casting rules and indexing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 516 17 Group By: split-apply-combine 519 17.1 Splitting an object into groups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 524 17.3 Selecting a group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 525 17 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 914 32.3 GROUP BY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 916

casting rules and indexing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 717 16 Group By: split-apply-combine 721 16.1 Splitting an object into groups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 729 16.3 Selecting a group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 730 16 of “nuisance” columns . . . . . . . . . . . . . . . . . . . . . . . . . . 746 16.9.2 NA and NaT group handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 747 16.9.3 Grouping

casting rules and indexing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 715 16 Group By: split-apply-combine 719 16.1 Splitting an object into groups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 727 16.3 Selecting a group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 728 16 of “nuisance” columns . . . . . . . . . . . . . . . . . . . . . . . . . . 744 16.9.2 NA and NaT group handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 745 16.9.3 Grouping

rules and indexing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 645 xii 17 Group By: split-apply-combine 647 17.1 Splitting an object into groups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 653 17.3 Selecting a group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 654 17 of “nuisance” columns . . . . . . . . . . . . . . . . . . . . . . . . . . 667 17.9.2 NA and NaT group handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 668 17.9.3 Grouping

casting rules and indexing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 647 17 Group By: split-apply-combine 649 17.1 Splitting an object into groups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 655 17.3 Selecting a group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 656 17 of “nuisance” columns . . . . . . . . . . . . . . . . . . . . . . . . . . 669 17.9.2 NA and NaT group handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 670 17.9.3 Grouping

casting rules and indexing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 748 16 Group By: split-apply-combine 751 16.1 Splitting an object into groups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 759 16.3 Selecting a group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 760 16 of “nuisance” columns . . . . . . . . . . . . . . . . . . . . . . . . . . 776 16.9.2 NA and NaT group handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 777 16.9.3 Grouping

showing output from a single group when function returns an Index (GH28652) • Bug in DataFrame.groupby() with multiple groups where an IndexError would be raised if any group contained all NA values (GH20519) Series, DataFrame, etc. automatically align the data for you in computations • Powerful, flexible group by functionality to perform split-apply-combine operations on data sets, for both ag- gregating and Grouping By “group by” we are referring to a process involving one or more of the following steps: • Splitting the data into groups based on some criteria • Applying a function to each group independently

on DataFrame evaluates first group only once The implementation of DataFrameGroupBy.apply() previously evaluated the supplied function consistently twice on the first group to infer if it is safe to use may have led to surprises. (GH2936, GH2656, GH7739, GH10519, GH12155, GH20084, GH21417) Now every group is evaluated only a single time. In [20]: df = pd.DataFrame({"a": ["x", "y"], "b": [1, 2]}) In [21]: (continued from previous page) 1 y 2 [2 rows x 2 columns] In [22]: def func(group): ....: print(group.name) ....: return group ....: Previous behavior: In [3]: df.groupby('a').apply(func) x x y Out[3]:

on DataFrame evaluates first group only once The implementation of DataFrameGroupBy.apply() previously evaluated the supplied function consistently twice on the first group to infer if it is safe to use may have led to surprises. (GH2936, GH2656, GH7739, GH10519, GH12155, GH20084, GH21417) Now every group is evaluated only a single time. In [20]: df = pd.DataFrame({"a": ["x", "y"], "b": [1, 2]}) In [21]: (continued from previous page) 1 y 2 [2 rows x 2 columns] In [22]: def func(group): ....: print(group.name) ....: return group ....: Previous behavior: In [3]: df.groupby('a').apply(func) x x y Out[3]:

Exponentially weighted windows . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 686 2.16 Group by: split-apply-combine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . groups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 699 2.16.3 Selecting a group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 700 2.16.4 Aggregation Series, DataFrame, etc. automatically align the data for you in computations • Powerful, flexible group by functionality to perform split-apply-combine operations on data sets, for both ag- gregating and