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# pandas.Categorical.remove_unused_categories

Categorical.remove_unused_categories()

  Remove categories which are not used.

  This method is useful when working with datasets that undergo dynamic changes where categories may no longer be relevant, allowing to maintain a clean, efficient data structure.

  Returns:
    Categorical

      Categorical with unused categories dropped.

  See also

  `rename_categories`

    Rename categories.

  `reorder_categories`

    Reorder categories.

  `add_categories`

    Add new categories.

  `remove_categories`

    Remove the specified categories.

  `set_categories`

    Set the categories to the specified ones.

  Examples

      >>> c = pd.Categorical(["a", "c", "b", "c", "d"])
      >>> c
      ['a', 'c', 'b', 'c', 'd']
      Categories (4, str): ['a', 'b', 'c', 'd']

      >>> c[2] = "a"
      >>> c[4] = "c"
      >>> c
      ['a', 'c', 'a', 'c', 'c']
      Categories (4, str): ['a', 'b', 'c', 'd']

      >>> c.remove_unused_categories()
      ['a', 'c', 'a', 'c', 'c']
      Categories (2, str): ['a', 'c']