argparse & configparser

argparse builds CLI interfaces with --flags, subcommands, and help text. configparser reads INI-style files for layered defaults - pair with environment variables for twelve-factor apps.

Recipe

import argparse
 
parser = argparse.ArgumentParser(description="Demo CLI")
parser.add_argument("--verbose", action="store_true")
parser.add_argument("name")
args = parser.parse_args(["Ada", "--verbose"])

When to reach for this:

• Internal CLI tools and dev scripts
• Legacy INI config (setup.cfg, logging.ini)
• Typer/click not yet justified
• Subcommand-based tools (git-style)
• Default config file + CLI override pattern

Working Example

import argparse
import configparser
from pathlib import Path
 
def build_parser() -> argparse.ArgumentParser:
    p = argparse.ArgumentParser()
    p.add_argument("--config", type=Path, default=Path("app.ini"))
    p.add_argument("--port", type=int)
    return p
 
def load_config(path: Path) -> configparser.ConfigParser:
    cfg = configparser.ConfigParser()
    if path.exists():
        cfg.read(path, encoding="utf-8")
    return cfg
 
def resolve_port(args: argparse.Namespace, cfg: configparser.ConfigParser) -> int:
    if args.port is not None:
        return args.port
    if cfg.has_option("server", "port"):
        return cfg.getint("server", "port")
    return 8000
 
args = build_parser().parse_args(["--port", "9000"])
cfg = load_config(args.config)
print(resolve_port(args, cfg))

What this demonstrates:

• CLI overrides beat file config beat defaults
• configparser sections and typed getters (getint)
• Path type for argparse paths
• Encoding explicit on read for portability

Deep Dive

argparse Features

• nargs, choices, metavar
• add_subparsers for subcommands
• ArgumentDefaultsHelpFormatter shows defaults

configparser Notes

• Interpolation %(section)s optional
• ConfigParser does not support full TOML - use tomllib for pyproject

Gotchas

• Boolean flags wrong - type=bool broken. Fix: action="store_true".
• Required optional confusion - document positional vs optional clearly.
• Config silently missing - empty defaults surprise. Fix: validate required sections after read.
• Secrets in INI - world-readable files. Fix: env vars for secrets, not committed config.
• argparse in library import - parse_args on import side effect. Fix: guard in main().

Alternatives

FAQs

Related

• subprocess - CLI tools spawn processes
• pathlib & os - config paths
• Standard Library Overview - map
• Standard Library Best Practices - CLI/config policy

Stack versions: This page was written for Python 3.14.0 (stable 3.14, maintenance 3.13), FastAPI 0.115+, Django 5.2, Flask 3.1, Pydantic 2, PyTorch 2.6+, pandas 2.2+, Polars 1.x, ruff 0.9+, and uv 0.6+.