[Maya Mel/Python] look up array attribute connections

MEL

Consider this node and connections to the array (multi) attribute .matrix:

MEL command to list every DG nodes connected to a multi attribute (a.k.a array attribute):

string $joint_names[] = `listConnections ($skincluster+".matrix")`;
// example output: {'joint1', 'joint2'}

Now here is how to list the "logical" indices of an array attribute that are connected to a node:

int $joint_indices[] = `getAttr -multiIndices ($skincluster+".matrix")`;
// example output: {0, 2}

Note: you may recall that array attributes can be sparse, for instance:

    array_attribute[2] : [ joint1,  joint2]
    Logical indices    : {      0,       2} 
    Physical indices   : {      0,       1} 

Summary

string $joint_names[] = `listConnections ($source_skincluster+".matrix")`;
int $joint_indices[] = `getAttr -multiIndices ($source_skincluster+".matrix")`;

// Loop through the nodes connected to the attribute '$source_skincluster+".matrix[]"'
for( $i = 0; $i < size($joint_indices); ++$i) {    
    int $idx = $joint_indices[$i]; 
    string $name = $joint_names[$i];   
    // Reconnect joints to another skin cluster while preserving the original indices of the joints:
    connectAttr ($name+".worldMatrix[0]") ($destination_skincluster+".matrix["+$idx+"]")
}

Python 2.0 API

The above MEL code translates as follows:

from maya.api.OpenMaya import *

# get_MObject() and get_MPlug() are utilities I define below
obj = get_MObject("skinCluster1")
plug = get_MPlug(obj, "matrix")
for i in range(plug.numElements()):
    plug_element = plug.elementByPhysicalIndex(i)
    print( "name: " + plug_element.name() )
    print( "logical index: " + str(plug_element.logicalIndex()) )
    print( "physical index: " + str(i) )
    print( "" )

Output:

# name: skinCluster1.matrix[0]
# logical index: 0
# physical index: 0
#
# name: skinCluster1.matrix[2]
# logical index: 2
# physical index: 1

from maya.api.OpenMaya import *

def get_MObject(node_name):
    if isinstance(node_name, MObject):
        return node_name

    if isinstance(node_name, MDagPath):
        return node_name.node()

    slist = MSelectionList()
    try:
        slist.add(node_name)
    except RuntimeError:
        assert False, 'The node: "'+node_name+'" could not be found.'

    matches = slist.length()
    assert (matches == 1), 'Multiple nodes found for the same name: '+node_name

    obj = slist.getDependNode(0)
    return obj
# -----------------

def get_MPlug(obj, attribute):
    if not isinstance(obj, MObject):
        assert False, "expected MObject"
    dep = MFnDependencyNode(obj)
    plug = MPlug()
    try:
        plug = dep.findPlug( attribute, True)
    except RuntimeError:
        assert False, 'The attribute: "'+attribute+'" could not be found.'
          
    assert (not plug.isNull), 'The attribute: "'+attribute+'" could not be found.'
    return plug
# -----------------

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