Cómo construir una aplicación web usando Flask e implementarla en la nube

Programación

Introducción

En cada sección, mostraré fragmentos de código para que los siga. Todo el código utilizado en el tutorial está disponible en este repositorio de GitHub.

¿Qué es HTTP y qué tiene que ver con Flask?

HTTP es el protocolo para sitios web. Internet lo usa para interactuar y comunicarse con computadoras y servidores. Déjame darte un ejemplo de cómo lo usas todos los días.

Cuando escribe el nombre de un sitio web en la barra de direcciones de su navegador y presiona enter. Lo que sucede es que se ha enviado una solicitud HTTP a un servidor.

Por ejemplo, cuando voy a mi barra de direcciones y escribo google.com, luego presiono enter, se envía una solicitud HTTP a un servidor de Google. El servidor de Google recibe la solicitud y necesita averiguar cómo interpretar esa solicitud. El servidor de Google envía una respuesta HTTP que contiene la información que recibe mi navegador web. Luego, muestra lo que solicitó en una página del navegador.

¿Cómo participa Flask?

Escribiremos código que se encargará del procesamiento del lado del servidor. Nuestro código recibirá solicitudes. Averiguará de qué tratan esas solicitudes y qué están pidiendo. También averiguará qué respuesta enviar al usuario.

Para hacer todo esto usaremos Flask.

¿Qué es Flask?

Simplifica el proceso de diseño de una aplicación web. Matraz nos deja enfocarsobre lo que solicitan los usuarios y qué tipo de respuesta dar.

Obtenga más información sobre micro frameworks.

¿Cómo funciona una aplicación Flask?

El código nos permite ejecutar una aplicación web básica que podemos servir, como si fuera un sitio web.

from flask import Flask app = Flask(__name__) @app.route("/") def home(): return "Hello, World!" if __name__ == "__main__": app.run(debug=True)

Este fragmento de código se almacena en nuestro main.py.

Línea 1: Aquí estamos importando el módulo Flask y creando un servidor web Flask desde el módulo Flask.

Línea 3: __name__ significa este archivo actual . En este caso, será main.py. Este archivo actual representará mi aplicación web.

Estamos creando una instancia de la clase Flask y la llamamos aplicación. Aquí estamos creando una nueva aplicación web.

Línea 5: Representa la página predeterminada. Por ejemplo, si voy a un sitio web como "google.com/" sin nada después de la barra. Entonces esta será la página predeterminada de Google.

Línea 6–7 : Cuando el usuario va a mi sitio web y va a la página predeterminada (nada después de la barra), se activará la siguiente función.

Línea 9: cuando ejecuta su secuencia de comandos de Python, Python asigna el nombre "__main__" a la secuencia de comandos cuando se ejecuta.

Si importamos otro script, la instrucción if evitará que se ejecuten otros scripts. Cuando ejecutamos main.py, cambiará su nombre a __main__ y solo entonces se activará la instrucción if.

Línea 10: Esto ejecutará la aplicación. Tener debug=Truepermite que aparezcan posibles errores de Python en la página web. Esto nos ayudará a rastrear los errores.

Intentemos ejecutar main.py

En su Terminal o Símbolo del sistema, vaya a la carpeta que contiene su main.py.Entonces haz py main.pyo python main.py. En su terminal o símbolo del sistema, debería ver este resultado.

La parte importante es donde dice Running on //127.0.0.1:5000/.

127.0.0.1 significa esta computadora local. Si no conoce el significado de esto (como yo no lo sabía cuando comencé, este artículo es realmente útil), la idea principal es que 127.0.0.1 y localhost se refieren a esta computadora local.

Vaya a esa dirección y debería ver lo siguiente:

Más diversión con frasco

Anteriormente viste lo que sucedió cuando ejecutamos main.py con una ruta que era app.route (“/”).

