.

      With the development server running, use your browser to navigate to the /create route:

      http://127.0.0.1:5000/create
      

      Fill in the form with some data and submit it.

      You’ll be redirected to the index page where you’ll see your new book review.

      Next, you’ll add a link to the Create page in the navigation bar. Open base.html:

      Edit the file to look as follows:

      flask_app/templates/base.html

      
      <!DOCTYPE html>
      <html lang="en">
      <head>
          <meta charset="UTF-8">
          <title>{% block title %} {% endblock %} - FlaskApp</title>
          <style>
              nav a {
                  color: #d64161;
                  font-size: 3em;
                  margin-left: 50px;
                  text-decoration: none;
              }
      
              .book {
                  padding: 20px;
                  margin: 10px;
                  background-color: #f7f4f4;
              }
      
              .review {
                      margin-left: 50px;
                      font-size: 20px;
              }
      
          </style>
      </head>
      <body>
          <nav>
              <a href="https://www.digitalocean.com/community/tutorials/{{ url_for("index') }}">FlaskApp</a>
              <a href="https://www.digitalocean.com/community/tutorials/{{ url_for("create') }}">Create</a>
              <a href="#">About</a>
          </nav>
          <hr>
          <div class="content">
              {% block content %} {% endblock %}
          </div>
      </body>
      </html>
      

      Save and close the file.

      Here, you add a new <a> link to the navigation bar that points to the Create page.

      Refresh your index page and you’ll see the new link in the navigation bar.

      You now have a page with a web form for adding new book reviews. For more on web forms, see How To Use Web Forms in a Flask Application. For a more advanced and more secure method of managing web forms, see How To Use and Validate Web Forms with Flask-WTF.

      Conclusion

      You built a small web application for book reviews that communicates with a PostgreSQL database. You have basic database functionality in your Flask application, such as adding new data to the database, retrieving data, and displaying it on a page.

      If you would like to read more about Flask, check out the other tutorials in the Flask series.



      Source link

      How To Build and Deploy a Flask Application Using Docker on Ubuntu 20.04


      The author selected the Tech Education Fund to receive a donation as part of the Write for DOnations program.

      Introduction

      Docker is an open-source application that allows administrators to create, manage, deploy, and replicate applications using containers. Containers can be thought of as a package that houses dependencies that an application requires to run at an operating system level. This means that each application deployed using Docker lives in an environment of its own and its requirements are handled separately.

      Flask is a web micro-framework that is built on Python. It is called a micro-framework because it does not require specific tools or plug-ins to run. The Flask framework is lightweight and flexible, yet highly structured, making it especially popular for small web apps written in Python.

      Deploying a Flask application with Docker will allow you to replicate the application across different servers with minimal reconfiguration.

      In this tutorial, you will create a Flask application and deploy it with Docker. This tutorial will also cover how to update an application after deployment.

      Prerequisites

      To follow this tutorial, you will need the following:

      Step 1 — Setting Up the Flask Application

      To get started, you will create a directory structure that will hold your Flask application. This tutorial will create a directory called TestApp in /var/www, but you can modify the command to name it whatever you’d like.

      • sudo mkdir /var/www/TestApp

      Move in to the newly created TestApp directory:

      Next, create the base folder structure for the Flask application:

      • sudo mkdir -p app/static app/templates

      The -p flag indicates that mkdir will create a directory and all parent directories that don’t exist. In this case, mkdir will create the app parent directory in the process of making the static and templates directories.

      The app directory will contain all files related to the Flask application such as its views and blueprints. Views are the code you write to respond to requests to your application. Blueprints create application components and support common patterns within an application or across multiple applications.

      The static directory is where assets such as images, CSS, and JavaScript files live. The templates directory is where you will put the HTML templates for your project.

      Now that the base folder structure is complete, you need to create the files needed to run the Flask application. First, create an __init__.py file inside the app directory using nano or a text editor of your choice. This file tells the Python interpreter that the app directory is a package and should be treated as such.

      Run the following command to create the file:

      • sudo nano app/__init__.py

      Packages in Python allow you to group modules into logical namespaces or hierarchies. This approach enables the code to be broken down into individual and manageable blocks that perform specific functions.

      Next, you will add code to the __init__.py that will create a Flask instance and import the logic from the views.py file, which you will create after saving this file. Add the following code to your new file:

      /var/www/TestApp/app/__init__.py

      from flask import Flask
      app = Flask(__name__)
      from app import views
      

      Once you’ve added that code, save and close the file. You can save and close the file by pressing Ctrl+X, then when prompted, Y and Enter.

      With the __init__.py file created, you’re ready to create the views.py file in your app directory. This file will contain most of your application logic.

      Next, add the code to your views.py file. This code will return the hello world! string to users who visit your web page:

      /var/www/TestApp/app/views.py

      from app import app
      
      @app.route('/')
      def home():
         return "hello world!"
      

      The @app.route line above the function is called a decorator. Decorators are a Python language convention that are widely used by Flask; their purpose is to modify the functions immediately following them. In this case, the decorator tells Flask which URL will trigger the home() function. The hello world text returned by the home function will be displayed to the user on the browser.

