python-development

name	python-development
description	General Python development skill for designing, implementing, refactoring, debugging, testing, validating, and reviewing Python code. Use for Python 3.11+ projects, especially modern pyproject-based repos using uv, Ruff, pytest, mypy or pyright, Pydantic v2, Typer, or Loguru. Adapt to the target repository before applying defaults.

Python Development

Use this as the default Python development playbook. It is intentionally broader than a review rubric: it guides how to understand a repository, design the smallest useful API, write the code, test the behavior, validate the change, and review the result.

The skill is opinionated but not blind. First discover the target project's Python version, tools, commands, and local conventions. Then apply the defaults below only where they fit.

When to use this skill

Use this for Python work involving:

• feature implementation
• bug fixes and debugging
• refactoring and cleanup
• test design and test repair
• API, data model, CLI, or package design
• code review and self-review
• migration to modern Python tooling
• questions about Python best practices, clean code, or maintainability

If a domain-specific skill is also relevant, such as web deployment, browser automation, ML inference, data science, or HPC, use it as an overlay. This skill remains the baseline for Python quality.

Operating rule

Do not start by coding. Start by discovering the local contract.

1. Read local instructions and the user request.
2. Inspect project metadata and configuration.
3. Inspect nearby code and tests.
4. Identify the exact behavior or design contract.
5. Choose the narrowest useful implementation and validation path.

When the user asks for planning, advice, review, assessment, explanation, or any other no-edit task, do not edit files unless they explicitly ask for changes.

Sources of truth

Follow these in order:

1. The user's explicit request.
2. Local agent or repository instructions.
3. Project configuration, such as pyproject.toml, setup.cfg, tox.ini, noxfile.py, pytest.ini, .pre-commit-config.yaml, CI workflows, and lock files.
4. Existing code and tests.
5. This skill and its references.
6. General ecosystem practice.

If project configuration conflicts with this skill, follow the project and note the tradeoff only when it matters.

Modern Python defaults

Use these as defaults when the project does not say otherwise:

• Python: target the version declared by the project. For new code, prefer Python 3.12+ when the project allows it.
• Packaging and metadata: prefer pyproject.toml as the central configuration file.
• Environment and package manager: prefer uv for new projects or projects already using it. Otherwise follow the existing tool, such as pip, pip-tools, Poetry, Hatch, PDM, tox, or nox.
• Formatting and linting: prefer Ruff formatter plus Ruff linting when the project has not standardized elsewhere.
• Type checking: prefer strict or gradually strict mypy or pyright. Follow the checker already configured in the repo.
• Testing: prefer pytest for new test suites unless the project uses unittest or another runner.
• Validation: run focused checks first, then broader checks before declaring a substantial change complete.

Never hardcode commands from this skill if the repository defines its own. Discover and use the repo's actual commands.

Development loop

1. Orient

• Find the package layout, source roots, tests, and configured tools.
• Read the nearest existing code before inventing new patterns.
• Read tests around the behavior before changing behavior.
• For large or unfamiliar areas, build a short map: inputs, outputs, side effects, dependencies, callers, tests, and failure modes.

2. Define the contract

Before implementation, state or infer:

• what the function, module, CLI, or service accepts
• what it returns or produces
• what side effects it has
• what errors it raises
• what invariants must hold
• what tests or checks will prove it works

Prefer the smallest API current callers need. Do not add hooks, modes, flags, or configuration for hypothetical futures.

3. Test the behavior

When behavior changes, add or update tests before implementation when practical. At minimum, know what focused validation will fail before the fix or protect the new behavior after the change.

Good tests assert observable behavior. Weak tests assert only that an internal method was called.

4. Implement simply

• Keep functions small and single-purpose.
• Keep abstraction levels separate: orchestration reads like intent, helpers do low-level work.
• Prefer guard clauses over deep nesting.
• Prefer explicit errors over None, -1, empty strings, or silent fallbacks as failure signals.
• Use plain data structures until a named model improves clarity or validation.
• Prefer the standard library or an existing dependency over custom code when it cleanly solves the problem.
• Add abstractions only when there are real current call sites or invariants.

