In Python, set default values for ArugmentParser, optionally read in a config file or override with CLI arguments.

app.ini

[defaults]
option2:Overridden #2 in config
option3:Overridden #3 in config
flag: 0
number_int: 6
number_float: 0.2

argparse_configparser_override.py

#!/usr/bin/env python3
# Thanks to: http://blog.vwelch.com/2011/04/combining-configparser-and-argparse.html
# Features:
#  * Cascading overrides (defaults, then config file, then command line arguments)
#  * bool/int/float parse correctly as long as we have a default set.

import argparse
import configparser
from typing import TypeVar, Callable, Generic, Dict

T = TypeVar('T')


class _ConfigType(Generic[T]):
    def __init__(self, name: str, parse_func: Callable[[str, str], T]):
        self.name = name
        self.parse_func = parse_func


def parse_config_and_args(arg_parser: argparse.ArgumentParser):
    # Add option for our config file path
    arg_parser.add_argument("-c", "--config", help="Specify config file", metavar="FILE")
    args, remaining_argv = arg_parser.parse_known_args()  # Parse for config file path

    if args.config:
        config = configparser.ConfigParser()
        config.read([args.config])

        first_section = config.sections()[0]  # Just assume first section for this example.

        config_values = dict(config.items(first_section))
        defaults = vars(arg_parser.parse_args([]))
        merged_config_file = {
            **defaults,
            **config_values
        }
        # Try and parse out config with defaults
        type_mapping: Dict[type, _ConfigType] = {
            bool: _ConfigType[bool]("boolean", config.getboolean),
            int: _ConfigType[int]("integer", config.getint),
            float: _ConfigType[float]("float", config.getfloat)
        }
        for key, value in defaults.items():
            type_config = None
            try:
                type_config = type_mapping[type(value)]
                merged_config_file[key] = type_config.parse_func(first_section, key)

            except ValueError:
                if isinstance(type_config, _ConfigType):
                    raise ValueError("\"%s\" in config wasn't set to a %s. Either omit or set to a valid "
                                     "%s." % (key, type_config.name, type_config.name))
                else:
                    raise ValueError(
                        "\"%s\" wasn't the correct type. Please double check the type in the config." % key)
            except KeyError:
                pass
            except configparser.NoOptionError:
                pass

        arg_parser.set_defaults(**merged_config_file)

    return arg_parser.parse_args()  # Parse our full list of CLI opts now.


if __name__ == '__main__':
    parser = argparse.ArgumentParser(
        description="Testing app to quickly check layering of config file + cli arguments."
    )
    parser.add_argument("--option1", default="opt1 default", help="some option help text")
    parser.add_argument("--option2", default="opt2 default", help="some other option help text")
    parser.add_argument("--option3", default="opt3 default", help="some other option")
    parser.add_argument("--no-default", help="No default")
    parser.add_argument("--flag", default=False, action="store_true")  # A boolean, True if flag present.
    parser.add_argument("--number-int", dest="number_int", type=int, default=42, help="Ensure we have an int")
    parser.add_argument("--number-float", dest="number_float", type=float, default=0.5,
                        help="Ensure we have a float")
    args = parse_config_and_args(parser)
    print(args)

# Tests:
# $ ./arg_test.py
# > Namespace(config=None, flag=False, no_default=None, number_float=0.5, number_int=42, option1='opt1 default', option2='opt2 default', option3='opt3 default')

# $ ./arg_test.py -c ./app.ini --option3 "Hello World"
# > Namespace(config='./app.ini', flag=False, no_default=None, number_float=0.2, number_int=6, option1='opt1 default', option2='Overridden #2 in config', option3='Hello World')

# $ ./arg_test.py -c ./app.ini --option3 "Hello World" --flag --number-int 7
# > Namespace(config='./app.ini', flag=True, no_default=None, number_float=0.2, number_int=7, option1='opt1 default', option2='Overridden #2 in config', option3='Hello World')