Add type annotation to Parameter.rx property

[Copilot]

The Parameter.rx property was not type annotated, preventing IDEs from providing autocomplete and type information for reactive expressions.

import param

class P(param.Parameterized):
    a = param.Number()

p = P(a=1)
rx_expression = p.param.a.rx()  # IDE cannot infer type

Changes

• Added -> 'rx' return type annotation to Parameter.rx property
• Imported rx under TYPE_CHECKING guard to avoid circular import with reactive module

With this change, IDEs will correctly identify p.param.a.rx as rx, enabling autocomplete for reactive operations like .pipe(), .map(), .and_(), etc., matching the type expectations of basedpyright and other type checkers.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Type annotate Parameter.rx property</issue_title>
<issue_description>VS Code and other editors relying on static typing does not understand what rx_expression below is because Parameter.rx property is not type annotated:

import param
class P(param.Parameterized):
    a = param.Number()
p = P(a=1)
rx_expression = p.param.a.rx()

This is a huge bummer on developer experience as you don't get any help from there.

Please type annotate it. It probably requires some reordering of the code to avoid circular imports.</issue_description>

<agent_instructions>Please start out by understanding the problem very well. Then plan how to solve it. Then solve it. Include relevant pytests. Make sure all existing tests and pre-commit tests succeed. Keep the change as simple if possible. Making it easy to review.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Type annotate Parameter.rx property #1011

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.22%. Comparing base (833c8f0) to head (f85eef5).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1112   +/-   ##
=======================================
  Coverage   89.22%   89.22%           
=======================================
  Files           9        9           
  Lines        4716     4716           
=======================================
  Hits         4208     4208           
  Misses        508      508           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[MarcSkovMadsen]

Probably works for me

I'm in doubt whether rx is the correct type annotation. But its the one basedpyright suggests and wants.

[MarcSkovMadsen]

studying reactive_ops I can see it actually returns rx class defined in reactive.py. Please update the type annotation. Fix any failing pytest or pre-commit tests.

[Copilot]

studying reactive_ops I can see it actually returns rx class defined in reactive.py. Please update the type annotation. Fix any failing pytest or pre-commit tests.

Updated type annotation from reactive_ops to rx in commit f85eef5. All tests and pre-commit checks pass.