blender/release/scripts/templates/script_stub.py

# This stub runs a python script relative to the currently open
# blend file, useful when editing scripts externally.

import bpy
import os

# Use your own script name here:
filename = "my_script.py"

filepath = os.path.join(os.path.dirname(bpy.data.filepath), filename)
global_namespace = {"__file__": filepath, "__name__": "__main__"}
file_handle = open(filepath, 'rb')
exec(compile(file_handle.read(), filepath, 'exec'), global_namespace)
file_handle.close()