# pandas.Index.union

`Index.``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 : 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')

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