param

Param

Correct patterns and common pitfalls for Param — the reactive parameter library that underpins Panel, HoloViews, and the HoloViz ecosystem.

Contents

• Lookup — site search
• Parameterized Classes
• Reactive Dependencies (@param.depends)
• Dependent Parameters
• Parameter Types
• Reactive Expressions (rx)
• .watch() vs @param.depends vs .link()
• allow_refs
• Custom Parameter Types

Lookup

Web-search https://param.holoviz.org/search.html?q=<topic> for anything not covered below; the Reactive Expressions guide is the deepest reference for rx.

Parameterized Classes

• Add # pyright: reportAssignmentType=false at the top — Param's descriptors conflict with static type checkers.
• Add type annotations (target: str = param.String(...)) for IDE autocomplete — Param doesn't enforce them at runtime.
• Never use name as a parameter. Param won't stop you, but name is constant — it can only be set in the constructor, and obj.name = "x" afterwards raises TypeError: Constant parameter 'name' cannot be modified. You also lose the auto-generated unique instance name (DataConfig00042) that Param uses in reprs and error messages.
• Never name a param in UPPERCASE_SNAKE_CASE. A param is reactive, settable, watchable instance state, so an all-caps name misreads as a frozen module constant — self.MAX_ROWS = 50 looks illegal and isn't. It also leaks into UIs: Param's default label formatter only upper-cases the first character (pname[:1].upper() + pname[1:]) rather than title-casing, so MAX_ROWS becomes the shouty widget label "MAX ROWS" while max_rows becomes "Max rows". Use lowercase_snake_case for params and keep genuine constants at module level.
• self.param.param_key is the Parameter object; self.param_key is the current value. Use self.param.param_key with .from_param() and pane constructors.