pandas.Series.cat.remove_unused_categories
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Series.cat.remove_unused_categories(*args, **kwargs)[source] - 
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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    https://pandas.pydata.org/pandas-docs/version/0.23.4/generated/pandas.Series.cat.remove_unused_categories.html