Data Science Python Stack

Opinionated stack — one library per job, organized into four tiers plus an orthogonal agent feature:

1. Mandatory — installed at project start, no exceptions.
2. User choice (competing-library jobs) — multiple valid libraries

for the same job; the user picks via AskUserQuestion before any import lands.

1. Optional — install only when the project's task requires it.
2. Transitive — already pulled in by the mandatory tier; do not

install explicitly, but know they're available.

1. Agent feature (orthogonal) — deps that the agent uses

to audit a workspace and to power the editor LSP integration (ipython, pyright), kept out of the production-shape runtime via a manager-specific scope. Install logistics owned by python-env-manager § "Agent feature"; consumed by audit-ml-pipeline and the opencode LSP integration.

Stop conditions — read before naming any library

• No silent pick on a competing-library job. Whenever the stack

offers two or more libraries for the same job (see § "Competing libraries — general rule" and the Tier 2 table), the user picks via AskUserQuestion before any Write that imports the library and before any install command runs. "Already pulled in transitively" / "user said 'quick'" / "the folder has no preference signalled" are not waivers. A silent pick is a Stop-condition violation, full stop.

• No substitute when import fails. When code in this stack needs

a library but import fails, install it; do not rewrite to a non-stack equivalent (see § "Missing dependency"). The most common silent-rewrite path — import skrub fails → rewrite as sklearn.Pipeline, import skore fails → rewrite as crossvalscore — silently undoes the workflow skills' contract.

• **Harness-level "no clarifying questions" hints do NOT waive the

competing-library AskUserQuestion.** The Tier 2 pick is an operating-contract gate, not a clarifying question. The same applies to user urgency phrasing: "quick baseline", "just do it", "go fast", "you pick", "whatever" do NOT resolve a competing- library gate. See § "Free-text resolution" in the general rule below for what does resolve a gate.

• Post-hoc audit — required before ending the turn. Before

declaring the turn complete, verify each competing-library job invoked in this turn has either (a) an AskUserQuestion answer recorded this session, or (b) a matching row in journal/JOURNAL.md Status Workspace decisions. If any competing-library job ran without one of those, surface the non-compliance to the user explicitly as part of your final message — do not hide it.

Forbidden shortcuts (observed in real traces)

Competing libraries — general rule

This is the meta-rule that governs every "user choice" entry in this skill. It applies to the Tier 2 table below and to any new competing-library job added in the future. It also applies inside Tier 3 when two optional libraries cover the same job (e.g. pytorch vs keras as the deep-learning framework).

The rule

Whenever the stack offers two or more libraries for the same job:

1. AskUserQuestion before any import or install. Use the

options listed for the job in the competing-jobs table; do not editorialize the option labels.

1. **Persist the answer in journal/JOURNAL.md Status under

Workspace decisions. This block is immutable until the user explicitly pivots. On future sessions, read Status first**; do not re-ask a recorded decision. The persistence contract lives in iterate-ml-experiment's JOURNAL.md template — the Workspace decisions block is the source of truth for cross- session continuity.

1. No silent default. Even when one option is "free"

(already pulled in transitively) and the other costs an install, never pick silently. The free option becoming the pick is fine; the picking happens via AskUserQuestion.

Free-text resolution

A user message resolves a competing-library gate only if it names one of the listed options for the job. Apply in priority order:

• Exact match (case-insensitive, whitespace-trimmed) to an

option label: resolves the gate. ("use polars", "let's go with pytorch", "pandas please" → resolved.)

• Library named in a free-text intent ("rewrite the loader

in polars", "I want a keras model"): resolves the gate for that job.

• No library named ("make it fast", "you pick", "whatever",

"no preference", "quick baseline"): does NOT resolve. Fall through to the structured AskUserQuestion.

• "You pick" / "no preference" specifically — surface the

default-on-no-preference for the job (from the Tier 2 table) and ask for confirmation. Never silently pick; never skip the confirmation step.

Adding a new contested job

When a new job appears in the stack with two viable libraries, add a row to the Tier 2 competing-jobs table. Every row must name an explicit Default-on-no-preference — rows without one are forbidden, because they re-create the silent-pick loophole this rule exists to close. If a sensible default cannot be named, the job does not belong in the table; surface the gap to the user and pick per-project via a free-form AskUserQuestion.

When to invoke this skill

Two events trigger this skill before any other action:

1. A library import fails in the stack's domain. The answer is

install (see § "Missing dependency" below), never substitute.