      With the views.py file in place, you’re ready to create the uwsgi.ini file. This file will contain the uWSGI configurations for our application. uWSGI is a deployment option for Nginx that is both a protocol and an application server; the application server can serve uWSGI, FastCGI, and HTTP protocols.

      To create this file, run the following command:

      Next, add the following content to your file to configure the uWSGI server:

      /var/www/TestApp/uwsgi.ini

      [uwsgi]
      module = main
      callable = app
      master = true
      

      This code defines the module that the Flask application will be served from. In this case, this is the main.py file, referenced here as main. The callable option instructs uWSGI to use the app instance exported by the main application. The master option allows your application to keep running, so there is little downtime even when reloading the entire application.

      Next, create the main.py file, which is the entry point to the application. The entry point instructs uWSGI on how to interact with the application.

      Next, copy and paste the following into the file. This imports the Flask instance named app from the application package that was previously created.

      /var/www/TestApp/main.py

      from app import app
      

      Finally, create a requirements.txt file to specify the dependencies that the pip package manager will install to your Docker deployment:

      • sudo nano requirements.txt

      Add the following line to add Flask as a dependency:

      /var/www/TestApp/requirements.txt

      Flask>=2.0.2
      

      This specifies the version of Flask to be installed. At the time of writing this tutorial, 2.0.2 is the latest Flask version, and specifying >=2.0.2 will ensure you get version 2.0.2 or newer. Because you’re making a basic test app in this tutorial, the syntax is unlikely to go out of date due to future updates to Flask, but if you wanted to be safe and still receive minor updates, you could specify that you don’t want to install a future major version by specifying something like Flask>=2.0.2,<3.0. You can check for updates at the official website for Flask, or on the Python Package Index’s landing page for the Flask library.

      Save and close the file. You have successfully set up your Flask application and are ready to set up Docker.

      Step 2 — Setting Up Docker

      In this step you will create two files, Dockerfile and start.sh, to create your Docker deployment. The Dockerfile is a text document that contains the commands used to assemble the image. The start.sh file is a shell script that will build an image and create a container from the Dockerfile.

      First, create the Dockerfile.

      Next, add your desired configuration to the Dockerfile. These commands specify how the image will be built, and what extra requirements will be included.

      /var/www/TestApp/Dockerfile

      FROM tiangolo/uwsgi-nginx-flask:python3.8-alpine
      RUN apk --update add bash nano
      ENV STATIC_URL /static
      ENV STATIC_PATH /var/www/app/static
      COPY ./requirements.txt /var/www/requirements.txt
      RUN pip install -r /var/www/requirements.txt
      

      In this example, the Docker image will be built off an existing image, tiangolo/uwsgi-nginx-flask, which you can find on DockerHub. This particular Docker image is a good choice over others because it supports a wide range of Python versions and OS images.

      The first two lines specify the parent image that you’ll use to run the application and install the bash command processor and the nano text editor. It also installs the git client for pulling and pushing to version control hosting services such as GitHub, GitLab, and Bitbucket. ENV STATIC_URL /static is an environment variable specific to this Docker image. It defines the static folder where all assets such as images, CSS files, and JavaScript files are served from.

      The last two lines will copy the requirements.txt file into the container so that it can be executed, and then parses the requirements.txt file to install the specified dependencies.

      Save and close the file after adding your configuration.

      With your Dockerfile in place, you’re almost ready to write your start.sh script that will build the Docker container. Before writing the start.sh script, first make sure that you have an open port to use in the configuration. To check if a port is free, run the following command:

      • sudo nc localhost 56733 < /dev/null; echo $?

      If the output of the command above is 1, then the port is free and usable. Otherwise, you will need to select a different port to use in your start.sh configuration file.

      Once you’ve found an open port to use, create the start.sh script:

      The start.sh script is a shell script that will build an image from the Dockerfile and create a container from the resulting Docker image. Add your configuration to the new file:

      /var/www/TestApp/start.sh

      #!/bin/bash
      app="docker.test"
      docker build -t ${app} .
      docker run -d -p 56733:80 
        --name=${app} 
        -v $PWD:/app ${app}
      

      The first line is called a shebang. It specifies that this is a bash file and will be executed as commands. The next line specifies the name you want to give the image and container and saves as a variable named app. The next line instructs Docker to build an image from your Dockerfile located in the current directory. This will create an image called docker.test in this example.

      The last three lines create a new container named docker.test that is exposed at port 56733. Finally, it links the present directory to the /var/www directory of the container.

      You use the -d flag to start a container in daemon mode, or as a background process. You include the -p flag to bind a port on the server to a particular port on the Docker container. In this case, you are binding port 56733 to port 80 on the Docker container. The -v flag specifies a Docker volume to mount on the container, and in this case, you are mounting the entire project directory to the /var/www folder on the Docker container.

      Save and close the file after adding your configuration.

