maya_output_path.py

from pymel.core import *

# original setup
SCENE.defaultRenderGlobals.imageFilePrefix.set('/render_output/frame_<RenderLayer>')
SCENE.defaultRenderGlobals.animation.set(1)
SCENE.defaultRenderGlobals.outFormatControl.set(2)
SCENE.defaultRenderGlobals.putFrameBeforeExt.set(1)
SCENE.defaultRenderGlobals.extensionPadding.set(3)

# get path and add format expression
if SCENE.defaultRenderGlobals.animation.get():
    path = renderSettings(genericFrameImageName='{0:0%sd}' % SCENE.defaultRenderGlobals.extensionPadding.get(), fullPath=True)[0]
else:
    path = renderSettings(firstImageName=True, fullPath=True)[0]
# > /render_output/filename_{0:04d}.exr

# usage path via format
frame = 123
frame_path = path.format(frame)
# > /render_output/frame_0123.exr