Extensible chat app with command bots

Implement an in-memory chat app with one shared channel that people and bots post to. Users send text. Some messages are slash commands (/name arg ...), and bots can respond to those. Bots can also react to ordinary messages.

The main goal is extensibility. Adding a new bot type must not require changes to ChatApp.

API

class ChatApp:
    def __init__(self, bots: list[Bot] | None = None) -> None: ...
    def add_bot(self, bot: Bot) -> None: ...
    def send_message(self, user: str, text: str) -> list[str]: ...
    def get_log(self) -> list[str]: ...

• Log entries are rendered as "sender: text". Bot replies use the bot's name as the sender.
• send_message first appends the user's message to the log. Then bots may append replies, in the order the bots were registered. It returns the lines this call added.
• get_log returns the whole channel history in order.
• Bot replies never trigger other bots.
• A command that no bot owns is logged with no reply. Command names are case-insensitive.
• If two bots register the same command, add_bot raises ValueError.

Bots to support

AwayBot

• /away [reason] marks the sender away and replies "<user> is away: <reason>". The default reason is away.
• /back replies "welcome back, <user>", or "<user> was not away" if the sender was not away.
• When a plain (non-command) message mentions @name and that user is away, it replies "<name> is away: <reason>". It posts this once per away user, in the order they were mentioned, and never about the sender.

MeetBot

• /meet @a @b ... replies "meeting #<n> started: <sender>, a, b". Numbers start at 1. The sender comes first, duplicates are removed, and the rest keep the order they were mentioned.
• With no mentions, it replies "usage: /meet @user [@user ...]" and does not use up a meeting number.

TacoBot

• /taco @user [n] gives n tacos (default 1, allowed range 1-5) and replies "<giver> gave <n> taco(s) to <user> (<user> now has <total>)".
• An invalid amount or a missing target gets "usage: /taco @user [1-5]". Giving tacos to yourself gets "you can't give tacos to yourself".
• /tacos replies "tacos: u1=n1, u2=n2", sorted by count descending and then by name. If nobody has tacos yet, it replies "no tacos yet".

Example

app = ChatApp([AwayBot(), MeetBot(), TacoBot()])
app.send_message("dana", "/away dentist")
# ["dana: /away dentist", "AwayBot: dana is away: dentist"]
app.send_message("eli", "@dana can you review?")
# ["eli: @dana can you review?", "AwayBot: dana is away: dentist"]
app.send_message("eli", "/taco @fay 2")
# ["eli: /taco @fay 2", "TacoBot: eli gave 2 taco(s) to fay (fay now has 2)"]
app.send_message("fay", "/meet @eli @eli")
# ["fay: /meet @eli @eli", "MeetBot: meeting #1 started: fay, eli"]

Be ready to explain how a fourth bot would be added, and how the design would change for multiple channels.

Reconstructed from the reported question; details may differ from what interviewers ask.