to as resampling in this book and will receive separate treatment in groupby mechanics hadley wickhaman author of many popular packages for the programming languagecoined the term split-apply-combine for talking about group operationsand think that' good description of the process in the first stage of the processdata contained in pandas objectwhether seriesdataframeor otherwiseis split into groups based on one or more keys that you provide the splitting is performed on particular axis of an object for examplea dataframe can be grouped on its rows (axis= or its columns (axis= once this is donea function is applied to each groupproducing new value finallythe results of all those function applications are combined into result object the form of the resulting object will usually depend on what' being done to the data see figure - for mockup of simple group aggregation figure - illustration of group aggregation each grouping key can take many formsand the keys do not have to be all of the same typea list or array of values that is the same length as the axis being grouped value indicating column name in dataframe data aggregation and group operations www it-ebooks info