---
id: param
version: "2.4.1"
license: BSD-3-Clause
license_treatment: permissive
maintenance: active
---

# param — Declarative parameters for robust Python classes and a rich API for reactive programming

License: permissive · Maintenance: active · Downloads: 2.8M/mo

## What it is and what it does

Param is a zero-dependency library for building classes with rich, validated attributes and reactive programming capabilities. You define attributes as Parameter objects—Integer, String, Selector, Event, and many others—each with metadata like bounds, default values, documentation, and nullability constraints. Param enforces these constraints at runtime, raising errors when invalid values are assigned. The library also provides a reactive API: you can declare that methods should automatically run when specific attributes change, enabling data-driven workflows where UI updates, computations, or side effects trigger automatically.

Param serves as the foundation for HoloViz's Panel and HoloViews libraries, which rely on its combination of declarative attributes and reactive bindings to power interactive visualizations and responsive applications. It is conceptually similar to Pydantic or dataclasses but emphasizes class-level attribute configuration, parameter inheritance across subclasses, and automatic reactivity rather than just input validation.

Use it for:

- Build configuration classes with validated, documented parameters that can be introspected by tools to generate UIs or CLIs automatically.
- Create interactive applications where method calls or state updates trigger automatically when watched parameters change.
- Define hierarchies of classes where subclasses inherit and selectively override parameter metadata from parent classes.
- Implement data pipelines where reactive dependencies ensure downstream computations stay synchronized with upstream changes.
- Construct scientific or data-analysis workflows with rich parameter metadata for reproducibility and documentation.

## Worth the install?

AI-flagged interpretation of the facts above — verify before relying.

Param provides declarative class attributes with runtime validation and a reactive programming API for automatic updates when attribute values change.

Yes. Param is production-stable, actively maintained, has zero runtime dependencies, and solves a real problem: combining declarative attribute validation with reactive programming. Install it if you need runtime parameter validation with class-level configuration, or if you're building interactive systems where automatic synchronization matters. The BSD-3-Clause license poses no barrier.

## Install

pip install param
uv add param
poetry add param

## Installing param

Before you install: Installation is straightforward with zero runtime dependencies and a pure Python wheel. The package is actively maintained with recent releases and a stable codebase.

License in practice: BSD-3-Clause is a permissive license allowing commercial and private use with minimal restrictions, making it suitable for most projects.

Quickstart:

import param

class MyClass(param.Parameterized):
    value = param.Integer(default=5, bounds=(0, 10))
    name = param.String(default='test')

obj = MyClass(value=7)
print(obj.param.values())

Requires Python 3.10 or later.

Verify before relying:

- Whether reactive watchers scale well with hundreds or thousands of interdependent parameters.
- Performance characteristics when used in high-frequency update scenarios.
- Integration patterns with async/await code.

## Package facts

- License: BSD-3-Clause (permissive)
- Python support: supports_current
- Install friction: low
- Maintenance: active
- Downloads: 2.8M/month (top 5,000 on PyPI)
- Known vulnerabilities: none known

## Tags

declarative class attributes, runtime type validation, reactive programming, parameter metadata, automatic attribute updates, data validation framework, class attribute binding, validation, reactive-programming, declarative-api

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