      Execute the start.sh script to create the Docker image and build a container from the resulting image:

      Once the script finishes running, use the following command to list all running containers:

      You will receive output that shows the containers:

      Output

      CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES 58b05508f4dd docker.test "/entrypoint.sh /sta…" 12 seconds ago Up 3 seconds 443/tcp, 0.0.0.0:56733->80/tcp docker.test

      You will find that the docker.test container is running. Now that it is running, visit the IP address at the specified port in your browser: http://ip-address:56733

      You’ll see a page similar to the following:

      the home page

      In this step you have successfully deployed your Flask application on Docker. Next, you will use templates to display content to users.

      Step 3 — Serving Template Files

      Templates are files that display static and dynamic content to users who visit your application. In this step, you will create a HTML template to create a homepage for the application.

      Start by creating a home.html file in the app/templates directory:

      • sudo nano app/templates/home.html

      Add the code for your template. This code will create an HTML5 page that contains a title and some text.

      /var/www/TestApp/app/templates/home.html

      
      <!doctype html>
      
      <html lang="en-us">   
        <head>
          <meta charset="utf-8">
          <meta http-equiv="x-ua-compatible" content="ie=edge">
          <title>Welcome home</title>
        </head>
      
        <body>
          <h1>Home Page</h1>
          <p>This is the home page of our application.</p>
        </body> 
      </html>
      

      Save and close the file once you’ve added your template.

      Next, modify the app/views.py file to serve the newly created file:

      First, add the following line at the beginning of your file to import the render_template method from Flask. This method parses an HTML file to render a web page to the user.

      /var/www/TestApp/app/views.py

      from flask import render_template
      ...
      

      At the end of the file, you will also add a new route to render the template file. This code specifies that users are served the contents of the home.html file whenever they visit the /template route on your application.

      /var/www/TestApp/app/views.py

      ...
      
      @app.route('/template')
      def template():
          return render_template('home.html')
      

      The updated app/views.py file will look like this:

      /var/www/TestApp/app/views.py

      from flask import render_template
      from app import app 
      
      @app.route('/')
      def home():
          return "Hello world!"
      
      @app.route('/template')
      def template():
          return render_template('home.html')
      

      Save and close the file when done.

      In order for these changes to take effect, you will need to stop and restart the Docker containers. Run the following command to rebuild the container:

      • sudo docker stop docker.test && sudo docker start docker.test

      Visit your application at http://your-ip-address:56733/template to see the new template being served.

      homepage

      In this you’ve created a Docker template file to serve visitors on your application. In the next step you will see how the changes you make to your application can take effect without having to restart the Docker container.

      Step 4 — Updating the Application

      Sometimes you will need to make changes to the application, whether it is installing new requirements, updating the Docker container, or HTML and logic changes. In this section, you will configure touch-reload to make these changes without needing to restart the Docker container.

      Python autoreloading watches the entire file system for changes and refreshes the application when it detects a change. Autoreloading is discouraged in production because it can become resource intensive very quickly. In this step, you will use touch-reload to watch for changes to a particular file and reload when the file is updated or replaced.

      To implement this, start by opening your uwsgi.ini file:

      Next, add the highlighted line to the end of the file:

      /var/www/TestApp/uwsgi.ini

      module = main
      callable = app
      master = true
      touch-reload = /app/uwsgi.ini
      

      This specifies a file that will be modified to trigger an entire application reload. Once you’ve made the changes, save and close the file.

      To demonstrate this, make a small change to your application. Start by opening your app/views.py file:

      Replace the string returned by the home function:

      /var/www/TestApp/app/views.py

      from flask import render_template
      from app import app
      
      @app.route('/')
      def home():
          return "<b>There has been a change</b>"
      
      @app.route('/template')
      def template():
          return render_template('home.html')
      

      Save and close the file after you’ve made a change.

      Next, if you open your application’s homepage at http://ip-address:56733, you will notice that the changes are not reflected. This is because the condition for reload is a change to the uwsgi.ini file. To reload the application, use touch to activate the condition:

      Reload the application homepage in your browser again. You will find that the application has incorporated the changes:

      Homepage Updated

      In this step, you set up a touch-reload condition to update your application after making changes.

      Conclusion

      In this tutorial, you created and deployed a Flask application to a Docker container. You also configured touch-reload to refresh your application without needing to restart the container.

      With your new application on Docker, you can now scale with ease. To learn more about using Docker, check out their official documentation.



      Source link

      How to Use an SQLite Database in a Flask Application


      The author selected the Free and Open Source Fund to receive a donation as part of the Write for DOnations program.

      Introduction

      In web applications, you usually need a database, which is an organized collection of data. You use a database to store and maintain persistent data that can be retrieved and manipulated efficiently. For example, in a social media application, you have a database where user data (personal information, posts, comments, followers) is stored in a way that can be efficiently manipulated. You can add data to a database, retrieve it, modify it, or delete it, depending on different requirements and conditions. In a web application, these requirements might be a user adding a new post, deleting a post, or deleting their account, which might or might not delete their posts. The actions you perform to manipulate data will depend on specific features in your application. For example, you might not want users to add posts with no titles.

