pandas.Index.union

final 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.

Parameters:

otherIndex or array-like

Index or an array-like object containing elements to form the union with the original Index.

sortbool 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.

• True : Sort the result (which may raise TypeError).

Returns:

Index

Returns a new Index object with elements from both the original Index and the other Index.

If either index contains duplicate entries, the result preserves duplicates up to the maximum number of occurrences in either index.

See also

Index.unique

Return unique values in the index.

Index.intersection

Form the intersection of two Index objects.

Index.difference

Return a new Index with elements of index not in other.

Examples

Union matching dtypes

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

Union with duplicate values

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

MultiIndex case

>>> idx1 = pd.MultiIndex.from_arrays(
...     [[1, 1, 2, 2], ["Red", "Blue", "Red", "Blue"]]
... )
>>> idx1
MultiIndex([(1,  'Red'),
    (1, 'Blue'),
    (2,  'Red'),
    (2, 'Blue')],
   )
>>> idx2 = pd.MultiIndex.from_arrays(
...     [[3, 3, 2, 2], ["Red", "Green", "Red", "Green"]]
... )
>>> idx2
MultiIndex([(3,   'Red'),
    (3, 'Green'),
    (2,   'Red'),
    (2, 'Green')],
   )
>>> idx1.union(idx2)
MultiIndex([(1,  'Blue'),
    (1,   'Red'),
    (2,  'Blue'),
    (2, 'Green'),
    (2,   'Red'),
    (3, 'Green'),
    (3,   'Red')],
   )
>>> idx1.union(idx2, sort=False)
MultiIndex([(1,   'Red'),
    (1,  'Blue'),
    (2,   'Red'),
    (2,  'Blue'),
    (3,   'Red'),
    (3, 'Green'),
    (2, 'Green')],
   )