This will display help for scripts with just one command but that are called with no commands.

example.py

#!/usr/bin/env python
import typer

app = typer.Typer()

@app.callback(invoke_without_command=True)
def default(ctx: typer.Context):
  typer.echo(ctx.get_help())

  
@app.command()
def trigger():
    """calls trigger"""
    print("work done here")


if __name__ == "__main__":
    app()