bugfix-union-7da79789134a3028.yaml

---
fixes:
  - |
    Previously, :func:`~retworkx.digraph_union` would incorrectly keep or delete edges
    if argument ``merge_edges`` is set to true. This has been fixed and an edge from
    the second graph will be skipped if both its endpoints were merged to nodes from
    the first graph and these nodes already share an edge with equal weight data.
    Fixed `#432 <https://github.com/Qiskit/retworkx/issues/432>`__
features:
  - |
    Add a new function :func:`~retworkx.graph_union` that returns the union
    of two :class:`~retworkx.PyGraph` objects. This is the equivalent to 
    :func:`~retworkx.digraph_union` but for a :class:`~retworkx.PyGraph`
    instead of for a :class:`~retworkx.PyDiGraph`. A new unified function
    :func:`~retworkx.union` was also added that supports both
    :class:`~retworkx.PyDiGraph` and :class:`~retworkx.PyGraph`.
    For example:

    .. jupyter-execute::

      import retworkx
      from retworkx.visualization import mpl_draw

      first = retworkx.generators.path_graph(3, weights=["a_0", "node", "a_1"])
      second = retworkx.generators.cycle_graph(3, weights=["node", "b_0", "b_1"])
      graph = retworkx.graph_union(first, second, merge_nodes=True)
      mpl_draw(graph)
  - |
    The kwargs ``merge_nodes`` and ``merge_edges`` of :func:`~retworkx.digraph_union` are
    now optional and by default are set `False`.
  - |
    Add a new :meth:`~retworkx.PyGraph.find_node_by_weight` that finds the index
    of a node given a specific weight.