pyclig

A Python Click wrapper with Command Line Interface Guidelines (CLIG) paradigms.

click owns parsing, help, shell completion & the test runner. pyclig adds the conventions on top: STDOUT for output & STDERR for messages, exit codes, error rendering, logging, output primitives, run flags & standard flags. Options, arguments & types are plain click.

CLIG is treated as a contract: a CLI conforms with no application code wherever pyclig can do it alone, gets one obvious interface for each rule that needs application input, and pyclig.testing.assert_conforms reports the rest by rule ID. docs/clig-coverage.md has the rule-by-rule status and docs/roadmap.md what each release adds. Pin pyclig~=0.3.0 while it is pre-1.0: a minor release may break, and ~=0.3 would admit 0.4.

Usage

import click
import pyclig

from dkais import exceptions


@pyclig.group(
    "dkais",
    short_help="DMA AIS data manager",
    examples=["dkais list --start 2026-10"],
    errors=(exceptions.Error,),
    epilog="For more information see https://github.com/andyrids/dkais.",
)
def cli() -> None:
    """Download DMA AIS archives and convert them to Parquet."""


@cli.command("list", examples=["dkais list --start 2026-10 --json"])
@pyclig.options.json_output()
@pyclig.options.plain()
@click.option("--start", metavar="DATE", help="The range start.")
@pyclig.pass_context
def list_(ctx: pyclig.Context, start: str | None, json_output: bool) -> int:
    """List bucket archives & local state."""
    if ctx.format == "human":
        ctx.out.print(f"from {start}")
    else:
        click.echo(f"start\t{start}")
    return pyclig.ExitCode.OK


if __name__ == "__main__":
    cli()

Point the entry point at the group: [project.scripts] dkais = "dkais.__main__:cli". Always name the group - the name is the program name, also under python -m, and sets the shell completion variable (_DKAIS_COMPLETE). A single-command tool uses pyclig.command() instead, and gets everything a command of a group does.

Commands take examples= too: their --help leads with them under EXAMPLES:.

What every command gets

ctx.obj is left to the application.

Run flags

Run flags are about how a run behaves, never what a command does, so they are on every command and act as soon as they are parsed - -d applies even when a later option is invalid:

A command's own option with the same spelling wins: copy -d /tmp reaches the app, and --debug still works there. The root's --help lists the run flags under GLOBAL OPTIONS:; a subcommand's names them on one line.

An application declares its own with cls=pyclig.GlobalOption on the command whose subtree accepts it, usually the root group. The callback is a plain click callback. It runs only when the flag was given - on the command line or through envvar= - and may run once per level of a nested command line that sees it (each level reads the environment), so it must be idempotent:

def _trace(ctx: click.Context, param: click.Parameter, value: bool) -> None:
    ctx.meta["trace"] = True


@pyclig.group("dkais")
@click.option(
    "--trace", cls=pyclig.GlobalOption, callback=_trace, help="Trace the run."
)
def cli() -> None: ...

Errors & exit codes

Commands return an exit code (or None for 0). Anything else is mapped by the runner, with messages on STDERR:

pyclig.ExitCode names the codes: OK, FAILURE, USAGE, INTERNAL & INTERRUPT. The bug report hint names the distribution's Issues project URL. Messages are never parsed as Rich markup: user input such as [/b] or :warning: prints as typed.

hint & exit_code are read by attribute, so the application's exceptions need no pyclig import - a class attribute on the exception type is enough:

class ToolNotFoundError(Error):
    hint = "Run the hook in the environment pkgdx is installed in, e.g. via `uv run`"

A command may catch Ctrl-C or its own errors to print a tailored message first - a returned code passes through untouched. Wrap third-party failures (e.g. network errors) in an errors= type, so users see a message instead of a crash. errors= goes on the root; add_command rejects it on a subcommand, where the runner would ignore it.

Output

ctx.out (STDOUT) & ctx.err (STDERR) are Rich consoles created for each run, with markup, emoji codes & highlighting off. Pass ctx.err to any Progress or Status, and gate animation on ctx.err.is_interactive. Each console tells the truth about its own stream: is_terminal is the real TTY check, so FORCE_COLOR means colour only - a piped console keeps its colour but stays one line per message, never wraps and never animates. NO_COLOR, TERM=dumb & --no-color drop colour; TTY_COMPATIBLE=1 makes a pipe a terminal.

ctx.format says what a command producing records should print: human on a terminal, otherwise plain; the per-command flags --plain (pyclig.options.plain()) & --json (pyclig.options.json_output()) override it, --json winning in any order. Only commands that produce records take the flags, so help never advertises a flag a command ignores.

