Learn Configuration Precedence With Empty, Missing, and Invalid Values

Configuration bugs often hide inside one question: does an empty value mean “unset,” “zero,” or “invalid”?

Build a resolver with four layers:

default < config file < environment < command line

Do not use truthiness to choose a value. 0, false, and "" are different inputs with different validation rules.

from dataclasses import dataclass

@dataclass
class Resolved:
    value: int
    source: str

def parse_port(raw, source):
    if raw is None:
        return None
    if raw == "":
        raise ValueError(f"empty port from {source}")
    value = int(raw)
    if not 0 <= value <= 65535:
        raise ValueError(f"invalid port from {source}")
    return Resolved(value, source)

Resolve from highest to lowest precedence, stopping at the first value that is present, then validate it. Do not silently fall through when a high-priority source is malformed; that hides deployment mistakes.

Table-driven tests make the contract visible:

cases = [
    ({}, 8080, "default"),
    ({"PORT": "0"}, 0, "environment"),
    ({"PORT": ""}, ValueError, "environment"),
    ({"PORT": "abc"}, ValueError, "environment"),
]

Add --explain-config so users can inspect the winning source without exposing secrets:

port=8080 source=config-file
host=127.0.0.1 source=default
api_key=<redacted> source=environment

Python’s argparse documentation covers command-line defaults and parsing. The learning goal here is to keep parsing, precedence, validation, and explanation separate so each can be tested.

Extension exercise: add a --no-color boolean and distinguish an omitted flag from an explicit false value. Then serialize the resolved, non-secret settings in a startup test and compare them with an approved snapshot. That small change catches many production configuration mistakes before a framework obscures them.