---
name: programming
description: "Applies strict, modern language practice (typed errors, exhaustive match, tests that can fail) for Python, Rust, TypeScript, JavaScript, and Go. Use for work on .py, .rs, .ts, .js, or .go files, and whenever a test or spec file is written, reshaped, reviewed, or deleted: the four-question test authoring gate and the junk-test classes live here."
---

# Programming

**You own this code.** What it is for and how it is built are your call, so change it the way its owner would: fit new behavior into the structure that is there and leave no duplicate or dead code behind. **A defect in the code you are changing is yours to fix now, at its root cause; never excuse it as pre-existing, someone else's, or the tool's fault.** One elsewhere goes in your summary as a follow-up.

This skill is an index. The hard per-language rules live under `references/`. Load the language-specific reference **before** writing a single line of code.

---

## PHASE 0 — LANGUAGE GATE (RUN THIS FIRST, EVERY TIME)

**DO NOT WRITE OR EDIT A SINGLE LINE OF CODE BEFORE COMPLETING THIS GATE.**

1. **Identify the language** from the file extension or the user's request.
2. **STOP** and read the matching reference set:

   | File / Language | MANDATORY reading (load `Read` tool on every file below) |
   |---|---|
   | `.py`, `.pyi`, "Python" | `references/python/README.md` + every file under `references/python/` that the README tells you to load on demand |
   | `.rs`, `Cargo.toml`, "Rust" | `references/rust/README.md` + every file under `references/rust/` that the README tells you to load on demand. **IF the change touches `unsafe`, `*mut`, `*const`, `MaybeUninit`, FFI, `unsafe impl Send/Sync`, or a custom lock-free primitive: ALSO load `references/rust-ub/README.md` plus every file under `references/rust-ub/`.** |
   | `.ts`, `.tsx`, `.mts`, `.cts`, `.js`, `.jsx`, `.mjs`, `.cjs`, "TypeScript" / "JavaScript" | `references/typescript/README.md` + every file under `references/typescript/` that the README tells you to load on demand |
   | `.go`, `go.mod`, `go.sum`, `.golangci.yml`, `*.proto` next to a Go module, "Go" / "Golang" | `references/go/README.md` + every file under `references/go/` that the README tells you to load on demand |
   | any test file (`*.test.*`, `*.spec.*`, `*_test.go`, `test_*.py`, `*_test.py`, under `test/`, `tests/` or `__tests__/`), or a change that adds, reshapes, or deletes a test | the language row above PLUS `references/testing/README.md` (the authoring gate) and `references/testing/junk-classes.md` (the table the gate reviewer applies) |

3. Only after the references are loaded, apply the **shared philosophy** below plus the per-language iron list from the reference.

**No exceptions for "small" or "one-off" code.** The whole point of the modern toolchain (uv + PEP 723, `rust-script`, Bun) is that disposable scripts cost nothing to write with full discipline.

---

## Shared philosophy (all four languages)

These are not style preferences. They are the seven axioms every recipe in `references/` derives from.

0. **The best code is the code never written.** Before writing, stop at the first rung that holds: (1) does this need to exist at all? (YAGNI) (2) does this codebase already have it? — reuse the helper or pattern, do not re-implement. (3) does the standard library do it? (4) does a native platform feature cover it? (5) does an installed dependency solve it? (6) can it be one line? (7) only then, write the minimum that works. Climb the ladder *after* you understand the problem and trace the real flow end to end — the smallest diff in the wrong place is a second bug. The ladder is a fast decision, not a written essay: pick the rung and move. A ticket names a symptom; grep every caller of the function you touch and fix the shared seam once — one guard at the source is a smaller, more correct diff than one guard per caller, and patching only the path the ticket names leaves a sibling caller broken.

1. **The type system is your proof system.** Make illegal states unrepresentable. The compiler / type checker is the cheapest test you will ever run. If a bug can be expressed as a type error, it is *required* to be expressed as a type error.

