What is reindexing only valid with uniquely valued index objects?

Reindexing is a process in pandas that involves changing the row labels or column labels of a DataFrame. This is often necessary when new data is added to a DataFrame or when the existing data is not properly ordered.

One important consideration when reindexing is the uniqueness of the index values. When using reindexing, it is only valid to use uniquely valued index objects. This means that each index label must be unique and cannot be duplicated.

If there are duplicate index values in a DataFrame, reindexing can lead to unexpected results or errors. To avoid these issues, it is important to first remove any duplicate index values before reindexing.

Overall, reindexing is a powerful tool for managing data in pandas, but it is important to follow best practices to ensure accuracy and avoid errors.