Add .nunique method to DataFrame and .groupby

[topper-123]

Currently, pd.DataFrame does not have a .nunique method

DataFrames have various aggregation methods that they share with series, examples being .sum, .count etc. Likewise with DataFrame.groupby-objects.

In many cases, a .nunique would be useful as on DataFrame and DataFrame.groupby-objects, for example when you're trying to assess the diversity of data in a DataFrame.

Expected Output

When you're using DataFrame.nunique, with n columns, you should get back a series where the index values are the dataframe.columns values, and the series values are the count of unique values, for each column.

[jreback]

you mean like this? #14376

this will be in 0.20.0 (soon)

[jreback]

duplicate of #14336

[topper-123]

you mean like this? #14376

Yes, thanks. I only looked through open issues, should have considered closed ones since 19.2 also.