Bootstraps ShotGrid in Maya from a standalone launch.

shotgrid_boostrap_maya.py

import sys

# assumes a specific tk-core version is installed, other tk-core acquisition methods are available
tk_core_path = "C:/Program Files/Shotgun/Resources/Desktop/Python/bundle_cache/app_store/tk-core/v0.20.12/python"
# the project id to use for the context
project_id = 369

sys.path.append(tk_core_path)

import sgtk

# set authenticated user, requires a user to log in or be logged into ShotGrid desktop app
auth = sgtk.authentication.ShotgunAuthenticator()
user = auth.get_user()
sgtk.set_authenticated_user(user)

# create a boostrap manager, using same plugin_id that's used when Maya is launched from desktop app
mgr = sgtk.bootstrap.ToolkitManager(user)
mgr.plugin_id = 'basic.desktop'
project = {'type': 'Project', 'id': project_id}

# start tk-maya engine
engine = mgr.bootstrap_engine('tk-maya', entity=project)