Agreguemos más rutas para que pueda ver la diferencia.

from flask import Flask app = Flask(__name__) @app.route("/") def home(): return "Hello, World!" @app.route("/salvador") def salvador(): return "Hello, Salvador" if __name__ == "__main__": app.run(debug=True)

En las líneas 9-11 . agregamos una nueva ruta, esta vez a / salvador.

Ahora ejecute main.py nuevamente y vaya a// localhost: 5000 / salvador.

Hasta ahora hemos estado devolviendo texto. Hagamos que nuestro sitio web se vea mejor agregando HTML y CSS.

HTML, CSS y entornos virtuales

HTML y plantillas en matraz

Primero cree un nuevo archivo HTML. Llamé al mío home.html.

Aquí hay un código para comenzar.

    Flask Tutorial   
 My First Try Using Flask 
 Flask is Fun

Punto importante para recordar

Flask Framework busca archivos HTML en una carpeta llamada plantillas. Usted necesita crear un plantillas de carpeta y poner todos los archivos HTML en ese país.

Ahora necesitamos cambiar nuestro main.py para que podamos ver el archivo HTML que creamos.

from flask import Flask, render_template app = Flask(__name__) @app.route("/") def home(): return render_template("home.html") @app.route("/salvador") def salvador(): return "Hello, Salvador" if __name__ == "__main__": app.run(debug=True) We made two new changes

Line 1: We imported render_template() method from the flask framework. render_template() looks for a template (HTML file) in the templates folder. Then it will render the template for which you ask. Learn more about render_templates() function.

Line 7: We change the return so that now it returns render_template(“home.html”). This will let us view our HTML file.

Now visit your localhost and see the changes: //localhost:5000/.

Let’s add more pages

Let’s create an about.html inside the templates folder.

    About Flask   
 About Flask 
 Flask is a micro web framework written in Python.
 Applications that use the Flask framework include Pinterest, LinkedIn, and the community web page for Flask itself.

Let’s make a change similar to what we did before to our main.py.

from flask import Flask, render_template app = Flask(__name__) @app.route("/") def home(): return render_template("home.html") @app.route("/about) def about(): return render_template("about.html") if __name__ == "__main__": app.run(debug=True)

We made three new changes:

Line 9: Change the route to"/about".

Line 10: Change the function so it is nowdef about():

Line 11: Change the return so that now it returns render_template("about.html").

Now see the changes: //localhost:5000/about.

Let’s Connect Both Pages with a Navigation

To connect both pages we can have a navigation menu on the top. We can use Flask to make the process of creating a navigation menu easier.

First, let’s create a template.html. This template.html will serve as a parent template. Our two child templates will inherit code from it.

     Flask Parent Template 
First Web App

     
    
      
  • Home
    • 
      
  • About
    • 
     
 {% block content %} {% endblock %}

Line 13–14: We use the function calledurl_for(). It accepts the name of the function as an argument. Right now we gave it the name of the function. More information on url_for() function.

The two lines with the curly brackets will be replaced by the content of home.html and about.html. This willdepend on the URL in which the user is browsing.

These changes allow the child pages (home.html and about.html) to connect to the parent (template.html). This allows us to not have to copy the code for the navigation menu in the about.html and home.html.

Content of about.html:

    About Flask   {% extends "template.html" %} {% block content %} 
 About Flask 
 Flask is a micro web framework written in Python.
 Applications that use the Flask framework include Pinterest, LinkedIn, and the community web page for Flask itself.
 {% endblock %}

Content of home.html:

    Flask Tutorial   {% extends "template.html" %} {% block content %} 
 My First Try Using Flask 
 Flask is Fun 
 {% endblock %}

Let’s try adding some CSS.

Adding CSS to Our Website

An important note to remember

In the same way as we created a folder called templates to store all our HTML templates, we need a folder called static.

In static, we will store our CSS, JavaScript, images, and other necessary files. That is why it is important that you should create a CSSfolder to store your stylesheets. After you do this, your project folder should look like this:

Linking our CSS with our HTML file

Our template.html is the one that links all pages. We can insert the code here and it will be applicable to all child pages.