Arguments after the console are keyword-only. Control characters, including terminal escapes, are dropped from messages. Print machine-readable output - JSON, tab-separated rows - with click.echo, which neither wraps nor expands tabs. Use rich.text.Text with a style for styled output, and TextColumn(markup=False) in a Rich Progress.

Standard flags

pyclig.options holds the flags with standard names, each taking a help= override:

Prompts & confirmation

A prompt needs a terminal on STDIN to answer it and on STDERR to see it, and --no-input refuses every prompt; ctx.can_prompt is that test. pyclig.confirm() asks before a large or destructive action, and --force skips the question:

@cli.command()
@pyclig.options.force()
@pyclig.pass_context
def purge(ctx: pyclig.Context, force: bool) -> None:
    """Delete every local archive."""
    pyclig.confirm(ctx, message="Delete every local archive?", force=force)

A script cannot answer the prompt, so the run fails rather than read an answer from a pipe, and with 2>log it fails rather than hide the question in the log. Call confirm before starting a Progress or Status. A click option with prompt= follows the same rule: it prompts on STDERR, and when the run cannot prompt it takes its default, or fails naming the option when it has none - so --no-input never lies.

Logging

Log through the package logger - logging.getLogger(__name__) in any module of the package that defines the group. Libraries add only a NullHandler, in the package __init__.py:

logging.getLogger(__name__).addHandler(logging.NullHandler())

During a run, warnings & errors print as warning: / error: notices on ctx.err. With -d, DEBUG records, times & call sites are added - laid out by a RichHandler when STDERR is a terminal. Otherwise each record is one line that never wraps, with any traceback after it in plain text:

[2026-10-31T09:14:03+0000] DEBUG    [convert] converting 'nope'  types.py:47

The logger's handlers, level & propagation are restored when the run ends.

Testing

from pyclig.testing import assert_conforms, invoke


def test_list_rows_when_piped() -> None:
    """Assert `list` prints tab-separated rows when STDOUT is a pipe."""
    result = invoke(cli, ["list"], out_tty=False, err_tty=True)
    assert result.exit_code == 0
    assert "\t" in result.stdout


def test_cli_conforms() -> None:
    """Assert the CLI follows every guideline pyclig can check."""
    assert_conforms(cli, ignore={"ARG-16": "`--token` also reads a file"})

invoke runs any pyclig.Command or Group and returns click's Result, with stdout & stderr apart. out_tty, err_tty, color & width set up each console; env sets (or, with None, unsets) variables. Colour & terminal variables from the developer's shell are cleared. Never patch Rich private attributes. in_tty=True & err_tty=True let pyclig.confirm & prompt= options prompt and read their answer from input; click echoes that answer to STDERR after the prompt (... [y/N]: y).

assert_conforms walks the CLI and reports deviations by the rule IDs of docs/clig-coverage.md: flags without a long form or help text, standard short flags with another meaning, secret-looking flags & environment variables, a root without examples, plain click children, and help or unknown-flag probes that misuse exit codes or streams. It never runs a command body. check returns the Findings instead of asserting; ignore needs a reason per rule.

Parser-less entry points

pyclig.run() gives a pre-commit hook shim, which leaves sys.argv to the tool it wraps, the same exit codes, error rendering & logging. It enters a pyclig.Context, so the function may be a pyclig.pass_context function using ctx.err:

def ruff_format() -> NoReturn:
    pyclig.run(
        lambda: core_ruff_format(sys.argv[1:]),
        errors=(exceptions.Error,),
    )

Migrating from argparse

• Each subcommand becomes a @cli.command(); shared options become one decorator stacking click.options. Commands are listed in the order they are added.
• click shows a command's docstring as its help: put \f after the help paragraph, so the Args: & Returns: sections stay out of --help. Pass help= for a description that is not an imperative docstring line.
• Custom argument types become click.ParamType[T] subclasses calling self.fail().
• parser.error() becomes raise click.UsageError(...) (or ctx.fail()).
• A type= function checking a minimum becomes click.IntRange(min=1).
• An environment variable default becomes envvar=, with show_envvar=True so a missing value names it.
• Module-level consoles become ctx.out / ctx.err, passed to helpers as arguments.
• argparse accepted unambiguous option prefixes (--st for --start); click does not.