5. Validate

Use the narrowest useful command first, for example one changed test module or one type-check target. Then run the broader project check before declaring substantial work complete, or state why it was not run.

Validation evidence should include the command, result, and any known limitation.

6. Self-review

Before completion, review the diff for:

• correctness and failure modes
• API clarity
• typing and data model fit
• test signal quality
• hidden side effects
• unnecessary abstraction
• IO and resource safety
• performance and concurrency hazards in hot paths
• consistency with project conventions

Mode playbooks

Feature work

• Identify the public contract and callers first.
• Add focused tests for the new behavior.
• Implement the smallest useful path.
• Prefer obvious data flow over clever reuse.
• Validate focused tests, then broader checks.

Bug fixing

• Reproduce the bug or locate the failing path with evidence.
• Add a regression test when practical.
• Fix the root cause, not only the symptom.
• Verify the original path and the regression test.
• Note any adjacent risks not fixed.

Refactoring

• Separate mechanical moves from behavior changes.
• Preserve behavior with existing tests or characterization tests.
• Keep the public API stable unless the task is an API migration.
• Rename for clarity when it reduces future mistakes.
• Delete dead code instead of routing around it.

Debugging

• Capture the exact error, input, environment, and stack trace.
• Reduce to the smallest reproducer.
• Inspect runtime state when source reading is insufficient.
• Form one hypothesis at a time and test it.
• Remove temporary probes before finishing.

Testing

• Prefer tests that exercise real behavior through public or stable seams.
• Mock at boundaries, not inside the code being tested.
• Use fixtures to remove setup noise, not to hide essential inputs.
• Use parametrization for variations.
• Use property-based tests for pure logic with clear invariants.

Review

• Triage first. Route depth by risk, not habit. A tiny isolated change needs a different review than an auth, data, dependency, migration, concurrency, or public API change.
• Form expectations before reading the diff. Compare the stated goal with what changed, what did not change, and what tests prove.
• Check the premise before polishing implementation. Ask whether the change should exist, whether existing code already solves it, and whether a standard library or current dependency should replace custom code.
• Read the relevant code, tests, and configuration before judging.
• Separate configured tool failures from human judgment findings.
• Report only actionable issues with a concrete fix.
• Do not invent findings to appear thorough.
• End substantial reviews with what you did not verify, such as commands not run, call sites not checked, or integration paths not exercised.
• If there are no findings, say what was checked, what was not checked, and why the result is acceptable.

Review finding schema

Use this JSON-friendly shape for review mode. The canonical schema lives in references/review-schema.md; keep this summary and the examples in sync with that file.

{
  "file": "src/package/module.py",
  "line": 42,
  "symbol": "optional function or class name",
  "category": "correctness|api|design|typing|errors|testing|io|performance|concurrency|security|packaging|maintainability|style|tooling",
  "severity": "high|medium|low",
  "title": "Short problem statement",
  "why": "Concrete risk, broken contract, or violated rule",
  "fix": "Smallest useful change",
  "validation": "Test, check, or manual verification that should cover the fix"
}

Severity:

• high: correctness bug, silent failure, data loss, security issue, broken test signal, API misuse, or likely production failure.
• medium: unclear API, fragile design, weak test, wrong abstraction, or maintainability issue likely to affect near-term work.
• low: local readability or consistency issue that materially improves comprehension.

Rules:

• One issue per record.
• Include why it matters and how to fix it.
• Do not report issues already enforced by configured tools unless those tools are not being run.
• Locate by content when line numbers are approximate.

High-signal checklist

API design

• Ambiguous options and flags are keyword-only.
• Return types match consumption: reusable collection versus one-shot iterator.
• None is not overloaded when it is a valid input or output.
• Data crossing boundaries has a named shape when that improves clarity.
• Public APIs avoid speculative parameters.

Typing

• Type syntax matches the project's supported Python version.
• Use built-in generics and | unions when supported.
• Use Protocol for structural dependency boundaries.
• Use overloads only when they make caller types more precise.
• Avoid type tricks that make runtime behavior hard to understand.

