fix: Validate that reactive and render decorated functions have no required parameters#2200
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schloerke merged 6 commits intoposit-dev:mainfrom Mar 26, 2026
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…quired parameters Add runtime validation to @reactive.calc, @reactive.effect, and @render.* decorators that raises TypeError when the decorated function has required parameters. Reactive and render functions should take no arguments. Closes posit-dev#1819 Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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Mar 26, 2026
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@mvanhorn Thank you (and Claude)! 😅
Request:
- If default parameters are supplied, could we also display a "anti-pattern" warning?
- This will help steer users away from adding parameters that will never be leveraged... Or to wrap their functions in lambda to clearly define the contract.
- In the future, we could turn it into an error. 1 year?
- Add a CHANGELOG.md entry
Add a UserWarning in validate_no_params when functions have parameters with default values, since Shiny never supplies arguments to reactive or render decorated functions. Also add comprehensive unit tests for validate_no_params covering all logic paths (skip_names, var args, error/warning messages) and a CHANGELOG entry. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Add a `stacklevel` keyword argument so callers with deeper call stacks can point warnings at the correct user code frame. Also add a `_shiny_params_validated` flag on validated functions to prevent duplicate warnings when decorators are stacked (e.g., @reactive.poll internally uses @reactive.calc via @functools.wraps). Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
- Pass stacklevel=5 from Calc_.__init__ and Effect_.__init__ so
warnings point to user code, not internal create_calc/create_effect.
- Pass stacklevel=5 from Renderer.__call__ to account for the extra
frame through subclass __init__ -> super().__init__ -> self(_fn).
- Use f"render.{type(self).__name__}" so error/warning messages say
@render.text instead of @text.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Extend validate_no_params coverage to all decorators that expect zero-argument functions: - reactive.poll (in wrapper) - reactive.file_reader (in wrapper) - reactive.event (in decorator) - render.download (in __call__, which overrides Renderer.__call__ and bypasses the base class validation) Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
…edup - Stacklevel tests for all 7 callsites verifying warnings point to the correct user file - Render prefix tests for error and warning messages - fn.__name__ in warning suggestion text - Validation tests (reject/accept/warn) for reactive.event, reactive.poll, reactive.file_reader, and render.download - Stacked decorator tests verifying exactly one warning when @calc/@effect is combined with @event, and when @poll/@file_reader internally chains through @reactive.calc Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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** Implementing my requests. Yay Claude |
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Thank you, @mvanhorn ! |
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Summary
Adds runtime validation to
@reactive.calc,@reactive.effect, and@render.*decorators to raise a clearTypeErrorwhen the decorated function has required parameters. These decorators expect zero-argument functions.Why this matters
Users who forget to remove parameters from their decorated functions get confusing runtime errors later (e.g., missing argument errors during reactive flush). This validation catches the mistake at decoration time with a clear message like:
@reactive.calc expected a function with no required parameters, but my_func() has required parameter(s): x, y.Per #1819, this is a common mistake for users who don't enable type checking.
Changes
shiny/_utils.py: Addedvalidate_no_params(fn, decorator_name)utility usinginspect.signature(). Accepts functions with*args,**kwargs, or default-valued parameters.shiny/reactive/_reactives.py: Added validation toCalc_.__init__andEffect_.__init__shiny/render/renderer/_renderer.py: Added validation toRenderer.__call__tests/pytest/test_reactives.py: 6 tests covering calc/effect with required params, no params, and default paramstests/pytest/test_renderer.py: 3 tests covering render.text with the same patternsTesting
Added 9 unit tests. Run with
pytest tests/pytest/test_reactives.py tests/pytest/test_renderer.py.Fixes #1819
This contribution was developed with AI assistance (Claude Code).