'`css_classes` may be used as a key in _html_output() (class comes
from required_css_class in this case).'

def test_field_with_css_class(self):

    class SomeForm(Form, ):
        some_field = CharField()
        required_css_class = 'foo'
        def as_p(self):
            return self._html_output(normal_row='<p    class="%(css_classes)s"></p>', error_row='%s', row_ender='</p>', help_text_html='    %s', errors_on_separate_row=True)
    form = SomeForm()
    self.assertHTMLEqual(form.as_p(), '<p    class="foo"></p>')
