Use a subcommand to select a dry-run action

argparse subcommands make the selected operation visible in the command shape, instead of making a dry-run behavior depend only on a boolean flag. This example creates a dry-run subparser with one positional target. Its set_defaults() call stores a callable as action, so parsing the synthetic arguments dry-run cache produces a namespace containing the subcommand name, target, and selected action.

The action returns a preview string; it does not delete a cache or access the filesystem. The assertions verify that parsing selected dry-run, that the callable produced the expected preview, and that the output is exactly would-delete cache. This keeps parsing and dispatch close together while making the dry-run path explicitly testable.

add_subparsers() creates the command group. Passing required=True means parsing without a subcommand is an error; that option requires Python 3.7 or later. set_defaults() can attach arbitrary values to the parsed namespace, but it does not ensure that actions from several subcommands accept compatible arguments. Define and test that interface when adding commands. More generally, a dry run is only a preview: it cannot establish that a later real operation will succeed when permissions, concurrent changes, or external state have changed.

Python added argparse in 3.2. The official add_subparsers() reference documents subcommand creation, and set_defaults() documents namespace defaults.

AI assistance disclosure: This article was drafted with AI assistance and should be adapted to the application's command contract.

import argparse


def preview_delete(target):
    return f"would-delete {target}"


def build_parser():
    parser = argparse.ArgumentParser(prog="cachectl")
    commands = parser.add_subparsers(dest="command", required=True)

    dry_run = commands.add_parser("dry-run")
    dry_run.add_argument("target")
    dry_run.set_defaults(action=preview_delete)
    return parser


arguments = build_parser().parse_args(["dry-run", "cache"])
result = arguments.action(arguments.target)

assert arguments.command == "dry-run"
assert result == "would-delete cache"
print(result)
would-delete cache

Comments

Post a Comment

Popular posts from this blog

Compare Two Text Files in Python Without Modifying Them

Batu Lab Notes · Batu When comparing two versions of a configuration or notes file, “are they the same?” is only the first question. The bytes may be identical, the visible text may be equal despite different line endings, or the content may contain a real change. For small UTF-8 text files, Python’s standard library can show these distinctions without writing to either input. Quick answer For small, stable UTF-8 files, read bounded bytes, compare them first, then decode and produce a bounded unified diff. Decide explicitly whether newline differences matter. Keep both inputs read-only and review the diff for private data before sharing it. Set a limit before reading Loading an input of unknown size directly into memory is a poor default. This example checks both sizes with Path.stat() , then performs a bounded read and applies a 256 KiB limit per file. It rejects directories and other non-regular paths. If reading raises OSError , the result includes the exception ty...
Read more

Seven CSV Quality Checks to Run Before Importing Data

Batu Lab Notes · Batu A CSV file is not ready for import merely because it opens. The wrong delimiter can collapse an entire row into one column. An unexpected encoding can damage non-ASCII characters. A repeated identifier can overwrite an existing record in the destination. A short, read-only check makes these problems visible before data reaches a spreadsheet or application. Quick answer Before importing a CSV, set its encoding and delimiter, validate the exact header and every row width, distinguish empty IDs, duplicate IDs, and duplicate rows, and keep the check read-only. 1. Specify the encoding Python may open a text file with the platform’s default encoding, which can differ across machines. If the expected encoding is UTF-8, encoding="utf-8-sig" is a practical choice: it reads ordinary UTF-8 and skips an optional UTF-8 byte order mark at the start. If decoding fails, verify the source application’s export settings instead of silently trying unrelat...
Read more

Explain why SQLite transactions cannot make an HTTP call atomic

Direct answer A SQLite transaction can atomically commit database statements handled by that connection. It cannot include an HTTP service in the same commit protocol. The example commits an outbox row first, then simulates a network failure. The durable row remains pending , which is exactly the information a later delivery worker needs instead of pretending both actions succeeded or failed together. The edge case is failure after the database commit but before remote acknowledgement. Retrying then requires an idempotency design owned by the remote endpoint or a stable request key; it is not supplied by SQLite’s COMMIT . Conversely, sending the request before storing a durable intent can leave no local evidence when the process stops. This demonstration intentionally does not make an HTTP request, implement an outbox dispatcher, or prove delivery exactly once. It shows the boundary that must be designed around. Use transactions to protect the local state transition, and test remote...
Read more