How to 'ls' shading group node?

[g2m.agent]

I want to list all shading group node, the manuals said:

materials(mat) boolean
List materials or shading groups.

so my script is:

import maya.cmds as cmds
cmds.ls(mat=True)

the result is:

# Result: [u'lambert1',
u'particleCloud1',
u'blinn2'] #

there is no SG node at all.

why??

[chaneyx · Accepted solution]

in MEL: 
ls -type "shadingEngine"