1. A library choice has to be made — for tabular data at project

start, or any time code is about to introduce a new dependency (deep learning, model serving, notebooks, …).

In both cases, read the whole SKILL.md before deciding. The tier structure below determines whether a library should already be present, needs a user prompt, or is opt-in — that decision can't be made from a single index entry.

Missing dependency — install, do not substitute

When code in this stack needs a library but import fails, the answer is install it, not substitute. Specifically:

• Surface the missing dependency to the user with the exact install

command. Invoke python-env-manager to detect the project's environment manager (pixi / uv / poetry / hatch / conda / pip+venv) and produce the right install command — don't infer the manager from memory; the project may not use the default. Stop and wait for confirmation before doing anything else.

• Do not rewrite the code to use a non-stack equivalent

(sklearn.Pipeline for skrub, crossvalscore + handwritten metric prints for skore. Substitution silently breaks the contract that the workflow skills (build-ml-pipeline, evaluate-ml-pipeline, organize-ml-workspace) rely on.

• This rule overrides "make the code run". If the user prefers a

substitute, they will say so — until they do, install. Reaching for a substitute because the dependency is missing is the most common way the stack gets silently undone, so treat the missing import as a hard stop.

How to use this skill

1. Read this whole SKILL.md before picking — the tier structure

determines whether the library should already be installed, needs a user-choice prompt, or is opt-in.

1. Match the task to an entry in the right tier.
2. Read the linked references/<library>.md for the chosen library's

scope and tradeoffs before introducing it.

1. Install via pixi by default. If the project already uses a

different manager (pip+venv, uv, conda), follow that instead.

1. Don't substitute libraries silently. If no entry fits the task,

surface the tradeoff to the user.

Tier 1 — Mandatory (install at project start)

These five libraries are always installed in a data-science / ML project. The first three co-own the modeling workflow: scikit-learn provides the estimators, skrub provides the data-cleaning + DataOps layer that sits before them, skore evaluates the result and persists it as a project on disk. The fourth, ruff, owns lint + format and is non-negotiable: every project Claude touches should pass ruff check. The fifth, pytest, runs the smoke test that every approved experiment must have per the test-ml-pipeline / smoke-test-ml-pipeline contract — without pytest the smoke-test gate can't enforce predict-time correctness, so pytest stays mandatory even when no other tests have been written yet. Each is named explicitly even when transitively present, because the workflow skills (build-ml-pipeline, evaluate-ml-pipeline, python-code-style, test-ml-pipeline) depend on them directly and should not silently lose them if upstream packaging changes.

• [scikit-learn](references/scikit-learn.md) — tabular ML

algorithms, preprocessing, model-selection helpers. Use HistGradientBoosting{Classifier,Regressor} instead of pulling in xgboost or lightgbm. Evaluation, cross-validation reports, and model comparison are owned by skore — don't inline crossvalscore / classification_report for analysis output.

• [skrub](references/skrub.md) — wrap custom dataframe operations

in a sklearn-compatible computation graph that replays deterministically across train and test splits. Use for the data-cleaning + feature-engineering layer that sits before the sklearn pipeline.

• [skore](references/skore.md) — predictive-model evaluation built

on top of scikit-learn (evaluate, EstimatorReport, CrossValidationReport, ComparisonReport) and experiment tracking via the Project API (skore.Project(...), project.put(...), project.get(...)). Replaces ad-hoc crossvalscore + handwritten metric printouts; replaces mlflow for tracking. Brings numpy, pandas, matplotlib, seaborn, plotly, joblib, and others transitively (see Tier 4) — so static and interactive plotting are available without any extra install.

Install variant per mode. skore.Project(...) supports three mutually exclusive modes: local (artifacts on disk; no extra deps), hub (artifacts on Skore Hub; requires skore[hub] extra + skore.login() before first use), mlflow (artifacts in an MLflow tracking server; requires the skore[mlflow] extra plus an explicit mlflow>=3 pin — skore[mlflow] alone can resolve an unsupported mlflow 2.x). The choice is a workspace-level decision owned by organize-ml-workspace § "G-SKORE-MODE" — fired at scaffold alongside G-PKG-NAME / G-TABULAR / G-ENV-MGR. python-env-manager § "Tier 1 install: skore variant per mode" maps the recorded decision to the right install command per env manager. Default-on-no-preference: local.