      Flask is a lightweight Python web framework that provides useful tools and features for creating web applications in the Python Language. SQLite is a simple and fast open source SQL engine that can be used with Python to store and manipulate application data. SQLite works well with Python because the Python standard library provides the sqlite3 module, which you can use to interact with any SQLite database without having to install anything. Using SQLite with Python also requires minimal setup compared to other database engines.

      In this tutorial, you’ll build a small web application that demonstrates how to use SQLite with Flask to perform basic data manipulation covering CRUD: Create, Read, Update, and Delete. The web application will be a basic blog that displays posts on the index page. Users can create, edit, and delete individual posts.

      Prerequisites

      Step 1 — Setting up the Database

      In this step, you’ll set up the SQLite database you’ll use to store your data (the blog posts for your application). You’ll then populate the database with a few example entries.

      You will use the sqlite3 module to interact with the database, which is readily available in the standard Python library.

      Data in SQLite is stored in tables and columns, so you first need to create a table called posts with the necessary columns. You’ll create a .sql file that contains SQL commands to create the posts table with a few columns. You’ll then use this schema file to create the database.

      Open a database schema file called schema.sql inside your flask_app directory:

      Type the following SQL commands inside this file:

      flask_app/schema.sql

      DROP TABLE IF EXISTS posts;
      
      CREATE TABLE posts (
          id INTEGER PRIMARY KEY AUTOINCREMENT,
          created TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
          title TEXT NOT NULL,
          content TEXT NOT NULL
      );
      

      Save and close the file.

      In this schema file, you first delete the posts table if it already exists. This avoids the possibility of another table named posts existing, which might result in confusing behavior (for example, if it has different columns). This isn’t the case here, because you haven’t created the table yet, so the SQL command won’t be executed. Note that this will delete all of the existing data whenever you execute this schema file. For our purposes, you will only execute this schema once, but you might want to execute it again to delete whatever data you inserted and start with an empty database again.

      Next, you use CREATE TABLE posts to create the posts table with the following columns:

      • id: An integer that represents a primary key. This key will get assigned a unique value by the database for each entry (that is, each blog post). AUTOINCREMENT automatically increments the post IDs, so that the first post will have an ID of 1, and the post added after it will have an ID of 2, and so on. Each post will always have the same ID, even if other posts are deleted.
      • created: The time the blog post was created. NOT NULL signifies that this column should not be empty, and the DEFAULT value is the CURRENT_TIMESTAMP value, which is the time at which the post was added to the database. Just like id, you don’t need to specify a value for this column, as it will be automatically filled in.
      • title: The post title. NOT NULL signifies that this column can’t be empty.
      • content: The post content. NOT NULL signifies that this column can’t be empty.

      Now, you’ll use the schema.sql file to create the database. To do so, you’ll create a Python file that will generate an SQLite .db database file based on this schema.sql file. Open a file named init_db.py inside your flask_app directory:

      Add the following code to it:

      flask_app/init_db.py

      import sqlite3
      
      connection = sqlite3.connect('database.db')
      
      
      with open('schema.sql') as f:
          connection.executescript(f.read())
      
      cur = connection.cursor()
      
      cur.execute("INSERT INTO posts (title, content) VALUES (?, ?)",
                  ('First Post', 'Content for the first post')
                  )
      
      cur.execute("INSERT INTO posts (title, content) VALUES (?, ?)",
                  ('Second Post', 'Content for the second post')
                  )
      
      connection.commit()
      connection.close()
      

      You first import the sqlite3 module. You open a connection to a database file named database.db, which will be created once you run the Python file. Then you use the open() function to open the schema.sql file. Next you execute its contents using the executescript() method that executes multiple SQL statements at once, which will create the posts table. You create a Cursor object that allows you to process rows in a database. In this case, you’ll use the cursor’s execute() method to execute two INSERT SQL statements to add two blog posts to your posts table. Finally, you commit the changes and close the connection.

      Save and close the file and then run it in the terminal using the python command:

      Once the file finishes execution, a new file called database.db will appear in your flask_app directory. This means you’ve successfully set up your database.

      Next, you’ll create a small Flask application, retrieve the two posts you inserted into the database, and display them on the index page.

      Step 2 — Displaying Posts

      In this step, you will create a Flask application with an index page where the blog posts you have in your database are displayed.

      With your programming environment activated and Flask installed, open a file called app.py for editing inside your flask_app directory:

      This file will set up your database connection and create a single Flask route to use that connection. Add the following code to the file:

      flask_app/app.py

      import sqlite3
      from flask import Flask, render_template
      
      app = Flask(__name__)
      
      def get_db_connection():
          conn = sqlite3.connect('database.db')
          conn.row_factory = sqlite3.Row
          return conn
      
      
      @app.route('/')
      def index():
          conn = get_db_connection()
          posts = conn.execute('SELECT * FROM posts').fetchall()
          conn.close()
          return render_template('index.html', posts=posts)
      

      Save and close the file.

