Python's standard library has modules for configuration file parsing (configparser), environment variable reading (os.environ), and command-line argument parsing (argparse). I want to write a program that does all those, and also:
Has a cascade of option values:
Allows one or more configuration file locations specified on the command line with e.g. --config-file foo.conf, and reads that (either instead of, or additional to, the usual configuration file). This must still obey the above cascade.
Allows option definitions in a single place to determine the parsing behaviour for configuration files and the command line.
Unifies the parsed options into a single collection of option values for the rest of the program to access without caring where they came from.
Everything I need is apparently in the Python standard library, but they don't work together smoothly.
How can I achieve this with minimum deviation from the Python standard library?
It seems the standard library doesn't address this, leaving each programmer to cobble configparser and argparse and os.environ all together in clunky ways.