Start background instances of blender to do rendering

multirender.py

#!/usr/bin/env python3

import os
import subprocess as sp

# this script is sent to each instance to do the render
# use str.format() to fill in the {}'s
render_scr = """import bpy
scn = bpy.context.scene
scn.render.use_sequencer = False
scn.render.engine = 'CYCLES'
scn.cycles.samples = 2
scn.frame_end = 5
scn.use_nodes = True

for node in scn.node_tree.nodes:
    scn.node_tree.nodes.remove(node)
renderNode = scn.node_tree.nodes.new('CompositorNodeRLayers')

# Depth map
depthOutputNode = scn.node_tree.nodes.new('CompositorNodeOutputFile')
depthOutputNode.format.file_format = 'OPEN_EXR'
depthOutputNode.format.color_depth = '32'
depthOutputNode.format.color_mode = 'RGB'
depthOutputNode.format.exr_codec = 'ZIP'
depthOutputNode.base_path = '//somepath/{}/'
depthOutputNode.file_slots[0].path = 'fileNameDepth#'

# Link
scn.node_tree.links.new(renderNode.outputs[2], depthOutputNode.inputs[0])

bpy.ops.import_scene.obj(filepath="{}")
bpy.ops.render.render(write_still=True, animation=True)
"""

# adjust this if blender isn't in your $PATH
blender_cmd = 'blender'

objPaths = ['obj1.obj', 'obj2.obj', 'obj3.obj', 'obj4.obj']

for op in objPaths:
    # sp.Popen will start the process and leave it running in the background
    # sp.run can be used to wait for completion before going to next file
    sp.Popen([blender_cmd,
            '--factory-startup', '--background', 'pystart.blend',
            '--python-expr', render_scr.format(op,op)])