How to perform a cumulative sum of distinct values in pandas dataframe

Question

I have a dataframe like this:

id    date         company    ......
123   2019-01-01        A
224   2019-01-01        B
345   2019-01-01        B
987   2019-01-03        C
334   2019-01-03        C
908   2019-01-04        C
765   2019-01-04        A
554   2019-01-05        A
482   2019-01-05        D

and I want to get the cumulative number of unique values over time for the 'company' column. So if a company appears at a later date they are not counted again.

My expected output is:

date            cumulative_count
2019-01-01      2
2019-01-03      3
2019-01-04      3
2019-01-05      4

I've tried:

df.groupby(['date']).company.nunique().cumsum()

but this double counts if the same company appears on a different date.

Answer 1

Using duplicated + cumsum + last

m = df.duplicated('company')
d = df['date']

(~m).cumsum().groupby(d).last()

date
2019-01-01    2
2019-01-03    3
2019-01-04    3
2019-01-05    4
dtype: int32