      In the code above, you first import the sqlite3 module to use it to connect to your database. Then you import the Flask class and the render_template() function from the flask package. You make a Flask application instance called app. You define a function called get_db_connection(), which opens a connection to the database.db database file you created earlier, and sets the row_factory attribute to sqlite3.Row so you can have name-based access to columns. This means that the database connection will return rows that behave like regular Python dictionaries. Lastly, the function returns the conn connection object you’ll be using to access the database.

      You then use the app.route() decorator to create a Flask view function called index(). You use the get_db_connection() function to open a database connection. Then you execute an SQL query to select all entries from the posts table. You use the fetchall() method to fetch all the rows of the query result, this will return a list of the posts you inserted into the database in the previous step.

      You close the database connection using the close() method and return the result of rendering the index.html template. You also pass the posts object as an argument, which contains the results you got from the database. This will allow you to access the blog posts in the index.html template.

      To display the posts you have in your database on the index page, you will first create a base template, which will have all the basic HTML code other templates will also use to avoid code repetition. Then you’ll create the index.html template file you rendered in your index() function. To learn more about templates, see How to Use Templates in a Flask Application.

      Create a templates directory, then open a new template called base.html:

      • mkdir templates
      • nano templates/base.html

      Add the following code inside the base.html file:

      flask_app/templates/base.html

      <!DOCTYPE html>
      <html lang="en">
      <head>
          <meta charset="UTF-8">
          <title>{% block title %} {% endblock %}- FlaskApp</title>
          <style>
              .post {
                  padding: 10px;
                  margin: 5px;
                  background-color: #f3f3f3
              }
      
              nav a {
                  color: #d64161;
                  font-size: 3em;
                  margin-left: 50px;
                  text-decoration: none;
              }
          </style>
      </head>
      <body>
          <nav>
              <a href="https://www.digitalocean.com/community/tutorials/{{ url_for('index') }}">FlaskApp</a>
              <a href="#">About</a>
          </nav>
          <hr>
          <div class="content">
              {% block content %} {% endblock %}
          </div>
      </body>
      </html>
      

      Save and close the file.

      This base template has all the HTML boilerplate you’ll need to reuse in your other templates. The title block will be replaced to set a title for each page, and the content block will be replaced with the content of each page. The navigation bar has two links, one for the index page where you use the url_for() helper function to link to the index() view function, and the other for an About page if you choose to include one in your application.

      Next, open a template called index.html. This is the template you referenced in the app.py file:

      • nano templates/index.html

      Add the following code to it:

      flask_app/templates/index.html

      {% extends 'base.html' %}
      
      {% block content %}
          <h1>{% block title %} Posts {% endblock %}</h1>
          {% for post in posts %}
              <div class="post">
                  <p>{{ post['created'] }}</p>
                  <h2>{{ post['title'] }}</h2>
                  <p>{{ post['content'] }}</p>
              </div>
          {% endfor %}
      {% endblock %}
      

      Save and close the file.

      In the code above, you extend the base.html template and replace the contents of the content block. You use an <h1> heading that also serves as a title.

      You use a Jinja for loop in the line {% for post in posts %} to go through each post in the posts list. You access the creation date via {{ post['created'] }}, the title via {{ post['title'] }}, and the post content via {{ post['content'] }}.

      While in your flask_app directory with your virtual environment activated, tell Flask about the application (app.py in this case) using the FLASK_APP environment variable:

      Then set the FLASK_ENV environment variable to development to run the application in development mode and get access to the debugger. For more information about the Flask debugger, see How To Handle Errors in a Flask Application. Use the following commands to do this (on Windows, use set instead of export):

      • export FLASK_ENV=development

      Next, run the application:

      With the development server running, visit the following URL using your browser:

      http://127.0.0.1:5000/
      

      You’ll see the posts you added to the database on the first initiation.

      Index Page

      You’ve displayed the posts in your database on the index page. You now need to allow users to add new posts. You’ll add a new route for adding posts in the next step.

      Step 3 — Creating Posts

      In this step, you will add a new route to your Flask application that allows users to add new blog posts to the database, which will then appear on the index page.

      You’ll add a page with a web form where users enter the post title and post content. This form will be validated to make sure users don’t submit an empty form. To inform users the form is invalid, you’ll use a flash message which will only be shown once and will disappear on the next request (if you navigate to another page for example).

      Leave the development server running and open a new terminal window.

      First, open your app.py file:

      For handling the web form, you’ll need to import a few things from the flask package:

      • The global request object to access submitted data.
      • The url_for() function to generate URLs.
      • The flash() function to flash a message if a request is invalid.
      • The redirect() function to redirect users to the index page after adding the posts to the database.

      Add these imports to the first line in the file:

      flask_app/app.py

      from flask import Flask, render_template, request, url_for, flash, redirect
      
      # ...
      

      The flash() function stores flashed messages in the client’s browser session, which requires setting a secret key to secure sessions that remember information from one request to another. You must never allow anyone to access your secret key. See the Flask documentation for sessions for more information.

