Click is actually doing what it is supposed to do. You created a group cli and added the single command read_csv. So click needs to know the command to know what to invoke because there could possibly be more and even nested commands. You could create a standalone click command read_csvand register it in your setup.py file like so:

# app.py
import click

# as a standalone command
@click.command()
@click.option('--readpower', type=str, default="")
def read_csv(readpower):
    click.echo("doing stuff...")
    if readpower:
        click.echo("reading {}...".format(readpower))

@click.group()
def cli():
    pass

@cli.command()
def do_stuff():
    click.echo("doing stuff...")

# as a subcommand in a group
@cli.command()
@click.option('--readpower', type=str, default="")
def read_csv(readpower):
    click.echo("doing something...")
    if readpower:
        click.echo("reading {}...".format(readpower))

And the setup:

# setup.py
from setuptools import setup

setup(
    name='app',
    version='0.1',
    py_modules=['app'],
    install_requires=['Click'],
    entry_points='''
        [console_scripts]
        read_csv=app:read_csv
        cli=app:cli
    ''',
)

So you can set multiple entry points to your application and mix multiple standalone commands with grouped (and even possibly nested) commands.

This use-case is described in the docs.

Sometimes, it might be interesting to invoke one command from another command. This is a pattern that is generally discouraged with Click, but possible nonetheless. For this, you can use the Context.invoke() or Context.forward() methods.

cli = click.Group()

@cli.command()
@click.option('--count', default=1)
def test(count):
    click.echo('Count: %d' % count)

@cli.command()
@click.option('--count', default=1)
@click.pass_context
def dist(ctx, count):
    ctx.forward(test)
    ctx.invoke(test, count=42)

They work similarly, but the difference is that Context.invoke() merely invokes another command with the arguments you provide as a caller, whereas Context.forward() fills in the arguments from the current command.