pandas.Index.groupby
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Index.groupby(values)[source]
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Group the index labels by a given array of values. Parameters: values : array Values used to determine the groups. Returns: groups : dict {group name -> group labels} 
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Licensed under the 3-clause BSD License.
    https://pandas.pydata.org/pandas-docs/version/0.23.4/generated/pandas.Index.groupby.html