'Raise an appropriate OAuthValidationError error if the field is missing'

def _require_oauth_field(self, field_name):

    field_val = self.cleaned_data.get(field_name)
    if (not field_val):
        raise OAuthValidationError({'error': 'invalid_request', 'error_description': '{}    is    required'.format(field_name)})
    return field_val
