The Ultimate Flask Front-End – Part 2

by Real Python Mar 25, 2015 flask front-end python

Welcome to part 2! Again, here’s what we’re covering: Let’s look at the small, yet powerful JavaScript UI library ReactJS in action, as we build a basic web application.

This app is powered by Python 3 and the Flask framework in the back-end and React in the front. In addition, we will use gulp.js (task runner), bower (front-end package manager), and Browserify (JavaScript dependency bundler).


  • 05/22/2016: Upgraded to the latest version of React (v15.0.1).

React – Round Two

Hello World is great and all, but let’s build something a bit more fun - a dynamic search tool. This Component is used to filter down a list of items based on user input.

Need the code? Grab it from the repo. Download v2.2 to start where we left off from part 1.

Add the following code to project/static/scripts/jsx/main.js:

var DynamicSearch = React.createClass({

  // sets initial state
  getInitialState: function(){
    return { searchString: '' };

  // sets state, triggers render method
  handleChange: function(event){
    // grab value form input box
    console.log("scope updated!");

  render: function() {

    var countries = this.props.items;
    var searchString = this.state.searchString.trim().toLowerCase();

    // filter countries list by value from input box
    if(searchString.length > 0){
      countries = countries.filter(function(country){
        return searchString );

    return (
        <input type="text" value={this.state.searchString} onChange={this.handleChange} placeholder="Search!" />
          {{ return <li>{} </li> }) }


// list of countries, defined with JavaScript object literals
var countries = [
  {"name": "Sweden"}, {"name": "China"}, {"name": "Peru"}, {"name": "Czech Republic"},
  {"name": "Bolivia"}, {"name": "Latvia"}, {"name": "Samoa"}, {"name": "Armenia"},
  {"name": "Greenland"}, {"name": "Cuba"}, {"name": "Western Sahara"}, {"name": "Ethiopia"},
  {"name": "Malaysia"}, {"name": "Argentina"}, {"name": "Uganda"}, {"name": "Chile"},
  {"name": "Aruba"}, {"name": "Japan"}, {"name": "Trinidad and Tobago"}, {"name": "Italy"},
  {"name": "Cambodia"}, {"name": "Iceland"}, {"name": "Dominican Republic"}, {"name": "Turkey"},
  {"name": "Spain"}, {"name": "Poland"}, {"name": "Haiti"}

  <DynamicSearch items={ countries } />,

What’s going on?

We created a Component called DynamicSearch, which updates the DOM when the value in the input box is changed. How does this work? Well, the handleChange() function is called when a value is added or removed from the input box, which, in turn, updates the state via setState(). This method then calls the render() function to re-render the Component.

The key takeaway here is that state changes only occur within the Component.

Test it out:

Since we added our JSX code to an external file, we need to trigger the transform, from JSX to vanilla JavaScript, outside the browser with Gulp.


Gulp is a powerful task runner/build tool that can be used to automate the transform process. We’ll also use it to watch for changes to our code (project/static/scripts/jsx/main.js) and automatically create new builds based on those changes.


Like bower, you can install gulp with npm. Install it globally, and then add it to the package.json file:

$ npm install -g gulp
$ npm install --save-dev gulp

Add a gulpfile.js at the root directory of your project:

// requirements
var gulp = require('gulp');

// tasks
gulp.task('default', function() {

This file tells gulp which tasks to run and in what order to run them in. You can see that our default task logs the string hello! to the console. You can run this task by simply running gulp. You should see something like:

$ gulp
[08:54:47] Using gulpfile ~/gulpfile.js
[08:54:47] Starting 'default'...
[08:54:47] Finished 'default' after 148 μs

We need to install the following gulp plugins:

To do this, simply update the package.json file:

  "name": "ultimate-flask-front-end",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  "repository": {
    "type": "git",
    "url": "git+"
  "author": "",
  "license": "ISC",
  "bugs": {
    "url": ""
  "homepage": "",
  "devDependencies": {
    "bower": "^1.7.9",
    "del": "^2.2.0",
    "gulp": "^3.9.1",
    "gulp-browser": "^2.1.4",
    "gulp-size": "^2.1.0",
    "reactify": "^1.1.1"

And then run npm install to install the plugins.

You can see these installed plugins within the “node_modules” directory. Make sure to include this directory in your .gitignore file.

