Typer CLI app with lazy loading of subcommands

app.py

from __future__ import annotations

import importlib
from typing import TYPE_CHECKING

import typer
from typer.core import TyperGroup

if TYPE_CHECKING:
    from click import Context
    from click.core import Command

LAZY_SUBCOMMANDS = {
    "db": "my_package.cli.commands.db_command",
    "www": "my_package.cli.commands.www_command",
}


class LazyGroup(TyperGroup):
    """A TyperGroup that allows for lazy loading of subcommands."""

    def __init__(
        self,
        *args,
        lazy_subcommands: dict[str, str] | None = None,
        **kwargs,
    ) -> None:
        """Initialize the LazyGroup."""
        super().__init__(*args, **kwargs)
        self.lazy_subcommands = lazy_subcommands or LAZY_SUBCOMMANDS

    def list_commands(self, ctx: Context) -> list[str]:
        base = super().list_commands(ctx)
        lazy = sorted(self.lazy_subcommands.keys())
        return base + lazy

    def get_command(self, ctx: Context, cmd_name: str) -> Command | None:
        if cmd_name in self.lazy_subcommands:
            cmd = self._lazy_load(cmd_name)
            cmd.info.name = cmd_name
            return typer.main.get_command(cmd)
        return super().get_command(ctx, cmd_name)

    def _lazy_load(self, cmd_name: str) -> typer.Typer:
        module_name = self.lazy_subcommands[cmd_name]
        module = importlib.import_module(module_name)
        app_object = getattr(module, "app", None)  # looks for a Typer instance named 'app'
        if not app_object:
            raise ValueError(
                f"Lazy loading of commands from module {module_name} failed."
            )
        return app_object

app = typer.Typer(no_args_is_help=True)

@app.callback(cls=LazyGroup)
def main() -> None:
    """My Package CLI."""

db_command.py

from __future__ import annotations

import typer

app = typer.Typer()


@app.command()
def upgrade() -> None:
    """Upgrade database schema."""


@app.callback()
def main() -> None:
    """Utilities to manage database."""

www_command.py

from __future__ import annotations

import typer

app = typer.Typer()


@app.command()
def start() -> None:
    """Start web server."""


@app.callback()
def main() -> None:
    """Utilities to manage web server."""