Maya MEL script that breaks up a mesh based on materials. Created by Andrew Coggeshall at Basenji Games.

SepScript.mel

// Poly Separate
// Copyright (C) 2012 Basenji Games
// Licensed under the MIT license

string $selection[] = `ls -sl`;
sepMat($selection[0]);

global proc sepMat(string $object){
string $shadingGroups[] = getSGsFromShape($object);
string $ParentName = ($object + "_lightMap_Group");
group -empty -n $ParentName;

	for ($i = 0; $i < size($shadingGroups) ; $i = $i + 1)
	{
		string $theMaterial[] = `listConnections -d off -s on $shadingGroups[$i]`;
		string $clone = ($object + "_" + $theMaterial[0]);
		duplicate -n $clone  $object;
		parent $clone $ParentName;
		
			string $material[] = `listConnections -d off -s on $shadingGroups[$i]`;
			select $clone; ConvertSelectionToFaces;
			string $tempset = `sets`;
			string $tempgrps[] = `listConnections -type shadingEngine $material[0]`;
			select `sets -int $tempset $tempgrps[0]`;
			
			
			InvertSelection;
			delete;
	}
	delete $object;
}



proc string[] getSGsFromShape( string $shape )
{
string $myShapeNode[] = `listRelatives -children -shapes $shape`;
  string $shadingEngines[];

  if ( `objExists $shape` )
  {
    string $dest[] = `listConnections -destination true -source false
      -plugs false -type "shadingEngine" $myShapeNode[0]`;

    if ( size( $dest ) )
    {
      string $select[] = `ls -sl`;
      select -r -ne $dest;
      $shadingEngines = `ls -sl`;
      select -r $select;
    }
  }

  return $shadingEngines;
}

Sorry, when I read your script, I found a command is cmds.ConvertSelectionToFaces. I didn't find this command when looking up the Maya documentation. Where can I find it? I would appreciate it if you could answer.

I don't think this is a documented command. However you can see it is a runtime command in the PyMel documentation: https://download.autodesk.com/global/docs/maya2014/en_us/PyMel/generated/functions/pymel.core.runtime/pymel.core.runtime.ConvertSelectionToFaces.html

Mel equivalent ConvertSelectionToFaces; is undocumented, however when you check your Script Editor, you can see this command calls PolySelectConvert 1; which is located in PolySelectConvert.mel file inside your main Maya application. Infact, if you monitor your script editor, select an object, CONTROL + Right Click > To Faces >To Faces, and you convert to faces this way, the script editor calls ConvertSelectionToFaces; which just runs PolySelectConvert 1;

With Python, you can also do:

import pymel.core as pm
pm.mel.PolySelectConvert(1)

Same thing. I guess in Python,

from maya import cmds
cmds.ConvertSelectionToFaces()

Will just call the pymel command:

import pymel.core as pm
pm.mel.PolySelectConvert(1)

But you're right, it's not really documented.

Thank you for your help. I have solved it perfectly.

Thanks, very useful

Thanks, I was looking for such a thing. I took the liberty to do some corrections and cleaning.
I cannot correctly paste code here, so this is the link: https://pastebin.com/5vhV2EeT

# coding=utf-8
from maya import cmds

def getSGs(in_obj_name):
    # type: (str) -> list[str]
    if not cmds.objExists(in_obj_name): return []
    
    shadingEngines = set()  # type: set[str]
    ShapeNodes=cmds.listRelatives(in_obj_name, shapes=1, children=1) or []# type: list[str]
    if not ShapeNodes : return []
    
    for shape in ShapeNodes:
        dest=cmds.listConnections(shape, source=False, plugs=False, destination=True, type="shadingEngine") # type: list[str]
        if not dest : continue
        shadingEngines.update(dest)
        
    return list(shadingEngines)

def sepMat(objectName, suffixName = "_Splitted"):
    # type: (str, str) -> None
    
    shadingGroups=getSGs(objectName)
    if shadingGroups.__len__() <= 1: return

    nodeParentName = (objectName + suffixName) # type: str
    if not cmds.objExists(nodeParentName): 
        nodeParentName = cmds.group(empty=1, n=nodeParentName) # type: str
    
    for i in range(shadingGroups.__len__()):
        curMatSg=shadingGroups[i]
        
        cloneObjName = (objectName + "_" + curMatSg) # type: str
        cloneObjName = cmds.duplicate(objectName, n=cloneObjName)[0] # type: str
        cloneObjName = cmds.parent(cloneObjName, nodeParentName)[0] # type: str
        
        polyFacesSet = cmds.sets(cloneObjName + '.f[0:]') # type: str
        sgFacesSet = cmds.sets(cmds.sets(curMatSg, un=polyFacesSet)) # type: str
        cmds.delete(cmds.sets(polyFacesSet , subtract=sgFacesSet), polyFacesSet, sgFacesSet)
    else: cmds.delete(objectName)


def runScript(*args, **kw):
    
    selection = cmds.ls(sl=True, objectsOnly=True, noIntermediate=True)
    for sel in selection:
        try: sepMat(sel)
        except : continue
        
    cmds.select(cl=True)

if __name__ == "__main__":
    runScript()

More fast && clean