.groupby()

The .groupby() function groups a DataFrame using a mapper or a series of columns and returns a GroupBy object. A range of methods, as well as custom functions, can be applied to GroupBy objects in order to combine or transform large amounts of data in these groups.

Syntax

dataframevalue.groupby(by, axis, level, as_index, sort, group_keys, observed, dropna)

.groupby() uses the following parameters:

• by: If a dictionary or Series is passed, the values will determine groups. If a list or ndarray with the same length as the selected axis is passed, the values will be used to form groups. A label or list of labels can be used to group by a particular column or columns.
• axis: Split along rows (0 or “index”) or columns (1 or “columns”). Default value is 0.
• level: If the axis is a MultiIndex, group by a particular level or levels. Value is int or level name, or sequence of them. Default value is None.
• as_index: Boolean value. True returns group labels as an index in aggregated output, and False returns labels as DataFrame columns. Default value is True.
• sort: Boolean value. True sorts the group keys. Default value is True.
• group_keys: Boolean value. Add group keys to index when calling apply. Default value is True.
• observed: Boolean value. If True, only show observed values for categorical groupers, otherwise show all values. Default value is False.
• dropna: Boolean value. If True, drop groups whose keys contain NA values. If False, NA will be used as a key for those groups. Default value is True.

Example

This example uses .groupby() on a DataFrame to produce some aggregate results.

import pandas as pd

df = pd.DataFrame({'Key' : ['A', 'A', 'A', 'B', 'B', 'C'],
                   'Value' : [15., 23., 17., 5., 8., 12.]})
print(df, end='\n\n')

print(df.groupby(['Key'], as_index=False).mean(), end='\n\n')

print(df.groupby(['Key'], as_index=False).sum())

This produces the following output:

  Key  Value
0   A   15.0
1   A   23.0
2   A   17.0
3   B    5.0
4   B    8.0
5   C   12.0

  Key  Value
0   A  18.333333
1   B   6.500000
2   C  12.000000

  Key  Value
0   A   55.0
1   B   13.0
2   C   12.0