Finally, update gulpfile.js:

// requirements

var gulp = require('gulp');
var gulpBrowser = require("gulp-browser");
var reactify = require('reactify');
var del = require('del');
var size = require('gulp-size');

// tasks

gulp.task('transform', function () {
  // add task

gulp.task('del', function () {
  // add task

gulp.task('default', function() {


Now let’s add some tasks, starting with transform to handle, well, the transform process from JSX to JavaScript.

First task – transform

gulp.task('transform', function () {
  var stream = gulp.src('./project/static/scripts/jsx/*.js')
    .pipe(gulpBrowser.browserify({transform: ['reactify']}))
  return stream;

The task() function takes two arguments - the task’s name and an anonymous function, which:

  • Defines the source directory,
  • Pipes the JSX files through the Browserify JSX transformer
  • Specifies the destination directory, and
  • Calculates the size of the created file(s).

Gulp utilizes pipes to stream data for processing. After grabbing the source file (main.js), the file is then “piped” to the browserify() function for transformation/bundling. This transformed and bundled code is then “piped” to the destination directory along with the size() function.

Curious about pipes and streams? Check out this excellent resource.

Ready for a quick test? Update index.html:

<!DOCTYPE html>
  <head lang="en">
    <meta charset="UTF-8">
    <title>Flask React</title>
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <!-- styles -->
    <link rel="stylesheet" type="text/css" href="{{ url_for('static', filename='bower_components/bootstrap/dist/css/bootstrap.min.css') }}">
    <link rel="stylesheet" type="text/css" href="{{ url_for('static', filename='css/style.css') }}">
    <div class="container">
      <h1>Flask React</h1>
      <div id="main"></div>
    <!-- scripts -->
    <script src="{{ url_for('static', filename='bower_components/react/react.min.js') }}"></script>
    <script src="{{ url_for('static', filename='bower_components/react/react-dom.min.js') }}"></script>
    <script src="{{ url_for('static', filename='scripts/js/main.js') }}"></script>

Then update the default task:

gulp.task('default', function () {


$ gulp
[08:58:39] Using gulpfile /gulpfile.js
[08:58:39] Starting 'default'...
[08:58:39] Starting 'transform'...
[08:58:39] Finished 'default' after 12 ms
[08:58:40] all files 1.99 kB
[08:58:40] Finished 'transform' after 181 ms

Did you notice the second to last line? This is the result of the size() function. In other words, the newly created JavaScript file (after the transform), project/static/scripts/js/main.js, is 1.99 kB in size.

Fire up the Flask server, and navigate to http://localhost:5000/. You should see all the countries along with the search box. Test out the functionality. Also, if you open the JavaScript console within Chrome Developer Tools, you’ll see the string, scope updated!, logged every time there’s a change in scope - which comes from the handleChange() function in the DynamicSearch Component.

Second task – clean

gulp.task('del', function () {
  return del(['./project/static/scripts/js']);

When this task is ran, we grab the source directory - the result of the transform task - and then run the del() function to remove the directory and its contents. It’s a good idea to run this before each new build to ensure you start fresh and clean.

Try running gulp del. This should remove “project/static/scripts/js”. Let’s add it to our default task so that it automatically runs before the transformation:

gulp.task('default', ['del'], function () {

Be sure to test this out before moving on.

Third task – watch

Finally, update the default task one last time, adding the ability to automatically re-run the transform task anytime changes are made to any .js files in the “project/static/scripts/jsx” directory:

gulp.task('default', ['del'], function () {
  gulp.start('transform');'./project/static/scripts/jsx/*.js', ['transform']);

Open up a new terminal window, navigate to the project root, and run gulp to generate a new build and activate the watcher functionality. In the other window run sh to run the Flask server. Your app should be running. Now if you comment out all the code in the project/static/scripts/jsx/main.js file, this will trigger the transform function. Refresh the browser to see the changes. Make sure to revert the changes when done.

Want to take this to the next level? Check out the Livereload plugin.


Here’s the end result after adding some custom styles to project/static/css/style.css:

Be sure to check out the official documentation along with the excellent tutorial for more information on React.

Grab the code from the repo. If you’d like to dig deeper into Flask and see how to build a complete web app with it from scratch, be sure to check out this video series:

Comment below if you have any questions or spot any errors. Also, what else would you like to see? We may add a third part if people are interested. Comment below.

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