def bot_says(channel, text, slack_client):
    return slack_client.api_call("chat.postMessage",
                                 channel=channel,
                                 text=text,
                                 as_user=True)


def compose_explanation(url):
    return "If you would like {} to be stored please react to it with a :+1:, \
if you would like it to be ignored use :-1:".format(url)


def ask_confirmation(message, slack_client):
    bot_says(message['channel'],
             compose_explanation(message['url']),
             slack_client)


def compose_url_list(url_cache_list):
    if len(url_cache_list) == 0:
        return "The list is empty"

    list_message = "The list of urls to be confirmed is: \n"

    for index in range(0, len(url_cache_list)):
        extra = "{} - {} \n".format(index, url_cache_list[index]['url'])
        list_message = list_message + extra

    return list_message


def list_cached_urls(url_cache_list, channel, slack_client):
    bot_says(channel,
             compose_url_list(url_cache_list),
             slack_client)

def bot_says(channel, text, slack_client):
    return slack_client.api_call("chat.postMessage",
                                 channel=channel,
                                 text=text,
                                 as_user=True)


def compose_explanation(url):
    return "If you would like {} to be stored please react to it with a :+1:, \
if you would like it to be ignored use :-1:".format(url)


def ask_confirmation(message, slack_client):
    bot_says(message['channel'],
             compose_explanation(message['url']),
             slack_client)


def compose_url_list(url_cache_list):
    if len(url_cache_list) == 0:
        return "The list is empty"

    intro = "The list of urls to be confirmed is: \n"
    options = ["{} - {}".format(i, v['url']) for i, v in enumerate(url_cache_list)]

    return intro + "\n".join(options)


def list_cached_urls(url_cache_list, channel, slack_client):
    bot_says(channel,
             compose_url_list(url_cache_list),
             slack_client)

Change iteration over a collection based on ags suggestion