map/doc/ReadablePropertyMap.html">Readable Property
Map</a>. The value type of the map must be an integer type. The vertex
descriptor type of graph 2 needs to be usable as the key type of the
map.<br>
<b>Default:</b> <tt>get(vertex_index, g2)</tt>
    Note: if you use this default, make sure your graph has
    an internal <tt>vertex_index</tt> property. For example,
    <tt>adjacency_list</tt> with <tt>VertexList=listS</tt> does
    not have an internal <tt>vertex_index</tt> property.
   <br>
<b>Python</b>: