Which is the best way to allow configuration options be overridden at the command line in Python?

Question

I have a Python application which needs quite a few (~30) configuration parameters. Up to now, I used the OptionParser class to define default values in the app itself, with

Answer 1

You can use ChainMap

A ChainMap groups multiple dicts or other mappings together to create a single, updateable view. If no maps are specified, a single empty dictionary is provided so that a new chain always has at least one mapping.

You can combine values from command line, environment variables, configuration file, and in case if the value is not there define a default value.

import os
from collections import ChainMap, defaultdict

options = ChainMap(command_line_options, os.environ, config_file_options,
               defaultdict(lambda: 'default-value'))
value = options['optname']
value2 = options['other-option']


print(value, value2)
'optvalue', 'default-value'