Argparse & Pika example for /r/learnpython

publish_date.py

"""
Help for /r/learnpython: https://www.reddit.com/r/learnpython/comments/7kilzt/cant_get_my_head_around_using_argparse_in_my/
"""

import argparse
import json
import pika
import sys

CONFIG_FILE = 'some_filename.json'

def get_argument_parser():
    """
    Get an argument parser with the arguments we care about.

    Params:
        None

    Returns:
        argparse.ArgumentParser

    """
    parser = argparse.ArgumentParser()
    parser.add_argument('--date', '-d', required=True)
    return parser


def parse_arguments(args):
    """
    Parse the arguments to the script into a python Namespace object

    Params:
        args: list of arguments to the Python script.

    Returns:
        Parsed arguments.

    """
    parser = get_argument_parser()
    return parser.parse_args(args)


def get_channel(config):
    """
    Get a publish-ready pika channel from a dict of configuration options.

    Params:
        config: dict of configuration

    Returns:
        pika.BlockingConnection: connection to specified RabbitMQ host

    """
    credentials = pika.PlainCredentials(config['user'], config['password'])
    params = pika.ConnectionParams(config['host'], config['port'],
            config['vhost'], credentials)
    
    connection = pika.BlockingConnection(params)
    channel = connection.channel()
    channel.queue_declare(queue=config['queue'])
    return channel

def get_config(config_file):
    """
    Load configuration info from a JSON config file.

    Params:
        config_file: a str file path

    Returns:
        dict: JSON loaded from file

    """
    with open(config_file) as fobj:
        return json.load(fobj)

def main():
    # First, we'll get our parsed arguments.
    # Args will be a Python class with attributes for each argument we added in
    # the get_argument_parser function above, such that args.date will be the
    # date passed in using the --date argument.
    args = parse_arguments(sys.argv[1:])

    # Get our config information
    config = get_config(CONFIG_FILE)

    # Get a channel 
    channel = get_channel(config)
    
    # Create the message body. I'm doing JSON because it's easier.
    body = json.dumps({'date': args.date})
    try:
        # Try to publish the message. If succeeds, exit with a success code.
        channel.basic_publish(exchange='', routing_key='message.test',
                body=body)
        exit(0)
    except:
        # please don't use a catch-all except in production code.
        exit(1)

if __name__ == '__main__':
    main()