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Project inspiration - Ditch Javascript Frameworks For Pure Python Joy

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Meld is a framework for Flask to meld your frontend and backend code. What does that mean? It means you can enjoy writing dynamic user interfaces in pure Python.

Less context switching. No need to write javascript. More fun!

Fastest way to get started

Install flask-meld to your Python environment: pip install flask-meld

# 1. Replace "project_name" with the name of your project
meld new project_name

# 2. Change to your project_name directory and install requirements
cd project_name; pip install -r requirements.txt

# 3. Run the flask-meld development server
flask run

Initialize Meld in an existing project (Alternative method to get started)

For the sake of example, here is a minimal Flask application to get things running:

from flask import Flask, render_template
from flask_meld import Meld

app = Flask(__name__)
app.config['SECRET_KEY'] = 'big!secret'

meld = Meld()

socketio = app.socketio

def index():
    return render_template("base.html")

if __name__ == '__main__':, debug=True)

Add {% meld_scripts %} to your base html template

This sets up the application and initializes Flask-Meld.

<!DOCTYPE html>
        <title>Meld Example</title>
        <!-- Add the line below to include the necessary meld scripts-->
        {% meld_scripts %}

        {% block content %}
            <!-- Using a component in your template is easy! -->
            {% meld 'counter' %}
        {% endblock %}


Components are stored in meld/components either within your application folder or in the base directory of your project.

Components are simple Python classes.

The counter component:

# app/meld/components/

from flask_meld.component import Component

class Counter(Component):
    count = 0

    def add(self):
        self.count = int(self.count) + 1

    def subtract(self):
        self.count = int(self.count) - 1


Create a component template in templates/meld/counter.html. By creating a file within the templates/meld directory just include {% meld 'counter' %} where you want the component to load.

Here is an example for counter:

<!-- templates/meld/counter.html -->
    <button meld:click="subtract">-</button>
    <input type="text" meld:model="count" readonly></input>
    <button meld:click="add">+</button>

Let's take a look at that template file in more detail.

The buttons use meld:click to call the add or subtract function of the Counter component. The input uses meld:model to bind the input to the count property on the Counter component.


Use modifiers to change how Meld handles network requests.

debounce: <input meld:model.debounce-500="search"> Delay network requests for an amount of time after a keypress. Used to increase performance and sync when the user has paused typing for an amount of time. debounce-250 will wait 250ms before it syncs with the server. The default is 150ms.

defer: <input meld:model.defer="search"> Pass the search field with the next network request. Used to improve performance when realtime databinding is not necessary.

prevent: Use to prevent a default action. The following example uses defer to delay sending a network request until the form is submitted. Idea of how this can be used: instead of adding a keydown event listener to the input field to capture the press of the enter key, a form with meld:submit.prevent="search" can be used to to invoke a component's search function instead of the default form handler on form submission.

<form meld:submit.prevent="search">
    <input meld:model.defer="search_text" type="text" name="name" id="name" placeholder="Search for name">
    <button meld:click="search">Search</button>

    <!-- To get the same functionality without using meld:submit.prevent="search" you
    would need to add an event listener for the enter key 
    <input meld:model.defer="search_text" meld:keydown.Enter="search" type="text" name="name" id="name" placeholder="Search for name">

Form Validation

A big part of creating web applications is using forms. Flask-Meld integrates with Flask-WTF to give you real-time form validation without writing any Javascript.

Use WTForms for validation

Define your form with Flask-WTF just as you always do.

from flask_wtf import FlaskForm
from wtforms import StringField, PasswordField
from wtforms.validators import DataRequired, Email, EqualTo

class RegistrationForm(FlaskForm):
    email = StringField('Email', validators=[DataRequired(), Email()])
    password = PasswordField('Password', validators=[DataRequired()])
    password_confirm = PasswordField('Confirm Password', validators=[DataRequired(), EqualTo('password')])

Create your template

Use WTForm helpers to create your form in your HTML template.

<!-- templates/meld/register.html -->
    <form method="POST">
            {{ }}
            {{ }}
            <span> {{ errors.password | first }} </span>

            {{ form.password.label }}
            {{ form.password }}
            <span> {{ errors.password | first }} </span>
            {{ form.password_confirm.label }}
            {{ form.password_confirm }}
            <span> {{ errors.password_confirm | first }} </span>
            {{ form.submit }}

Using the WTForm helpers saves you some typing. Alternatively, you can define your HTML form without using the helpers. For example, to make a field use <input id="email" meld:model="email" name="email" required="" type="text" value=""> Make sure that meld:model="name_of_field" exists on each field.

Define the form in the component

# meld/components/
from flask_meld import Component
from forms import RegistrationForm

class Register(Component):
    form = RegistrationForm()

Realtime form validation

To make your form validate as a user types use the updated function. This will provide the form field and allow you to validate on the fly. Simply call validate on the field.

# meld/components/
from flask_meld import Component
from forms import RegistrationForm

class Register(Component):
    form = RegistrationForm()

    def updated(self, field):

Your routes can stay the same when using real-time validation

You have options here, you can create a custom method on your component to handle submissions or you can use your regular old Flask routes.

@app.route('/register', methods=['GET', 'POST'])
def register():
    form = RegistrationForm()
    if form.validate_on_submit():
        # do anything you need with your form data...
        return redirect(url_for("index"))
    return render_template("register_page.html")

Pretty simple right? You can use this to create very dynamic user interfaces using pure Python and HTML. We would love to see what you have built using Meld so please share!


Meld is a full-stack framework for Flask that allows you to create dynamic frontends in Flask using Python and the Jinja2 templating engine.




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