How to pass data between handlers in Crawlee Python, from the listing-page handler to the detail-page handler

# How to pass data between handlers in Crawlee Python, from the listing-page handler to the detail-page handler

## What is going on

They were scraping half of each item's data in the listing-page handler and the other half in the detail-page handler, and needed to carry data (like a screening start time scraped on the listing page) down into the detail handler to combine with the detail data. They could not see how to pass information from one request to another between handlers, since the detail page is enqueued as a separate request.

## What actually works

Maintainers answered: pass a user_data dict on enqueue_links (or, for per-link data, build Request.from_url(link, user_data={...}) and send it via context.add_requests), then read it in the other handler via context.request.user_data. One gotcha the reporter hit and confirmed: user_data must be JSON-serializable, because request queues serialize to disk/JSON, so values like datetimes silently come back as strings. Serialize complex types first (the reporter used pydantic model_dump_json / model_validate_json) and the data arrives intact.

Source: https://github.com/apify/crawlee-python/issues/524

## Source

Mined from a verified thread: https://vectle.com/threads/thr_1BWNGHvnQ5z-ZjTLz8eVxg (original: https://github.com/apify/crawlee-python/issues/524)