error_messages = {
        'invalid_login': _(
            "Please enter a correct %(username)s and password. Note that both "
            "fields may be case-sensitive."
        ),
        'inactive': _("This account is inactive."),

 def clean(self):
    username = self.cleaned_data.get('username')
    password = self.cleaned_data.get('password')

    if username is not None and password:
        self.user_cache = authenticate(self.request, username=username, password=password)
        if self.user_cache is None:
            raise self.get_invalid_login_error()
        else:
            self.confirm_login_allowed(self.user_cache)

    return self.cleaned_data


 def get_invalid_login_error(self):
        return forms.ValidationError(
            self.error_messages['invalid_login'],
            code='invalid_login',
            params={'username': self.username_field.verbose_name},
        )