forms.py

class ClientForm(forms.ModelForm):
    class Meta:
        model = Client
        fields = ('name', 'email')
    def __init__(self, *args, **kwargs):
        super(ClientForm, self).__init__(*args, **kwargs)
        for field in iter(self.fields):
            self.fields[field].widget.attrs.update({'class': 'form-control'})