updateSettings.py

import argparse
import json
import os
import sys

parser = argparse.ArgumentParser(
    description='Updates output settings file based on a default file',
)

parser.add_argument(
    'input_file',
    type=str,
    help='Path to default settings json file',
)

parser.add_argument(
    'output_file',
    type=str,
    help='Path to output settings json file',
)

args = parser.parse_args()
default_settings = args.input_file
transmission_settings = args.output_file

# Fail if default settings file doesnt exist.
if not os.path.isfile(default_settings):
    sys.exit(
        'Invalid arguments, default settings file{file} does not exist'.format(
            file=default_settings,
        ),
    )


try:
    print('Attempting to use existing settings.json for Transmission')
    configuration_baseline = transmission_settings

    with open(configuration_baseline, 'r') as input_file:
        settings_dict = json.load(input_file)

    print('Successfully used existing settings.json', transmission_settings)

except (FileNotFoundError, json.JSONDecodeError):
    configuration_baseline = default_settings
    print('Could not read existing settings.json. Generating settings.json for '
          'Transmission from environment and defaults', default_settings)

    with open(configuration_baseline, 'r') as input_file:
        settings_dict = json.load(input_file)


def setting_as_env(setting: str) -> str:
    """Get a transmission settings environment variable name."""
    return 'TRANSMISSION_{setting}'.format(
        setting=setting.upper().replace('-', '_'),
    )


# For each setting, check if an environment variable is set to override it
for setting in settings_dict:
    setting_is_sensitive = setting == "rpc-password"
    setting_env_name = setting_as_env(setting)
    if setting_env_name in os.environ:
        env_value = os.environ.get(setting_env_name)
        env_log_value = "[REDACTED]" if setting_is_sensitive else env_value

        # Coerce env var values to the expected type in settings.json
        if type(settings_dict[setting]) == bool:
            env_value = env_value.lower() == 'true'
        else:
            setting_type = type(settings_dict[setting])
            try:
                env_value = setting_type(env_value)
            except ValueError:
                print(
                    'Could not coerce {setting_env_name} value {env_log_value} to expected type {setting_type}'.format(
                    setting_env_name=setting_env_name,
                    env_log_value=env_log_value,
                    setting_type=setting_type,
                    ),
                )
                raise

        print(
            'Overriding {setting} because {env_name} is set to {value}'.format(
                setting=setting,
                env_name=setting_env_name,
                value=env_log_value,
            ),
        )
        settings_dict[setting] = env_value

# Dump resulting settings to file
with open(transmission_settings, 'w') as fp:
    json.dump(settings_dict, fp)