pandas.Series.cat.remove_unused_categories
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Series.cat.remove_unused_categories(*args, **kwargs)[source]
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Removes categories which are not used. Parameters: inplace : boolean (default: False) Whether or not to drop unused categories inplace or return a copy of this categorical with unused categories dropped. Returns: cat : Categorical with unused categories dropped or None if inplace. 
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Licensed under the 3-clause BSD License.
    https://pandas.pydata.org/pandas-docs/version/0.20.3/generated/pandas.Series.cat.remove_unused_categories.html