pandas.Series.groupby#

Series.groupby(by=None, level=None, as_index=True, sort=True, group_keys=True, observed=False, dropna=True)[source]#

Group Series using a mapper or by a Series of columns.

A groupby operation involves some combination of splitting the object, applying a function, and combining the results. This can be used to group large amounts of data and compute operations on these groups.

Parameters:

bymapping, function, label, pd.Grouper or list of such: Used to determine the groups for the groupby. If by is a function, it’s called on each value of the object’s index. If a dict or Series is passed, the Series or dict VALUES will be used to determine the groups (the Series’ values are first aligned; see .align() method). If a list or ndarray of length equal to the selected axis is passed (see the groupby user guide), the values are used as-is to determine the groups. A label or list of labels may be passed to group by the columns in self. Notice that a tuple is interpreted as a (single) key.
levelint, level name, or sequence of such, default None: If the axis is a MultiIndex (hierarchical), group by a particular level or levels. Do not specify both by and level.
as_indexbool, default True: Return object with group labels as the index. Only relevant for DataFrame input. as_index=False is effectively “SQL-style” grouped output. This argument has no effect on filtrations (see the filtrations in the user guide), such as head(), tail(), nth() and in transformations (see the transformations in the user guide).
sortbool, default True: Sort group keys. Get better performance by turning this off. Note this does not influence the order of observations within each group. Groupby preserves the order of rows within each group. If False, the groups will appear in the same order as they did in the original DataFrame. This argument has no effect on filtrations (see the filtrations in the user guide), such as head(), tail(), nth() and in transformations (see the transformations in the user guide).

Changed in version 2.0.0: Specifying sort=False with an ordered categorical grouper will no longer sort the values.
group_keysbool, default True: When calling apply and the by argument produces a like-indexed (i.e. a transform) result, add group keys to index to identify pieces. By default group keys are not included when the result’s index (and column) labels match the inputs, and are included otherwise.

Changed in version 1.5.0: Warns that group_keys will no longer be ignored when the result from apply is a like-indexed Series or DataFrame. Specify group_keys explicitly to include the group keys or not.

Changed in version 2.0.0: group_keys now defaults to True.
observedbool, default True: This only applies if any of the groupers are Categoricals. If True: only show observed values for categorical groupers. If False: show all values for categorical groupers.

Changed in version 3.0.0: The default value is now True.
dropnabool, default True: If True, and if group keys contain NA values, NA values together with row/column will be dropped. If False, NA values will also be treated as the key in groups.

Returns:

pandas.api.typing.SeriesGroupBy: Returns a groupby object that contains information about the groups.

See also

resample: Convenience method for frequency conversion and resampling of time series.

Notes

See the user guide for more detailed usage and examples, including splitting an object into groups, iterating through groups, selecting a group, aggregation, and more.

The implementation of groupby is hash-based, meaning in particular that objects that compare as equal will be considered to be in the same group. An exception to this is that pandas has special handling of NA values: any NA values will be collapsed to a single group, regardless of how they compare. See the user guide linked above for more details.

Examples

>>> ser = pd.Series([390., 350., 30., 20.],
...                 index=['Falcon', 'Falcon', 'Parrot', 'Parrot'],
...                 name="Max Speed")
>>> ser
Falcon    390.0
Falcon    350.0
Parrot     30.0
Parrot     20.0
Name: Max Speed, dtype: float64

We can pass a list of values to group the Series data by custom labels:

>>> ser.groupby(["a", "b", "a", "b"]).mean()
a    210.0
b    185.0
Name: Max Speed, dtype: float64

Grouping by numeric labels yields similar results:

>>> ser.groupby([0, 1, 0, 1]).mean()
0    210.0
1    185.0
Name: Max Speed, dtype: float64

We can group by a level of the index:

>>> ser.groupby(level=0).mean()
Falcon    370.0
Parrot     25.0
Name: Max Speed, dtype: float64

We can group by a condition applied to the Series values:

>>> ser.groupby(ser > 100).mean()
Max Speed
False     25.0
True     370.0
Name: Max Speed, dtype: float64

Grouping by Indexes

We can groupby different levels of a hierarchical index using the level parameter:

>>> arrays = [['Falcon', 'Falcon', 'Parrot', 'Parrot'],
...           ['Captive', 'Wild', 'Captive', 'Wild']]
>>> index = pd.MultiIndex.from_arrays(arrays, names=('Animal', 'Type'))
>>> ser = pd.Series([390., 350., 30., 20.], index=index, name="Max Speed")
>>> ser
Animal  Type
Falcon  Captive    390.0
        Wild       350.0
Parrot  Captive     30.0
        Wild        20.0
Name: Max Speed, dtype: float64

>>> ser.groupby(level=0).mean()
Animal
Falcon    370.0
Parrot     25.0
Name: Max Speed, dtype: float64

We can also group by the ‘Type’ level of the hierarchical index to get the mean speed for each type:

>>> ser.groupby(level="Type").mean()
Type
Captive    210.0
Wild       185.0
Name: Max Speed, dtype: float64

We can also choose to include NA in group keys or not by defining dropna parameter, the default setting is True.

>>> ser = pd.Series([1, 2, 3, 3], index=["a", 'a', 'b', np.nan])
>>> ser.groupby(level=0).sum()
a    3
b    3
dtype: int64

To include NA values in the group keys, set dropna=False:

>>> ser.groupby(level=0, dropna=False).sum()
a    3
b    3
NaN  3
dtype: int64

We can also group by a custom list with NaN values to handle missing group labels:

>>> arrays = ['Falcon', 'Falcon', 'Parrot', 'Parrot']
>>> ser = pd.Series([390., 350., 30., 20.], index=arrays, name="Max Speed")
>>> ser.groupby(["a", "b", "a", np.nan]).mean()
a    210.0
b    350.0
Name: Max Speed, dtype: float64

>>> ser.groupby(["a", "b", "a", np.nan], dropna=False).mean()
a    210.0
b    350.0
NaN   20.0
Name: Max Speed, dtype: float64