• [ruff](references/ruff.md) — single-tool lint + format,

replaces black / isort / flake8 / pydocstyle. Install in the same feature/env as the rest of the Tier 1 stack so pixi run ruff works without extra activation. The configuration (rule selection, numpydoc convention, per-file ignores) and the rule "Claude runs ruff after generating code" are owned by the python-code-style skill, which also ships the canonical ruff.toml template.

• [pytest](references/pytest.md) — test runner for the

smoke-test gate enforced by test-ml-pipeline / smoke-test-ml-pipeline. Every approved experiment must have a passing tests/smoke/testNN<short_name>.py before its row in JOURNAL.md can flip to done; pytest is what runs that test, so the dependency is non-negotiable even on workspaces that haven't authored any tests yet. Install in the same feature/env as the rest of the Tier 1 stack.

Tier 2 — Competing-library jobs (user choice)

Jobs in this tier have more than one valid library in the stack. The user picks via AskUserQuestion before any import or install (see § "Competing libraries — general rule" above). Recorded picks live in journal/JOURNAL.md Status Workspace decisions and are read first on every subsequent session.

Competing-jobs table

Per-option detail:

• Tabular dataframe

- pandas (+ pyarrow) — established tabular library; pyarrow is the recommended Parquet engine + Arrow-backed dtype backend. pandas is already pulled in by skore (Tier 4), so the only explicit install for this option is pyarrow if Parquet IO is in scope. See [pandas](references/pandas.md) / [pyarrow](references/pyarrow.md). - polars — Arrow-native tabular library; faster on large frames, stricter type system. Requires an explicit install (not pulled in by anything in Tier 1). See [polars](references/polars.md).

• Deep-learning framework

- pytorch — tensor library with GPU / MPS support and autograd. Default DL framework in the stack. See [pytorch](references/pytorch.md). - keras — high-level, layer-oriented DL API; multi-backend (pytorch / TensorFlow / JAX). See [keras](references/keras.md).

• sklearn-compatible DL wrapper

- skorch — wraps a PyTorch nn.Module so it behaves like a sklearn estimator (fit / predict, GridSearchCV, pipelines). See [skorch](references/skorch.md). - keras — exposes a sklearn-compatible API directly via keras.wrappers.SKLearnClassifier / SKLearnRegressor. See [keras](references/keras.md).

• Static vs interactive plotting — both are already

available transitively via skore (matplotlib, seaborn, plotly all land without an explicit install). The ask is which output shape the user wants for this project, not which install to run. Pick by output medium: static reports / papers / static skore reports → matplotlib + seaborn; interactive notebooks / dashboards → plotly.

• Model serving / registry — only relevant when the

project's roadmap includes serving a trained model. Skip the gate entirely if serving is out of scope. When it is in scope, mlflow is the default (registry + REST out of the box); FastAPI + joblib is the lighter custom path.

How a Tier 2 gate fires in practice

• Project start (bootstrap). Every Tier 2 job the project

touches must have a pick recorded in Workspace decisions before the matching code is written. The tabular gate fires on every project; the others fire only when the project's roadmap brings them in scope.

• Mid-project (new job comes into scope). When a new Tier 2

job becomes relevant (e.g. the project pivots to add a DL model), the gate fires at that point; the existing Workspace decisions block is amended with the new row.

• Mid-project (user wants to pivot). Tier 2 decisions are

immutable unless the user explicitly says so. A pivot is a user-driven event; the skill never auto-pivots even if a newer library would obviously be a better fit.

Tier 3 — Optional (install on demand)

Add these only when the task calls for them. Do not pre-install.

Deep learning

For NLP, computer vision, or any task where deep learning is the right tool. None of these are mandatory; reach for them only when the project's modeling task requires DL.

**The which library pick (pytorch vs keras as framework, skorch vs keras as sklearn-compatible wrapper) is a competing-library job — owned by Tier 2.** This section only covers when to reach for DL at all; the framework choice has its own row in the Tier 2 competing-jobs table and fires an AskUserQuestion the first time DL comes into the project's scope.

Per-library reference material:

• [pytorch](references/pytorch.md) — tensor library with GPU /

MPS support and autograd; also the GPU alternative to numpy for raw numerical work.

• [keras](references/keras.md) — high-level, layer-oriented

deep-learning API; multi-backend (pytorch / TensorFlow / JAX).

• [skorch](references/skorch.md) — wraps a PyTorch nn.Module

so it behaves like a sklearn estimator (fit / predict, GridSearchCV, pipelines).