      Set a secret key by adding a SECRET_KEY configuration to your application via the app.config object. Add it next to the app instance definition.

      flask_app/app.py

      
      # ...
      app = Flask(__name__)
      app.config['SECRET_KEY'] = 'your secret key'
      

      Remember that the secret key should be a long random string. For more on web forms and the secret key configuration, see How To Use Web Forms in a Flask Application.

      Next, add the following route at the end of the app.py file:

      flask_app/app.py

      # ...
      
      @app.route('/create/', methods=('GET', 'POST'))
      def create():
          return render_template('create.html')
      

      Save and close the file.

      In this route, you pass the tuple ('GET', 'POST') to the methods parameter to allow both GET and POST requests. GET requests are used to retrieve data from the server. POST requests are used to post data to a specific route. By default, only GET requests are allowed. When the user first requests the /create route using a GET request, a template file called create.html will be rendered. You will later edit this route to handle POST requests for when users fill and submit the web form for creating new posts.

      Open the new create.html template:

      • nano templates/create.html

      Add the following code to it:

      flask_app/templates/create.html

      {% extends 'base.html' %}
      
      {% block content %}
          <h1>{% block title %} Add a New Post {% endblock %}</h1>
          <form method="post">
              <label for="title">Title</label>
              <br>
              <input type="text" name="title"
                     placeholder="Post title"
                     value="https://www.digitalocean.com/community/tutorials/{{ request.form['title'] }}"></input>
              <br>
      
              <label for="content">Post Content</label>
              <br>
              <textarea name="content"
                        placeholder="Post content"
                        rows="15"
                        cols="60"
                        >{{ request.form['content'] }}</textarea>
              <br>
              <button type="submit">Submit</button>
          </form>
      {% endblock %}
      

      Save and close the file.

      You extend the base template, set a heading as a title, and use a <form> tag with the attribute method set to post to indicate that the form will submit a POST request. You have a text field with the name title, which you’ll use to access the title data in your /create route. You set the value of the text field to request.form['title'] which is either empty or a saved version of the title if the form is invalid, so that the title does not get lost when things go wrong.

      After the title input field, you add a text area named content with the value {{ request.form['content'] }} to restore post content if the form is invalid.

      Last, you have a Submit button at the end of the form.

      Now, with the development server running, use your browser to navigate to the /create route:

      http://127.0.0.1:5000/create
      

      You will see an Add a New Post page with an input field for a post title, a text area for the post’s content, and a Submit button.

      Add New Post

      If you fill in the form and submit it, sending a POST request to the server, nothing happens because you did not handle POST requests on the /create route.

      Open app.py to handle the POST request the user submits:

      Edit the /create route to look as follows:

      flask_app/app.py

      # ...
      
      @app.route('/create/', methods=('GET', 'POST'))
      def create():
          if request.method == 'POST':
              title = request.form['title']
              content = request.form['content']
      
              if not title:
                  flash('Title is required!')
              elif not content:
                  flash('Content is required!')
              else:
                  conn = get_db_connection()
                  conn.execute('INSERT INTO posts (title, content) VALUES (?, ?)',
                               (title, content))
                  conn.commit()
                  conn.close()
                  return redirect(url_for('index'))
      
          return render_template('create.html')
      

      Save and close the file.

      You handle POST requests inside the if request.method == 'POST' condition. You extract the title and content the user submits from the request.form object. If the title is empty, you use the flash() function to flash the message Title is required!. You do the same in case of empty content.

      If both the title and the content are supplied, you open a database connection using the get_db_connection() function. You use the execute() method to execute an INSERT INTO SQL statement to add a new post to the posts table with the title and content the user submits as values. You use the ? placeholder to insert data into the table safely. You commit the transaction and close the connection. Lastly, you redirect the user to the index page where they can see their new post below existing posts.

      Warning: Never use Python string operations to dynamically create an SQL statement string. Always use the ? placeholder in your SQL statements to dynamically substitute values. Pass a tuple of values as the second argument to the execute() method to bind your values to the SQL statement. This prevents SQL injection attacks.

      With the development server running, use your browser to navigate to the /create route:

      http://127.0.0.1:5000/create
      

      Fill the form and submit it.

      You’ll be redirected to the index page where you’ll see your new post.

      If you submit a form without a title or one without any content, your post won’t be added to the database, you won’t be redirected to the index page, and you won’t receive any feedback for why that is. This is because you haven’t set up flashed messages to be displayed anywhere yet.

      Open base.html to add a link to the Create page in the navigation bar, and to display flashed messages below it.

      Edit the file to look as follows:

      flask_app/templates/base.html

      <!DOCTYPE html>
      <html lang="en">
      <head>
          <meta charset="UTF-8">
          <title>{% block title %} {% endblock %} - FlaskApp</title>
          <style>
              .post {
                  padding: 10px;
                  margin: 5px;
                  background-color: #f3f3f3
              }
      
              nav a {
                  color: #d64161;
                  font-size: 3em;
                  margin-left: 50px;
                  text-decoration: none;
              }
      
              .alert {
                  padding: 20px;
                  margin: 5px;
                  color: #970020;
                  background-color: #ffd5de;
              }
          </style>
      </head>
      <body>
          <nav>
              <a href="https://www.digitalocean.com/community/tutorials/{{ url_for("index') }}">FlaskApp</a>
              <a href="https://www.digitalocean.com/community/tutorials/{{ url_for("create') }}">Create</a>
              <a href="#">About</a>
          </nav>
          <hr>
          <div class="content">
              {% for message in get_flashed_messages() %}
                  <div class="alert">{{ message }}</div>
              {% endfor %}
      
              {% block content %} {% endblock %}
          </div>
      </body>
      </html>
      

      Save and close the file.

