configuration_parameters_precedence.py

## This script demonstrates how input parameters can be specified in multiple ways and 
## a recommended common approach and order of reading, and potentially overwriting these 
## parameters if multiple input sources provide a different value for the same parameter.

##  configuration parameter value set method precedence order:
##  cli args override everything 
##  env vars take presedence over a config file
##  If none of the three are supplied it is nice to fallback to hardcoded sane default values if applicable

##  Lower number configuration method overrides higher number

##  1 cli args
##  2 env vars
##  3 config file
##  4 sane default values hardcoded in application


## in some software, 2 and 3 are swapped. Check!


import argparse
import os
import configparser
import subprocess



# 4
# Set the default value for the variable
value = "default value declared in script"

# 3
# Read the value from a config file to use instead
config = configparser.ConfigParser()
config_file = "parameter_precedence_example_config.toml"
config.read(config_file)
if config.has_option('SETTINGS', 'value'):
    value = config['SETTINGS']['value']
    
# 2
# Check if the environment variable exists and override the value
if "ENV_VAR_VALUE" in os.environ:
    value = os.environ["ENV_VAR_VALUE"]

# 1
# If an optional command-line argument is provided, override the value
parser = argparse.ArgumentParser()
parser.add_argument("--arg", help="Override the default value")
args = parser.parse_args()

if args.arg:
    value = args.arg

# Print the variable with the final value


print("\033[0;35m" + F"{value = }" + "\033[0m")
#print(F"{value = }")
print()


if not os.getenv('CONF_DEMO_FIRST_RUN'):

    commands = [
        "echo -en \"[SETTINGS]\\nvalue = value read from config file\" > parameter_precedence_example_config.toml &&\\\npython configuration_parameters_precedence.py",
        "export ENV_VAR_VALUE=\"value set from env var\" &&\\\npython configuration_parameters_precedence.py",
        "python configuration_parameters_precedence.py --arg \"value overridden with cli arg\"",
        "rm ./parameter_precedence_example_config.toml"
    ]


    # we only want to let this script execute itself a few times, but not recursively
    os.environ['CONF_DEMO_FIRST_RUN'] = "1"

    for command in commands:
        print(command) # and print the command output in magenta ANSI color
        subprocess.run(command, shell=True, executable='/bin/bash') # needs bash instead of sh to use -e flag of echo command



#    print("echo -n \"[SETTINGS]\\nvalue = value read from config file\" > parameter_precedence_example_config.toml")
#    print("python configuration_parameters_precedence.py")
#    subprocess.run("echo -n \"[SETTINGS]\\nvalue = value read from config file\n\" > parameter_precedence_example_config.toml && python configuration_parameters_precedence.py", shell=True)

#    print("export ENV_VAR_VALUE=\"value set from env var\"")
#    print("python configuration_parameters_precedence.py")
#    subprocess.run("export ENV_VAR_VALUE=\"value set from env var\" && python configuration_parameters_precedence.py", shell=True)

#    print("python configuration_parameters_precedence.py --arg \"value overriden with cli arg\"")
#    subprocess.run("python configuration_parameters_precedence.py --arg \"value overriden with cli arg\"", shell=True)


#    subprocess.run("rm ./parameter_precedence_example_config.toml", shell=True)




# parameter_precedence_example_config.toml
# [SETTINGS]
# value = "value read from config file"

# It is also possible, and arguably better, to write the config file with the configparser library object also, or with python

#config_file_text = """
#[SETTINGS]
#value = "value read from config file"
#"""

#with open("./parameter_precedence_example_config.toml", 'w') as file:
#    file.write(config_file_text)