Add python-skill and improve simulation mechanics skill (#111)

* Add python-skill and improve simulation mechanics skill

- Add python-skill: uv + Python 3.13 standards, venv conflict resolution
- Expand simulation-mechanics-skill with correct import paths, venv setup,
  UK dataset loading (EFRS paths, ensure_datasets pattern), confirmed
  household variable reference (tenure_type values, domestic_energy_consumption),
  and direct data analysis example for decile/groupby/percentile work
- Register python-skill in country-models, analysis-tools, data-science, complete plugins

* Clarify ensure_datasets returns a dict and document key format

* Fix dataset loading: datasets is an exported module, not a pre-built dict

* Add US dataset loading pattern (same as UK)

* Simplify dataset loading: just import datasets dict, delete ./data to force regen

* Strengthen skill descriptions with ALWAYS LOAD and dense trigger phrases

* Replace partial variable list with pattern for inspecting variables at runtime

Author: nwoodruff-co

.claude-plugin/marketplace.json

@@ -88,6 +88,7 @@
         "./skills/technical-patterns/policyengine-code-style-skill",
         "./skills/technical-patterns/policyengine-code-organization-skill",
         "./skills/technical-patterns/policyengine-reform-patterns-skill",
+        "./skills/technical-patterns/python-skill",
         "./skills/documentation/policyengine-standards-skill",
         "./skills/documentation/policyengine-writing-skill"
       ]
@@ -183,6 +184,7 @@
         "./skills/domain-knowledge/policyengine-uk-skill",
         "./skills/domain-knowledge/policyengine-healthcare-skill",
         "./skills/analysis/policyengine-analysis-skill",
+        "./skills/technical-patterns/python-skill",
         "./skills/analysis/policyengine-district-analysis-skill",
         "./skills/data-science/microdf-skill",
         "./skills/documentation/policyengine-design-skill",
@@ -213,6 +215,7 @@
         "./skills/domain-knowledge/policyengine-us-skill",
         "./skills/domain-knowledge/policyengine-uk-skill",
         "./skills/domain-knowledge/policyengine-healthcare-skill",
+        "./skills/technical-patterns/python-skill",
         "./skills/documentation/policyengine-design-skill",
         "./skills/documentation/policyengine-standards-skill",
         "./skills/documentation/policyengine-writing-skill"
@@ -329,6 +332,7 @@
         "./skills/technical-patterns/policyengine-code-style-skill",
         "./skills/technical-patterns/policyengine-code-organization-skill",
         "./skills/technical-patterns/policyengine-reform-patterns-skill",
+        "./skills/technical-patterns/python-skill",
         "./skills/data-science/microdf-skill",
         "./skills/data-science/microimpute-skill",
         "./skills/data-science/microcalibrate-skill",

changelog.d/python-skill.added.md

@@ -0,0 +1 @@
+Add python-skill: uv + Python 3.13 environment setup and package management standards

changelog.d/simulation-mechanics-improvements.changed.md

@@ -0,0 +1 @@
+Expand policyengine-simulation-mechanics-skill with correct import paths, venv setup, UK dataset loading patterns, EFRS variable reference, and direct data analysis example; update python-skill with venv conflict resolution

skills/technical-patterns/python-skill/SKILL.md

@@ -0,0 +1,157 @@
+---
+name: python
+description: |
+  ALWAYS LOAD THIS SKILL before setting up any Python environment or installing packages.
+  Defines the standard: uv, Python 3.13, uv pip install, .venv at project root.
+  Triggers: "set up python", "install python", "create a venv", "virtual environment",
+  "pip install", "install packages", "uv pip", "uv venv", "python version",
+  "VIRTUAL_ENV", "venv conflict", "which python", "activate", "deactivate",
+  "run the script", "run with uv", "uv run", "pyproject.toml", "install dependencies",
+  "install requirements", "install the package", "editable install", "pip install -e".
+---
+
+# Python Environment — PolicyEngine Standard
+
+All PolicyEngine Python work uses **uv** with **Python 3.13** and a local `.venv`.
+
+## Required tools
+
+- [`uv`](https://docs.astral.sh/uv/) — fast Python package and project manager
+- Python 3.13 (installed/managed via uv)
+
+Install uv if not present:
+
+```bash
+curl -LsSf https://astral.sh/uv/install.sh | sh
+```
+
+---
+
+## Creating a new environment
+
+Always create a `.venv` at the project root using Python 3.13:
+
+```bash
+uv venv --python 3.13
+source .venv/bin/activate
+```
+
+On Windows:
+
+```bash
+uv venv --python 3.13
+.venv\Scripts\activate
+```
+
+---
+
+## Installing packages
+
+Always use `uv pip install`, never bare `pip install`:
+
+```bash
+# Install a package
+uv pip install policyengine
+
+# Install from requirements file
+uv pip install -r requirements.txt
+
+# Install the current project in editable mode
+uv pip install -e .
+
+# Install with extras
+uv pip install -e ".[dev]"
+```
+
+---
+
+## Running Python
+
+After activating the venv, run Python normally:
+
+```bash
+python script.py
+python -m pytest
+```
+
+Or run without activating using uv:
+
+```bash
+uv run python script.py
+uv run pytest
+```
+
+---
+
+## Checking the environment
+
+```bash
+# Confirm Python version (should be 3.13.x)
+python --version
+
+# Confirm uv is being used
+which pip   # should point to .venv
+uv pip list
+```
+
+---
+
+## Common patterns
+
+### New repo setup
+
+```bash
+uv venv --python 3.13
+source .venv/bin/activate
+uv pip install -e ".[dev]"
+```
+
+### Adding a dependency to pyproject.toml, then syncing
+
+```bash
+# Edit pyproject.toml to add the dependency, then:
+uv pip install -e .
+```
+
+### Running tests
+
+```bash
+uv run pytest
+# or after activating:
+pytest
+```
+
+---
+
+## Resolving venv conflicts
+
+If you see a warning like:
+```
+warning: `VIRTUAL_ENV=/Users/.../.venv` does not match the project environment path `.venv` and will be ignored
+```
+
+This means a *different* venv is active (e.g. a global one at `~/.venv`). Fix with:
+
+```bash
+# Option 1: Deactivate and use uv run
+deactivate
+uv run python script.py
+
+# Option 2: Use --active flag to force uv to use the active env
+uv run --active python script.py
+
+# Option 3: Activate the correct project venv explicitly
+source /path/to/project/.venv/bin/activate
+python script.py
+```
+
+**Do not** mix venvs between projects. Each project should have its own `.venv` at the repo root.
+
+## Rules
+
+- **Never** use `python -m venv` — always `uv venv`
+- **Never** use bare `pip install` — always `uv pip install`
+- **Always** target Python 3.13 (`--python 3.13`)
+- **Always** create the venv at the project root as `.venv`
+- Use `uv run <cmd>` as an alternative to activating the venv manually
+- If a venv conflict warning appears, use `deactivate` then `uv run`