Model serving

**The which library pick (mlflow.pyfunc vs FastAPI + joblib) is a competing-library job — owned by Tier 2.** This section only covers when serving is in scope.

Per-library reference material:

• [mlflow](references/mlflow.md) — model packaging, registry,

and REST serving (mlflow.pyfunc, mlflow models serve). Use only for serving and registry concerns; tracking belongs to skore.

Notebooks

For notebook-based work, prefer Python files with # %% cell markers (jupytext percent format) over .ipynb files. Python files are diffable and version-control friendly; jupytext handles the conversion to/from notebook format when needed.

• [jupyterlab](references/jupyterlab.md) + [ipykernel](references/ipykernel.md)

— ambient in the dev feature (alongside ruff + pytest, per python-env-manager § "Where does the package belong?"). Always installed; no per-project ask. The reference pages describe the tools' role, not an opt-in install.

• [jupytext](references/jupytext.md) — Tier 3 opt-in: sync

.ipynb ↔ .py (# %% markers) so the notebook source-of- truth stays version-control friendly. Install only when the project wants .ipynb interop with the # %% scripts.

Tier 4 — Transitive (already pulled in; do not install explicitly)

These land in the env as runtime dependencies of the mandatory tier (or of the chosen tabular library). Documented here so you don't add a redundant explicit dependency, and so you know what's available without an extra install.

• [numpy](references/numpy.md) — N-d arrays, numerical

primitives. Pulled in by scikit-learn and skore.

• [scipy](references/scipy.md) — scientific computing on top of

numpy (stats, optimize, sparse, signal). Supports the array API. Pulled in by scikit-learn.

• [matplotlib](references/matplotlib.md) — static plotting

foundation. Pulled in by skore (via seaborn).

• [seaborn](references/seaborn.md) — static statistical plots

(distributions, regression, faceting). Pulled in by skore.

• [plotly](references/plotly.md) — interactive plots (hover,

zoom, pan); browser-based, suited for dashboards and exploratory notebooks. Pulled in by skore — interactive viz is free, no extra install needed.

Agent feature — orthogonal to the four tiers

The audit flow owned by audit-ml-pipeline and the editor LSP integration both need agent-only tooling (ipython + pyright). These deps don't fit cleanly into the four tiers above:

• They are not Tier 1 mandatory — workspaces that don't run

audits and don't use opencode's LSP never need them.

• They are not Tier 2 user-choice — there is no competing-

library job; this is the one toolchain that powers the in-process audit runner AND the LSP.

• They are not Tier 3 optional in the same sense as

pytorch / mlflow — the agent feature is agent-orthogonal tooling, not a modelling library.

• They are not Tier 4 transitive — nothing in Tier 1 pulls

them in.

So they live in their own bucket: the agent feature, a manager-scoped install that composes alongside (not replaces) the data-science deps.

Install + config drop: owned by python-env-manager § "Agent feature". That skill carries the per-manager install table (pixi features / uv groups / poetry groups / hatch envs / conda envs / pip+venv extras) and the pyrightconfig.json placement step.

Consumed by audit-ml-pipeline and the LSP. When either consumer fires and the agent feature isn't present, the calling skill routes through python-env-manager's G-AGENT-FEATURE gate before proceeding.

No kernel registration. The in-process runner doesn't need a Jupyter kernel.

Distinct from the dev feature's notebook tooling. jupyterlab

• ipykernel are ambient in dev for interactive notebook

editing; jupytext stays Tier 3 opt-in. The agent feature (ipython + pyright) is its own bucket — ipython powers the in-process audit runner (no kernel), not user-facing notebook work. A workspace may have any combination of the three concerns.

Conventions

• Environment manager: detection + install commands are owned by

the python-env-manager skill — invoke it for any add / remove / upgrade. Default recommendation is pixi; if the project already uses a different manager (uv / poetry / hatch / conda / pip+venv), python-env-manager's detection table picks it up and never substitutes one manager for another.

• Versions: don't pin unless the user asks or there's a known

incompatibility. Exception — skore and skrub must always be the latest available release.

• One tool per job: don't introduce a second library for a task

already covered without explicit user request. (One library can own multiple jobs — skore covers both evaluation and tracking. The rule forbids piling a second tool onto a covered job, not a single tool covering multiple jobs.)

• Line width: wrap text at 88 chars where natural. Don't compress

content to fit; long inline links and code spans are fine to leave on longer lines.