2. **Parse, don't validate.** Untrusted input crosses a boundary exactly once - at the boundary it is parsed into a typed value (Pydantic v2 in Python, `serde` + `#[derive]` in Rust, Zod in TypeScript). Inside the boundary, code receives typed values and never re-validates. The boundary owns trust; the interior owns logic.

3. **One name = one concept.** A `UserId` is not a `string`. A `Seconds` is not a `Milliseconds`. Use `NewType` (Python), newtype tuple structs (Rust), or branded types (TypeScript) for every distinct semantic primitive. The compiler refuses to let two semantic units mix.

4. **Exhaustive variant matching, always.** Discriminated unions and enums are matched exhaustively. Python: `match` + `case unreachable: assert_never(unreachable)`. Rust: `match` (the compiler enforces). TypeScript: `switch` + `assertNever`. **`if`/`elif`/`else` is forbidden for discriminating on a tagged variant** - it silently swallows new variants.

5. **Trust framework guarantees. Validate only at boundaries.** No null checks for values the type system already proves non-null. No `try/except` around code that cannot raise. No `unwrap`/`!`/`as` to paper over a contract you should have encoded in types. No defensive layer for a scenario you cannot name.

6. **Tests are the behavior of record, and only tests that can fail count.** READ the tests covering the area BEFORE you change it: do they encode the intent, cover this path, pass? One wrong before your change is a FINDING — never edit it green. Reproduce a bug before fixing it. The run proves the change; add a test ONLY where the repository keeps tests for this behavior AND a regression would otherwise pass unnoticed — sized like its neighbors, never restating the change. The authoring gate and the junk-class table are in `references/testing/`.

---

## TEST DISCIPLINE

Tests are code: the language gate above applies to them, plus [`references/testing/README.md`](references/testing/README.md) (the contract every language's recipes assume) and [`references/testing/junk-classes.md`](references/testing/junk-classes.md) (the table the gate reviewer applies to every test in a diff). Read both before adding, reshaping, or deleting a test.

**The authoring gate.** Before a test exists, answer all four; a missing answer means do not add it yet.

1. What observable behavior, invariant, or independent contract does it protect?
2. What credible regression makes it fail?
3. Why does existing coverage not already catch that failure? One primary owner per contract, at the strongest boundary; extend a table row before adding a near-duplicate, a copy in another package, or a replay through a wrapper.
4. Does it need a production seam (export, flag, getter, wrapper, injection hook) no production caller needs? Then test at the real boundary instead.

The README owns the shape (Given / When / Then, the pyramid, the mock ladder, determinism, isolation, the prompt-test rule) and the "test your test" step: break the production owner, watch the test go red for the intended reason, restore byte for byte. A test that fails on the unchanged base is a bug to fix at its owner; one your own change turns red is fixed at the code or at the test; neither is deleted or edited green to unblock a run.

---

## Cross-language iron list

Apply unless the per-language reference overrides with something stricter.

