Overriding dictionaries

chainmap.py

d = defaults.copy()
d.update(os.environ)
d.update(command_line_arguments)

# PYTHON3 ONLY!!!
from collections import ChainMap
d = ChainMap(command_line_arguments, os.environ, defaults)

# it's view, not a real dictionary. if you want something equivalent to a new one
d = ChainMap({}, command_line_arguments, os.environ, defaults)

# writes are done on the first map in the list