Field ids should be generated automatically by django, to override other fields:

class ProductForm(ModelForm):
    class Meta:
        model = Product
        exclude = ('updated', 'created')

    def __init__(self, *args, **kwargs):
        super(ProductForm, self).__init__(*args, **kwargs)
        self.fields['name'].widget.attrs\
            .update({
                'placeholder': 'Name',
                'class': 'input-calss_name'
            })

I really like Dmitriy Sintsov’s answer but it doesn’t work. Here’s a version that does work:

def __init__(self, *args, **kwargs):
    super().__init__(*args, **kwargs)
    for field in iter(self.fields):
        self.fields[field].widget.attrs.update({
            'class': 'form-control'
    })

update

add this condition for better

if self.fields[field].widget.__class__.__name__ in ('AdminTextInputWidget' , 'Textarea' , 'NumberInput' , 'AdminURLFieldWidget', 'Select'): 
     self.fields[field].widget.attrs.update({ 'class': 'form-control' })

You can update forms.py as below

class ProductForm(ModelForm):
    class Meta:
        model = Product
        exclude = ('updated', 'created')
        widgets={
                   "name":forms.TextInput(attrs={'placeholder':'Name','name':'Name','id':'common_id_for_imputfields','class':'input-class_name'}),
                   "description":forms.TextInput(attrs={'placeholder':'description','name':'description','id':'common_id_for_imputfields','class':'input-class_name'}),
                }  

Slightly modified version of excellent mariodev answer, adding bootstrap class to all form fields, so I do not have to re-create form input widgets for each field manually (short Python 3.x super()):

class ProductForm(forms.ModelForm):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        for field in self.fields:
            self.fields[field].widget.attrs.update({
                'class': 'form-control'
            })

Adding to answer from Derick Hayes I created a class BasicForm which extends forms.ModelForm that adds the bootstrap classes to every form that extends it.

For my forms I just extend BasicForm instead of model form and automatically get bootstrap classes on all forms. I went a step further and append the classes to any custom css classes which may already be there.

class BaseModelForm(forms.ModelForm):

def __init__(self, *args, **kwargs):
    super(BaseModelForm, self).__init__(*args, **kwargs)
    # add common css classes to all widgets
    for field in iter(self.fields):
        #get current classes from Meta
        classes = self.fields[field].widget.attrs.get("class")
        if classes is not None:
            classes += " form-control"
        else:
            classes = "form-control"
        self.fields[field].widget.attrs.update({
            'class': classes
        })

add_class filter for adding a CSS class to form field:

 {% load widget_tweaks %}     
    <form enctype="multipart/form-data" action="{% url 'add_a_product' %}" method="post">
                 <div id="name">
                   {{form.name|add_class:"input-calss_name"}}
                 </div> 
                 <div id="description">
                   {{form.description|add_class:"input-calss_name"}}
                 </div> 
           </form> 

django-widget-tweaks library

You can do the following:

class ProductForm(ModelForm):
    name = forms.CharField(label='name ', 
            widget=forms.TextInput(attrs={'placeholder': 'name '}))

I know this is an old question but if someone is still looking to add custom class to all of his form fields, then you can use this one liner

class ProductForm(ModelForm):
    class Meta:
        model = Product
        exclude = ('updated', 'created')
def __init__(self, *args, **kwargs):
    super(ProductForm, self).__init__(*args, **kwargs)
    custom_attrs = {
        'class': 'form-control',
        'toggle-data': 'mydiv',
    }
    # adds our custom_attrs to each element of the form 
    [self.fields[i].widget.attrs.update(custom_attrs) for i in self.fields]