python - How to GroupBy a Dataframe in Pandas and keep Columns

Question

given a dataframe that logs uses of some books like this:

Name   Type   ID
Book1  ebook  1
Book2  paper  2
Book3  paper  3
Book1  ebook  1
Book2  paper  2

I need to get the count of all the books, keeping the other columns and get this:

Name   Type   ID    Count
Book1  ebook  1     2
Book2  paper  2     2
Book3  paper  3     1

How can this be done?

Thanks!

Answer 1

You want the following:

In [20]:
df.groupby(['Name','Type','ID']).count().reset_index()

Out[20]:
    Name   Type  ID  Count
0  Book1  ebook   1      2
1  Book2  paper   2      2
2  Book3  paper   3      1

In your case the 'Name', 'Type' and 'ID' cols match in values so we can groupby on these, call count and then reset_index.

An alternative approach would be to add the 'Count' column using transform and then call drop_duplicates:

In [25]:
df['Count'] = df.groupby(['Name'])['ID'].transform('count')
df.drop_duplicates()

Out[25]:
    Name   Type  ID  Count
0  Book1  ebook   1      2
1  Book2  paper   2      2
2  Book3  paper   3      1