Version 0.21.0

Multi-index columns support

We continue improving multi-index columns support. We made the following APIs support multi-index columns:

• nunique (#980)
• to_csv (#983)

Documentation

Now, we have installation guide, design principles and FAQ in our public documentation (#914, #944, #963, #964)

Other new features and improvements

We added the following new features:

koalas

• merge (#969)

koalas.DataFrame:

• keys (#937)
• ndim (#947)

koalas.Series:

• keys (#935)
• mode (#899)
• truncate (#928)
• xs (#921)
• where (#922)
• first_valid_index (#936)

koalas.Index:

• copy (#939)
• unique (#912)
• ndim (#947)
• has_duplicates (#946)
• nlevels (#945)

koalas.MultiIndex:

• copy (#939)
• ndim (#947)
• has_duplicates (#946)
• nlevels (#945)

koalas.Expanding

• count (#978)

Along with the following improvements:

• Fix passing options as keyword arguments (#968)
• Make is_monotonic~ work properly for index (#930)
• Fix Series.__getitem__ to work properly (#934)
• Fix reindex when all the given columns are included the existing columns (#975)
• Add datetime as the equivalent python type to TimestampType (#957)
• Fix is_unique to respect the current Spark column (#981)
• Fix bug when assign None to name as Index (#974)
• Use name_like_string instead of str directly. (#942, #950)