@Nullable
  private PsiElement resolveFormatString() {
    final PyArgumentList argumentList = getArgumentList(getElement());
    if (argumentList == null || argumentList.getArguments().length == 0) {
      return null;
    }
    return myChunk.getMappingKey() != null ? resolveKeywordFormat(argumentList).get() : resolvePositionalFormat(argumentList);
  }
@Nullable
  private PsiElement resolveFormatString()