    Flask Parent Template 
First Web App

     
    
      
  • Home
    • 
      
  • About
    • 
     
 {% block content %} {% endblock %}

Line 7: Here we are giving the path to where the template.css is located.

Now see the changes: //localhost:5000/about.

Moving Forward with Flask and virtualenv

Now that you are familiar with using Flask, you may start using it in your future projects. One thing to always do is use virtualenv.

Why use virtualenv?

You may use Python for others projects besides web-development.

Your projects might have different versions of Python installed, different dependencies and packages.

We use virtualenv to create an isolated environment for your Python project. This means that each project can have its own dependencies regardless of what dependencies every other project has.

Getting started with virtualenv

First, run this command on your command prompt or terminal:

pip install virtualenv

Second, do the following:

virtualenv “name of virtual environment”

Here you can give a name to the environment. I usually give it a name of virtual. It will look like this: virtualenv virtual.

After setting up virtual environment, check your project folder. It should look like this. The virtual environment needs to be created in the same directory where your app files are located.

Activating the virtual environment

Now go to your terminal or command prompt. Go to the directory that contains the file called activate. The file called activate is found inside a folder called Scripts for Windows and bin for OS X and Linux.

For OS X and Linux Environment:

$ name of virtual environmnet/bin/activate

For Windows Environment:

name of virtual environment\Scripts\activate

Since I am using a Windows machine, when I activate the environment it will look like this:

The next step is to install flask on your virtual environment so that we can run the application inside our environment. Run the command:

pip install flask

Run your application and go to //localhost:5000/

We finally made our web application. Now we want to show the whole world our project.

(More information on virtualenv can be found in the following guides on virtualenv and Flask Official Documentation)

Let’s send it to the Cloud

To show others the project we made, we will need to learn how to use Cloud Services.

Deploy Your Web Application to the Cloud

To deploy our web application to the cloud, we will use Google App Engine (Standard Environment). This is an example of a Platform as a Service (PaaS).

PaaS se refiere a la entrega de sistemas operativos y servicios asociados a través de Internet sin descargas ni instalación . El enfoque permite a los clientes crear e implementar aplicaciones sin tener que invertir en la infraestructura subyacente (más información sobre PaaS, consulte TechTarget).

Google App Engine es una plataforma como oferta de servicio que permite a los desarrolladores y empresas crear y ejecutar aplicaciones utilizando la infraestructura avanzada de Google: TechOpedia.

Antes de que empieces:

Necesitará una cuenta de Google . Una vez que cree una cuenta, vaya a Google Cloud Platform Console y cree un nuevo proyecto. Además, debe instalar el SDK de Google Cloud.

Al final de este tutorial, la estructura de su proyecto se verá así.

We will need to create three new files: app.yaml, appengine_config.py, and requirements.txt.

Content of app.yaml:

runtime: python27 api_version: 1 threadsafe: true handlers: - url: /static static_dir: static - url: /.* script: main.app libraries: - name: ssl version: latest

If you were to check Google’s Tutorial in the part where they talk about content of the app.yaml, it does not include the section where I wrote about libraries.

When I first attempted to deploy my simple web app, my deployment never worked. After many attempts, I learned that we needed to include the SSL library.

The SSL Library allows us to create secure connections between the client and server. Every time the user goes to our website they will need to connect to a server run by Google App Engine. We need to create a secure connection for this. (I recently learned this, so if you have a suggestions for this let me know!)

Content of appengine_config.py:

from google.appengine.ext import vendor # Add any libraries installed in the "lib" folder. vendor.add('lib')

Content of requirements.txt:

Flask Werkzeug

Now inside our virtual environment (make sure your virtualenv is activated),we are going to install the new dependencies we have in requirements.txt. Run this command:

pip install -t lib -r requirements.txt

-t lib: This flag copies the libraries into a lib folder, which uploads to App Engine during deployment.