      Here, you add a new <a> link to the navigation bar that points to the Create page.

      You use a Jinja for loop to go through the flashed messages. These are available in the get_flashed_messages() special function. Each message is displayed in a <div> tag with a CSS class called alert. You style this <div> tag inside the <style> tag in the <head> section.

      Refresh your index page and you’ll see the new link in the navigation bar.

      Click the Create link, and submit an empty form. You’ll receive the flashed message “Title is required!”

      Fill in the title field and leave the content text area empty. Submit the form again, and you’ll receive a “Content is required!” message. Notice how the “Title is required!” message is gone. This is because it’s a flashed message and not a permanent one.

      You now have a way of adding new posts. Next, you’ll add a new route for allowing users to edit existing posts.

      Step 4 — Editing Posts

      In this step, you will add a new route to the application to allow users to edit existing posts.

      First, to avoid code repetition and to isolate code and make it easier to maintain, you’ll add a new function that takes an ID and retrieves a post associated with it from the database. You’ll use this function to get the post data you want to edit, and you’ll use it to get the post for when you want to delete it in the next step.

      Open app.py:

      The function you’ll use to retrieve a post will respond with a 404 Not Found error if the ID of the requested post does not correspond with any of the existing posts. To do this, you’ll use the abort() function, which aborts a request and responds with an error message. For more, see How To Handle Errors in a Flask Application.

      Add the abort() function to the imports:

      flask_app/app.py

      from flask import Flask, render_template, request, url_for, flash, redirect, abort
      

      Add a new function called get_post() below your get_db_connection() function:

      flask_app/app.py

      
      # ...
      
      def get_db_connection():
          conn = sqlite3.connect('database.db')
          conn.row_factory = sqlite3.Row
          return conn
      
      def get_post(post_id):
          conn = get_db_connection()
          post = conn.execute('SELECT * FROM posts WHERE id = ?',
                              (post_id,)).fetchone()
          conn.close()
          if post is None:
              abort(404)
          return post
      
      # ...
      

      This new function has a post_id argument that determines what post to retrieve and return. You open a database connection with get_db_connection() and execute an SQL query to get the post associated with the given post_id value. You get the post with the fetchone() method, store it in the post variable, and close the connection.

      If the post variable has the value None, meaning no result was found in the database, you use the abort() function you imported earlier to respond with a 404 error code and the function will finish execution. If, however, a post was found, you return the value of the post variable.

      Next, add a new route for editing posts at the end of the file:

      flask_app/app.py

      # ...
      
      @app.route('/<int:id>/edit/', methods=('GET', 'POST'))
      def edit(id):
          post = get_post(id)
      
          if request.method == 'POST':
              title = request.form['title']
              content = request.form['content']
      
              if not title:
                  flash('Title is required!')
      
              elif not content:
                  flash('Content is required!')
      
              else:
                  conn = get_db_connection()
                  conn.execute('UPDATE posts SET title = ?, content = ?'
                               ' WHERE id = ?',
                               (title, content, id))
                  conn.commit()
                  conn.close()
                  return redirect(url_for('index'))
      
          return render_template('edit.html', post=post)
      

      Save and close the file.

      You use the route /<int:id>/edit/, with int: being a converter that accepts positive integers. And id is the URL variable that will determine the post you want to edit. For example, /2/edit/ will allow you to edit the post with the ID of 2. The ID is passed from the URL to the edit() view function. You pass the value of the id argument to the get_post() function to fetch the post associated with the provided ID from the database. Remember that this will respond with a 404 Not Found error if no post with the given ID exists.

      The last line renders a template file called edit.html, and passes in the post variable that has the post data. You’ll use this to display the existing title and content on the Edit page.

      The if request.method == 'POST' block handles the new data the user submits. Similar to adding a new post, you extract the title and content. You flash a message if the title or the content is not provided.

      If the form is valid, you open a database connection and use the UPDATE SQL statement to update the posts table by setting the new title and new content, where the ID of the post in the database is equal to the ID that was in the URL. You commit the transaction, close the connection, and redirect to the index page.

      Next you need to create a page where users can do the editing. Open a new edit.html template:

      Add the following code to it:

      flask_app/templates/edit.html

      {% extends 'base.html' %}
      
      {% block content %}
          <h1>{% block title %} Edit "{{ post['title'] }}" {% endblock %}</h1>
          <form method="post">
              <label for="title">Title</label>
              <br>
              <input type="text" name="title"
                     placeholder="Post title"
                     value="{{ request.form['title'] or post['title'] }}"></input>
              <br>
      
              <label for="content">Post Content</label>
              <br>
              <textarea name="content"
                        placeholder="Post content"
                        rows="15"
                        cols="60"
                        >{{ request.form['content'] or post['content'] }}</textarea>
              <br>
              <button type="submit">Submit</button>
          </form>
      {% endblock %}
      

      Save and close the file.

