# xarray.CFTimeIndex.union

`CFTimeIndex.``union`(other, sort=None)

  Form the union of two Index objects.

  If the Index objects are incompatible, both Index objects will be cast to dtype(‘object’) first.

  > Changed in version 0.25.0.

  Parameters:

  - other (Index or array-like) –
  - sort (bool or None, default None) –

    Whether to sort the resulting Index.

    - None : Sort the result, except when
      1. self and other are equal.
      2. self or other has length 0.
      3. Some values in self or other cannot be compared. A RuntimeWarning is issued in this case.
    - False : do not sort the result.

    New in version 0.24.0.

    Changed in version 0.24.1: Changed the default value from `True` to `None` (without change in behaviour).

  Returns: | union
  Return type: | Index

  Examples

  Union matching dtypes

      >>> idx1 = pd.Index([1, 2, 3, 4])
      >>> idx2 = pd.Index([3, 4, 5, 6])
      >>> idx1.union(idx2)
      Int64Index([1, 2, 3, 4, 5, 6], dtype='int64')

  Union mismatched dtypes

      >>> idx1 = pd.Index(['a', 'b', 'c', 'd'])
      >>> idx2 = pd.Index([1, 2, 3, 4])
      >>> idx1.union(idx2)
      Index(['a', 'b', 'c', 'd', 1, 2, 3, 4], dtype='object')