Your personal operating system for career transition.

Multi-turn conversation builder with JSON serialization

A conversation is a list of messages that must be assembled correctly and serialized for persistence. This exercise builds the foundation for multi-turn state management.

What you are building

Implement a Conversation class that manages a list of message dicts:

Methods:

• add_system(content: str) -> None — Add (or replace) the system message. The system message is always first.
• add_user(content: str) -> None — Append a user message.
• add_assistant(content: str) -> None — Append an assistant message.
• to_messages() -> list[dict] — Return the full messages list in order: system (if set), then user/assistant turns.
• to_json() -> str — Serialize to a JSON string.
• from_json(json_str: str) -> "Conversation" (classmethod) — Deserialize from a JSON string.
• token_estimate() -> int — Return a rough token estimate: sum(len(m["content"].split()) * 1.3) for all messages.
• last_role() -> str | None — Return the role of the last message, or None if empty.

Requirements

• A conversation can only have one system message (replacing it overwrites the previous one)
• to_messages() must always put the system message first, regardless of when add_system was called
• Maintain insertion order for non-system messages