      This is similar to the code in the create.html template, except for displaying the post title inside the page’s title in the line {% block title %} Edit "{{ post['title'] }}" {% endblock %}, the value of the input in {{ request.form['title'] or post['title'] }}, and the value of the text area in {{ request.form['content'] or post['content'] }}. This displays the data stored in the request if it exists; otherwise it displays the data from the post variable that was passed to the template containing current database data.

      With the development server running, use your browser to navigate to the following URL to edit the first post:

      http://127.0.0.1:5000/1/edit
      

      You’ll see a page that looks as follows:

      Edit Post

      Edit the post and submit the form. You’ll see your changes applied on the index page. If you submit a form without a title or without any content, you’ll receive a flashed message.

      You now need to add a link that points to the Edit page for each post on the index page. Open the index.html template file:

      • nano templates/index.html

      Edit the file to look exactly like the following:

      flask_app/templates/index.html

      
      {% extends 'base.html' %}
      
      {% block content %}
          <h1>{% block title %} Posts {% endblock %}</h1>
          {% for post in posts %}
              <div class="post">
                  <p>{{ post['created'] }}</p>
                  <h2>{{ post['title'] }}</h2>
                  <p>{{ post['content'] }}</p>
                  <a href="https://www.digitalocean.com/community/tutorials/{{ url_for("edit', id=post['id']) }}">Edit</a>
              </div>
          {% endfor %}
      {% endblock %}
      

      Save and close the file.

      You added an <a> tag that links to the edit() view function. You pass the post ID you have in post['id']) to the url_for() function to generate the post’s edit link. This adds a link to the Edit page of each post below it.

      Refresh the index page and click the Edit link to edit a post.

      You can now add new posts and edit existing ones. Next, you’ll add a button to allow users to delete existing posts.

      Step 5 — Deleting Posts

      In this step, you will add a Delete button to the Edit page to allow users to delete a post.

      First, you’ll add a new /id/delete route that accepts POST requests, similar to the edit() view function. Your new delete() view function will receive the ID of the post to be deleted from the URL, retrieve it using the get_post() function, and then delete it from the database if it exists.

      Open the app.py file:

      Add the following route at the end:

      flask_app/app.py

      # ...
      
      @app.route('/<int:id>/delete/', methods=('POST',))
      def delete(id):
          post = get_post(id)
          conn = get_db_connection()
          conn.execute('DELETE FROM posts WHERE id = ?', (id,))
          conn.commit()
          conn.close()
          flash('"{}" was successfully deleted!'.format(post['title']))
          return redirect(url_for('index'))
      

      Save and close the file.

      This view function only accepts POST requests in the methods parameter. This means that navigating to the /ID/delete route on your browser will return a 405 Method Not Allowed error, because web browsers default to GET requests. To delete a post, the user clicks on a button that sends a POST request to this route.

      The function receives the ID of the post to be deleted. You use this ID to retrieve the post using the get_post() function. This responds with a 404 Not Found error if no post with the given ID exists. You open a database connection and execute a DELETE FROM SQL command to delete the post. You use WHERE id = ? to specify the post you want to delete.

      You commit the change to the database and close the connection. You flash a message to inform the user that the post was successfully deleted and redirect them to the index page.

      Note that you don’t render a template file. This is because you’ll just add a Delete button to the Edit page.

      Open the edit.html template file:

      Then add the following <hr> and <form> tags directly before the {% endblock %} line:

      flask_app/templates/edit.html

              <button type="submit">Submit</button>
          </form>
      
      
          <hr>
          <form action="https://www.digitalocean.com/community/tutorials/{{ url_for("delete', id=post['id']) }}" method="POST">
              <input type="submit" value="Delete Post"
                      onclick="return confirm('Are you sure you want to delete this post?')">
          </form>
      {% endblock %}
      

      Save and close the file.

      Here, you have a web form that submits a POST request to the delete() view function. You pass post['id'] to specify the post that will be deleted. You use the confirm() method available in web browsers to display a confirmation message before submitting the request.

      Now navigate again to the Edit page of a post and try deleting it:

      http://127.0.0.1:5000/1/edit
      

      After you confirm the deletion, you’ll be redirected to the index page, and the post will no longer be there. A flash message will appear below the navigation bar informing you that the post was successfully deleted.

      You now have a way of deleting unwanted posts from the database in your Flask application.

      Conclusion

      You built a small web blog that communicates with an SQLite database. You have basic functionalities in your Flask application, such as adding new data to the database, retrieving data and displaying it on a page, and editing and deleting existing data.

      For more on how to use SQLite with Python and Flask, see the following tutorials:

      If you would like to read more about Flask, check out the other tutorials in the Flask series.



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