Aggregation in Pandas

How can I perform aggregation with Pandas?

Aggregation functions reduce the dimensionality of returned objects. Some common aggregation functions include mean(), sum(), size(), count(), std(), var(), and sem().

df1 = df.groupby(['A', 'B'], as_index=False)['C'].sum()

No DataFrame after aggregation! What happened?

If you group by two or more columns, you may need to specify as_index=False or use Series.reset_index() to convert a MultiIndex Series to columns.

How can I aggregate mainly strings columns (to lists, tuples, strings with separator)?

To aggregate string columns:

df1 = df.groupby('A')['B'].agg(list).reset_index()

For strings with a separator:

df2 = df.groupby('A')['B'].agg(','.join).reset_index()

How can I aggregate counts?

Use GroupBy.size or GroupBy.count.

df1 = df.groupby('A').size().reset_index(name='COUNT')

How can I create a new column filled by aggregated values?

Use GroupBy.transform.

df['C1'] = df.groupby('A')['C'].transform('sum')