Python | Form validation using django

Prerequisites: Django Installation | Introduction to Django
Django works on an MVT pattern. So there is a need to create data models (or tables). For every table, a model class is created. 
Suppose there is a form that takes Username, gender, and text as input from the user, the task is to validate the data and save it.
In django this can be done, as follows:

Python

from django.db import models

# model named Post
class Post(models.Model):
    Male = 'M'
    FeMale = 'F'
    GENDER_CHOICES = (
    (Male, 'Male'),
    (FeMale, 'Female'),
    )

    # define a username field with bound  max length it can have
    username = models.CharField( max_length = 20, blank = False,
                                 null = False)

    # This is used to write a post
    text = models.TextField(blank = False, null = False)

    # Values for gender are restricted by giving choices
    gender = models.CharField(max_length = 6, choices = GENDER_CHOICES,
                              default = Male)

    time = models.DateTimeField(auto_now_add = True)

After creating the data models, the changes need to be reflected in the database to do this run the following command:

python manage.py makemigrations

Doing this compiles the models and if it didn’t find any errors then, it creates a file in the migration folder. Later run the command given below to finally reflect the changes saved onto the migration file onto the database.

python manage.py migrate

Now a form can be created. Suppose that the username length should not be less than 5 and post length should be greater than 10. Then we define the Class PostForm with the required validation rules as follows: 

Python

from django.forms import ModelForm
from django import forms
from formValidationApp.models import *

# define the class of a form
class PostForm(ModelForm):
    class Meta:
        # write the name of models for which the form is made
        model = Post

        # Custom fields
        fields =["username", "gender", "text"]

    # this function will be used for the validation
    def clean(self):

        # data from the form is fetched using super function
        super(PostForm, self).clean()

        # extract the username and text field from the data
        username = self.cleaned_data.get('username')
        text = self.cleaned_data.get('text')

        # conditions to be met for the username length
        if len(username) < 5:
            self._errors['username'] = self.error_class([
                'Minimum 5 characters required'])
        if len(text) <10:
            self._errors['text'] = self.error_class([
                'Post Should Contain a minimum of 10 characters'])

        # return any errors if found
        return self.cleaned_data

Till now, the data models and the Form class are defined. Now the focus will be on how these modules, defined above, are actually used.
First, run on localhost through this command 

python manage.py runserver

Open http://localhost:8000/ in the browser, then it’s going to search in the urls.py file, looking for ‘ ‘ path
urls.py file is as given below: 

Python

from django.contrib import admin
from django.urls import path, include
from django.conf.urls import url
from django.shortcuts import HttpResponse
from . import views


urlpatterns = [
    path('', views.home, name ='index'),
]

Basically, this associates the ‘ ‘ url with a function home which is defined in views.py file.
views.py file: 

Python

from .models import Post
from .forms import PostForm
from .import views
from django.shortcuts import HttpResponse, render, redirect


def home(request):

    # check if the request is post
    if request.method =='POST':

        # Pass the form data to the form class
        details = PostForm(request.POST)

        # In the 'form' class the clean function
        # is defined, if all the data is correct
        # as per the clean function, it returns true
        if details.is_valid():

            # Temporarily make an object to be add some
            # logic into the data if there is such a need
            # before writing to the database
            post = details.save(commit = False)

            # Finally write the changes into database
            post.save()

            # redirect it to some another page indicating data
            # was inserted successfully
            return HttpResponse("data submitted successfully")

        else:

            # Redirect back to the same page if the data
            # was invalid
            return render(request, "home.html", {'form':details})
    else:

        # If the request is a GET request then,
        # create an empty form object and
        # render it into the page
        form = PostForm(None)
        return render(request, 'home.html', {'form':form})

home.html template file 

html

{% load bootstrap3 %}
{% bootstrap_messages %}
<!DOCTYPE html>
<html lang="en">

<head >

    <title>Basic Form</title>

    <meta charset="utf-8" />

    <meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">

<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css">
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.2.1/jquery.min.js">
</script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js">
</script>
</head>

<body  style="padding-top: 60px;background-color: #f5f7f8 !important;">
    <div class="container">
    <div class="row">
        <div class="col-md-4 col-md-offset-4">
        <h2>Form</h2>
             <form action="" method="post"><input type='hidden'/>
             {%csrf_token %}

                {% bootstrap_form form %}
<!-This is the form variable which we are passing from the function
of home in views.py file. That's the beauty of Django we
don't need to write much codes in this it'll automatically pass
all the form details in here
->
              <div class="form-group">
                <button type="submit" class="btn btn-default ">
                  Submit
                </button>

                  </div>

                </form>
        </div>
    </div>
</div>

</body>

</html>

Opening http://localhost:8000/ in the browser shows the following,

If a form with a username of length less than 5 is submitted, it gives an error at the time of submission and similarly for the Post Text filled as well. The following image shows how the form behaves on submitting invalid form data.