Method union

pandas/core/indexes/base.py:3025–3154

Form the union of two Index objects. If the Index objects are incompatible, both Index objects will be cast to dtype('object') first. Parameters ---------- other : Index or array-like Index or an array-like object containing elements to

(self, other, sort: bool | None = None)

    @final
    def union(self, other, sort: bool | None = 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
        ----------
        other : Index or array-like
            Index or an array-like object containing elements to form the union
            with the original Index.
        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.
            * True : Sort the result (which may raise TypeError).

        Returns
        -------
        Index
            Returns a new Index object with all unique elements from both the original
            Index and the `other` 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')

        MultiIndex case

        >>> idx1 = pd.MultiIndex.from_arrays(
        ...     [[1, 1, 2, 2], ["Red", "Blue", "Red", "Blue"]]
        ... )