Add class to form field Django ModelForm

Since it took me more hours, than I would like to (django newbie), to figure this out, I will place my outcome here aswell.

Setting widget to each field just to add one class over and over again is against programming rule of repeating and leads to many unneccessary rows. This especially happens when working with bootstrap forms.

Here is my (working) example for adding not only bootstrap classes:

forms.py

class CompanyForm(forms.Form):
    name = forms.CharField(label='Jméno')
    shortcut = forms.CharField(label='Zkratka')
    webpage = forms.URLField(label='Webové stránky')
    logo = forms.FileField(label='Logo')

templatetags/custom_tags.py

from django import template
from django.urls import reverse

register = template.Library()

@register.filter('input_type')
def input_type(ob):
    '''
    Extract form field type
    :param ob: form field
    :return: string of form field widget type
    '''
    return ob.field.widget.__class__.__name__


@register.filter(name='add_classes')
def add_classes(value, arg):
    '''
    Add provided classes to form field
    :param value: form field
    :param arg: string of classes seperated by ' '
    :return: edited field
    '''
    css_classes = value.field.widget.attrs.get('class', '')
    # check if class is set or empty and split its content to list (or init list)
    if css_classes:
        css_classes = css_classes.split(' ')
    else:
        css_classes = []
    # prepare new classes to list
    args = arg.split(' ')
    for a in args:
        if a not in css_classes:
            css_classes.append(a)
    # join back to single string
    return value.as_widget(attrs={'class': ' '.join(css_classes)})

reusable_form_fields.html (template)

{% load custom_tags %}

{% csrf_token %}
{% for field in form %}
    <div class="form-group row">
        {% if field|input_type == 'TextInput' %}
            <div for="{{ field.label }}" class="col-sm-2 col-form-label">
                {{ field.label_tag }}
            </div>
            <div class="col-sm-10">
                {{ field|add_classes:'form-control'}}
                {% if field.help_text %}
                    <small class="form-text text-muted">{{ field.help_text }}</small>
                {% endif %}
            </div>
        {% else %}
            ...
        {% endif %}
    </div>
{% endfor %}

If you can't use a third-party app and want to add a class (e.g., "form-control") to every field in a form in a DRY manner, you can do so in the form class __init__() method like so:

class ExampleForm(forms.Form):
    # Your declared form fields here
    ...

    def __init__(self, *args, **kwargs):
        super(ExampleForm, self).__init__(*args, **kwargs)
        for visible in self.visible_fields():
            visible.field.widget.attrs['class'] = 'form-control'

You might need to handle checking for existing classes in attrs too, if for some reason you'll be adding classes both declaratively and within __init__(). The above code doesn't account for that case.

Worth mentioning:

You specified that you don't want to use third-party packages. However, I'll take one second to mention that one of the simplest ways of automatically making forms render in the style of Bootstrap is to use django-crispy-forms, like this:

# settings.py
CRISPY_TEMPLATE_PACK = 'bootstrap3'

# forms.py
from crispy_forms.helper import FormHelper
class ExampleForm(forms.Form):
    # Your declared form fields here
    ...
    helper = FormHelper()

# In your template, this renders the form Bootstrap-style:
{% load crispy_forms_tags %}
{% crispy form %}

Crispy forms are the way to go . Tips for Bootstrap 4. Adding to @Christian Abbott's answer, For forms , bootstrap says, use form-group and form-control . This is how it worked for me .

My forms.py

class BlogPostForm(forms.ModelForm):
    class Meta:
        model = models.Post
        fields = ['title', 'text', 'tags', 'author', 'slug']
    helper = FormHelper()
    helper.form_class = 'form-group'
    helper.layout = Layout(
        Field('title', css_class='form-control mt-2 mb-3'),
        Field('text', rows="3", css_class='form-control mb-3'),
        Field('author', css_class='form-control mb-3'),
        Field('tags', css_class='form-control mb-3'),
        Field('slug', css_class='form-control'),
    )

My post_create.html

{% extends 'blog/new_blog_base.html' %}
{% load crispy_forms_tags %}
{% block content %}
<div class="container">

<form method='POST' enctype="multipart/form-data">
    {% csrf_token %}
    {{ form.media }}
    {% crispy form %}

<hr>
<input type="submit" name="Save" value="Save" class='btn btn-primary'> <a href="{% url 'home' %}" class='btn btn-danger'>Cancel</a>
</form>

</div>
{% endblock %}

Note : If you are using CK Editor RichTextField() for your model field , then that field wont be affected . If anyone knows about it , do update this .

you can add CSS classes in forms.py

subject = forms.CharField(label='subject', 
                          max_length=100,
                          widget=forms.TextInput(
                              attrs={'class': "form-control"}))