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CLAUDE.md

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+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Project Overview
+
+Param is a zero-dependency Python library providing declarative, validated parameters for Python classes and a reactive programming API (`rx`). It requires Python >= 3.10. Param is the backbone for Panel and HoloViews in the HoloViz ecosystem.
+
+## Development Setup
+
+This project uses **pixi** for environment and task management.
+
+```bash
+pixi run setup-dev    # Editable install + pre-commit hooks
+```
+
+## Common Commands
+
+```bash
+# Testing
+pixi run test-unit                    # Run all unit tests
+pixi run -e test-312 test-unit        # Run tests in a specific Python environment
+pytest tests/testparameterizedobject.py  # Run a single test file
+pytest tests/testparameterizedobject.py::TestParameterized::test_name  # Run a single test
+pixi run test-example                 # Run notebook/example tests
+
+# Linting
+pixi run lint                         # Run pre-commit hooks (ruff + misc checks)
+pixi run lint-install                 # Install pre-commit hooks
+
+# Docs
+pixi run -e docs docs-build           # Build docs with sphinx
+
+# Build
+pixi run -e build build-pip            # Build wheel/sdist
+pixi run -e build build-conda          # Build conda package
+```
+
+## Test Configuration
+
+- pytest with `filterwarnings = ["error"]` and `xfail_strict = true` — all warnings are errors and xfail tests must actually fail
+- `conftest.py` sets `PARAM_PARAMETER_SIGNATURE=1` and `param.parameterized.warnings_as_exceptions = True`
+- `asyncio_mode = "auto"` — async tests run automatically without `@pytest.mark.asyncio`
+- Test environments: `test-310` through `test-314`, `test-314t` (free-threading), `test-core` (minimal deps)
+
+## Code Architecture
+
+The `param/` package has a small number of tightly coupled modules:
+
+- **`parameterized.py`** — Core foundation. Defines `Parameter` (the descriptor base class), `Parameterized` (the base class for all param-using classes), and `ParameterizedMetaclass`. Contains the watch/callback event system, messaging/logging infrastructure, and `script_repr` serialization.
+
+- **`parameters.py`** — All concrete Parameter types (`Number`, `Integer`, `String`, `Boolean`, `List`, `Dict`, `Selector`, `ClassSelector`, `Path`, `Date`, `DataFrame`, `Array`, etc.). Each Parameter type implements validation, serialization, and type-specific behavior. Depends on `parameterized.py`.
+
+- **`reactive.py`** — The `rx` reactive expression framework and `bind()` function. Provides lazy evaluation, operator overloading, and dynamic dependency tracking for building reactive pipelines. Depends on `parameterized.py`.
+
+- **`depends.py`** — The `@depends` decorator for declaring method dependencies on parameters.
+
+- **`_utils.py`** — Internal utilities: type checking, validation helpers, `descendents`/`concrete_descendents`, `exceptions_summarized`.
+
+- **`serializer.py`** — Parameter serialization/deserialization support.
+
+- **`ipython.py`** — IPython/Jupyter integration.
+
+- **`__init__.py`** — Re-exports the full public API. The public API surface is entirely defined here.
+
+**`numbergen/`** — Separate package for number generation utilities, bundled with param.
+
+### Key Design Patterns
+
+- **Descriptor protocol**: `Parameter` subclasses are Python descriptors (`__get__`/`__set__`/`__delete__`) on `Parameterized` classes. They store per-instance values and enforce validation.
+- **Metaclass**: `ParameterizedMetaclass` handles parameter inheritance, merging parameter declarations from parent classes, and building the class parameter namespace.
+- **Observer/watch pattern**: `obj.param.watch(callback, ['param_name'])` registers callbacks that fire on parameter changes. This is the core of param's reactivity.
+
+## Linting
+
+Ruff with rules `D, W, E, F` (numpy docstring convention). Many rules are intentionally ignored — see `pyproject.toml [tool.ruff.lint]` for the ignore list. Pre-commit also runs `check-builtin-literals`, `check-case-conflict`, `end-of-file-fixer`, `trailing-whitespace`, and notebook cleaning.