Export/import shader assignment with Maya

export.py

import maya.cmds as cmds
import json

# Create the dictionary "data" with shadergroup as key
data = {}
shaders = cmds.ls(type='shadingEngine')
for shader in shaders:
    data[shader] = []

# Append all shapes to each shadergroup-key
for transform in cmds.ls(type='transform'):
    shapes = cmds.listRelatives(transform, fullPath=True)
    shaders = cmds.listConnections(shapes, type='shadingEngine')
    if not isinstance(shaders, type(None)):
        for shader in shaders:
            for shape in shapes:
                data[shader].append(shape)
                
with open('shader_assignment.json', 'w') as outfile:
    json.dump(data, outfile, sort_keys=True, indent=4, ensure_ascii=False)