Simple example of how to get shader assignment overrides in renderlayers in Maya. (This is built for renderlayers, *not!* for render setup)

maya_get_shader_overrides_in_renderlayer.py

import maya.cmds as cmds

def get_shader_overrides_in_renderlayer(layer):
    """Return shader assignment overrides in renderlayer"""
    
    attr = "{layer}.outAdjustments".format(layer=layer)
    indices = cmds.getAttr(attr, multiIndices=True)
    
    overrides = {}
    for index in indices:
        
        index_attr = "{0}[{index}]".format(attr, index=index)
        
        # For each adjustment check whether we have a valid out plug 
        # connection for which the override is intended. (In some
        # rare cases a disconnected override index remains to exist)
        connections = cmds.listConnections(index_attr + '.outPlug',
                                           shapes=True)
        if connections:
            node = connections[0]
            
            out = cmds.listConnections(index_attr + '.outValue')[0]
            overrides[node] = out
            
    return overrides

for layer in mc.ls(type="renderLayer"):
    print get_shader_overrides_in_renderlayer(layer)