-r requirements.txt: Tells pip to install everything from requirements.txt.

Deploying the Application

To deploy the application to Google App Engine, use this command.

gcloud app deploy

I usually include — project [ID of Project]

This specifies what project you are deploying. The command will look like this:

gcloud app deploy --project [ID of Project]

The Application

Now check the URL of your application. The application will be store in the following way:

"your project id".appspot.com

My application is here: //sal-flask-tutorial.appspot.com

Conclusion

From this tutorial, you all learned how to:

  • Use the framework called Flask to use Python as a Server Side Language.
  • Learned how to use HTML, CSS, and Flask to make a website.
  • Learned how to create Virtual Environments using virtualenv.
  • Use Google App Engine Standard Environment to deploy an application to the cloud.

What I learned

I learned three important things from this small project.

First, I learned about the difference between a static website and a web application

Static Websites:

  • Means that the server is serving HTML, CSS, and JavaScript files to the client. The content of the site does not change when the user interacts with it.

Web Applications:

  • A web application or dynamic website generates content based on retrieved data (most of the time is a database) that changes based on a user’s interaction with the site. In a web application, the server is responsible for querying, retrieving, and updating data. This causes web applications to be slower and more difficult to deploy than static websites for simple applications (Reddit).

Server Side and Client Side:

  • I learned that a web application has two sides. The client side and the server side. The client side is what the user interacts with and the server side is where the all the information that the user inputted is processed.

Second, I learned about Cloud Services

Most of my previous projects were static websites, and to deploy them I used GitHub Pages. GitHub Pages is a free static site hosting service designed to host projects from a GitHub Repository.

When working with web applications, I could not use GitHub Pages to host them. GitHub Pages is only meant for static websites not for something dynamic like a web application that requires a server and a database. I had to use Cloud Services such as Amazon Web Services or Heroku

Third, I learned how to use Python as a Server Side Language

To create the server side of the web application we had to use a server side language. I learned that I could use the framework called Flask to use Python as the Server Side Language.

Next Steps:

You can build all sorts of things with Flask. I realized that Flask helps make the code behind the website easier to read. I have made the following applications during this summer of 2018 and I hope to make more.

Personal Projects

  • A Twilio SMS App
  • My Personal Website

During my internship

  • Part of a project where I learned about Docker and Containers

Here is the list of resources that helped me create this tutorial:

  • “App Engine — Build Scalable Web & Mobile Backends in Any Language | App Engine | Google Cloud.” Google, Google, cloud.google.com/appengine/.
  • “Building a Website with Python Flask.” PythonHow, pythonhow.com/building-a-website-with-python-flask/.
  • “Flask — Lecture 2 — CS50’s Web Programming with Python and JavaScript.” YouTube, 6 Feb. 2018, youtu.be/j5wysXqaIV8.
  • “Getting Started with Flask on App Engine Standard Environment | App Engine Standard Environment for Python | Google Cloud.” Google, Google, cloud.google.com/appengine/docs/standard/python/getting-started/python-standard-env.
  • “Installation.” Welcome | Flask (A Python Microframework), flask.pocoo.org/docs/0.12/installation/.
  • “Python — Deploying Static Flask Sites for Free on Github Pages.” Reddit, www.reddit.com/r/Python/comments/1iewqt/deploying_static_flask_sites_for_free_on_github/.
  • Real Python. “Python Virtual Environments: A Primer — Real Python.” Real Python, Real Python, 7 Aug. 2018, realpython.com/python-virtual-environments-a-primer/.
  • “What Is Cloud Services? — Definition from WhatIs.com.” SearchITChannel, searchitchannel.techtarget.com/definition/cloud-services.
  • “What Is Google App Engine (GAE)? — Definition from Techopedia.” Techopedia.com, www.techopedia.com/definition/31267/google-app-engine-gae.

If you have any suggestions or questions, feel free to leave a comment.