Errors and logging

• Exceptions are specific and preserve useful causes.
• Runtime validation uses explicit raises, not assert.
• Failure modes do not pass silently.
• Logs add diagnostic context without noise.
• Hot paths avoid expensive disabled log formatting.

Imports and packaging

• Imports are absolute inside packages. Do not add relative imports.
• Runtime imports are not hidden under TYPE_CHECKING.
• Packaging changes are validated with the project's configured build or import smoke checks.

IO and resources

• Paths use pathlib unless the project has a reason not to.
• Text IO uses explicit encodings.
• Files, network handles, locks, tasks, and subprocesses are cleaned up.
• Serialization formats and schemas are explicit at boundaries.

Tests

• Tests assert outcomes, not implementation trivia.
• Exception tests match useful messages.
• Filesystem and environment changes are isolated.
• Mocks have specs when possible.
• Third-party libraries are wrapped behind seams before heavy mocking.

Security basics

• Inputs are validated at trust boundaries.
• Secrets are never logged or committed.
• Subprocesses avoid shell strings when argument lists work.
• Archive extraction and user-provided paths cannot write outside intended directories.

Maintainability

• Names reveal intent.
• Public functions, classes, modules, and non-obvious helpers have useful docstrings when the project expects them.
• Comments explain why, not what the code already says.
• Dead code is removed.
• Side effects are visible at the call site.
• Hard-to-explain code is simplified before it is documented.

Reference catalogue

Open only the files relevant to the task. When editing this skill itself, first read references/maintenance.md and quality/README.md, then run the quality harness before declaring the edit complete.

• references/project-discovery.md: how to detect Python version, tools, commands, package layout, and local conventions.
• references/tooling.md: formatters, linters, type checkers, pre-commit, and CI.
• references/development-loop.md: detailed workflow for writing code.
• references/validation.md: choosing checks by work mode and risk.
• references/api-design.md: functions, parameters, return values, and public contracts.
• references/typing.md: modern typing and version-aware annotation choices.
• references/data-modeling.md: dataclasses, Pydantic, TypedDict, enums, and plain collections.
• references/configuration.md: settings precedence, environment, files, and secrets.
• references/cli-apps.md: command shape, exit codes, stdout, stderr, and CLI tests.
• references/boundaries.md: web, database, external service, and adapter boundaries.
• references/errors-logging.md: exceptions, validation, chaining, and logs.
• references/observability.md: logs, metrics, tracing, health, and diagnostics.
• references/testing.md: pytest-oriented testing guidance and mocking seams.
• references/debugging.md: reproducing, isolating, and fixing bugs.
• references/refactoring.md: safe behavior-preserving change.
• references/review-schema.md: structured findings and severity policy.
• references/anti-patterns.md: common process, design, testing, and safety smells.
• references/io-filesystem.md: pathlib, encodings, temp files, JSON, CSV, and resources.
• references/imports-packaging.md: imports, pyproject, source roots, and build behavior.
• references/dependency-management.md: lockfiles, dependency groups, extras, and upgrades.
• references/ci-release.md: CI commands, builds, entry points, and package smoke tests.
• references/generated-code-and-migrations.md: generated outputs, schema changes, and data migrations.
• references/performance.md: profiling, batching, caching, and hot paths.
• references/concurrency.md: async, threads, subprocesses, cancellation, and cleanup.
• references/security.md: inputs, secrets, subprocesses, and unsafe deserialization.
• references/documentation.md: docstrings, comments, TODOs, and reader contracts.
• references/style-naming.md: names, builtin shadowing, and style review boundaries.
• references/notebooks.md: notebook reproducibility, review, and test extraction.
• references/maintenance.md: consistency checks when editing this skill.
• references/scientific-python.md: optional NumPy, pandas, and torch guidance.
• references/libraries.md: optional Pydantic, Typer, Loguru, FastAPI, and other ecosystem notes.

Examples live in examples/ and show review findings and validation summaries.