Apply error class to form elements django

forms.py

class MyForm(forms.Form):
    """
    Extend from this form so your widgets have an 'error' class if there's
    an error on the field. 
    """ 
    def __init__(self, *args, **kwargs):
        super(MyForm, self).__init__(*args, **kwargs)
        if self.errors:
            for f_name in self.fields:
                if f_name in self.errors:
                    classes = self.fields[f_name].widget.attrs.get('class', '')
                    classes += ' my_error_class'
                    self.fields[f_name].widget.attrs['class'] = classes