| Rule | Python | Rust | TypeScript | Go |
|---|---|---|---|---|
| Immutable by default | `@dataclass(frozen=True, slots=True)` / Pydantic `frozen=True` | every binding is `let` (not `let mut`) unless mutation is the documented purpose | every field is `readonly`; arrays are `readonly T[]` | value types, unexported fields, no mutation methods unless mutation is the purpose |
| Branded primitives | `UserId = NewType("UserId", int)` | `struct UserId(u64);` (newtype tuple) | `type UserId = Brand<string, "UserId">` | `type UserID string` + smart constructor with unexported field |
| Exhaustive variant matching | `match` + `assert_never` | `match` (compiler-enforced) | `switch` + `assertNever` | sealed interface + type switch + **`exhaustive` linter** (the compiler will not help) |
| No untyped escape hatches | no `Any` in public sigs, no `cast`, no `# type: ignore` | no `unwrap`/`expect` outside tests (invariant `expect` only behind `#[expect(clippy::expect_used, reason)]`), no numeric `as`, no `#[allow]` (silence with `#[expect(lint, reason)]`) | no `any`, no `as` (except `as const`, `satisfies`), no `!`, no `@ts-ignore`, no `@ts-expect-error` | no `interface{}` / bare `any` in domain sigs; no `_ = err`; no `//nolint` without reason |
| No bare error strings | typed exception dataclass with `__str__` | `thiserror` enum (lib) or `anyhow` with `.context(...)` (app) | `Error` subclass with typed fields | sentinel `errors.New` + typed `*XError` struct; wrap with `%w`; check via `errors.Is/As` |
| Boundary catch only | catch the exact exception you expect; broad `except Exception` only at a top-level boundary (`main()`/CLI entry, HTTP handler), with logging + re-raise | `?` everywhere; never `panic!` in library code | `catch` must narrow with `instanceof` and re-throw or convert; no empty catch | every `(T, error)` checked; `panic` only in `main`/tests; one `httperr.Write` funnel in handlers |
| Resources via RAII | `with` (sync) / `async with` (async) | `Drop` impl or RAII guard | `using`/`await using` (TC39 explicit resource management) | `defer x.Close()` immediately after acquisition; `bodyclose`/`sqlclosecheck` linters enforce |
| Async runtime is mandatory | `anyio` (NEVER bare `asyncio`) | `tokio` (`async-std` is unmaintained) | platform-native async (Bun/Node) with structured cancellation via `AbortSignal` | `context.Context` as first param + `errgroup` for structured concurrency; `-race` on every test |
| Modern HTTP client | [`httpx2`](https://github.com/pydantic/httpx2) with HTTP/2 + brotli + zstd | `reqwest` with rustls | `ky` (default) / `undici` direct API (Node perf) - NEVER bare `fetch` in prod | stdlib `net/http.Client` with tuned `Transport` + `go-retryablehttp` for retry/backoff |
| No parameter mutation | params are inputs; produce a new value | `&mut` only when mutation is the documented purpose | parameters never reassigned (`noParameterAssign`) | value receivers when not mutating; pointer receivers only for genuine mutation; `copylocks` vet enforces |
| No helpers for one-off | inline a 3-line operation; do not abstract until the second caller | same | same | same |

---

## Modern ecosystem - canonical libraries (2026)

Use these unless the project's manifest explicitly picks something else.

| Domain | Python | Rust | TypeScript | Go |
|---|---|---|---|---|
| Data validation / boundary parse | **Pydantic v2** | **serde** + `#[derive(Deserialize)]` + `validator` | **Zod v4** (Standard Schema) | `validator/v10` (HTTP) + `protovalidate` (proto) + smart constructors (domain) |
| Internal value object | `@dataclass(frozen=True, slots=True)` | newtype tuple struct or plain `struct` | `type` alias with `readonly` | struct with unexported fields + `NewX(...)` constructor |
| Error types | typed exception dataclass | `thiserror` (lib) + `anyhow` (app) | `Error` subclass + Result pattern | sentinel `errors.New` + typed `*XError` struct + `%w` wrap |
| HTTP client | [`httpx2`](https://github.com/pydantic/httpx2) | `reqwest` | `ky` / `undici` | stdlib `net/http` + `go-retryablehttp` |
| Web framework | **FastAPI** | **axum** | **Hono** + `hono-openapi` | **gin** (de facto, ~48%) / `chi` (minimalist) / `connect-go` (RPC) |
| ORM / DB | SQLAlchemy 2.x async + `asyncpg` | `sqlx` (compile-time checked) | **Drizzle** | **sqlc** (codegen from `.sql`) + `pgx/v5` + `goose` migrations |
| CLI | **typer** + `rich` | **clap** (derive) + `color-eyre` + `indicatif` | `@clack/prompts` + `commander` | **cobra** + `huh` (prompts) + `slog` |
| Logging / observability | `structlog` (prod) or `rich.logging` (dev) | **tracing** + `tracing-subscriber` | `pino` (structured JSON) | stdlib **`log/slog`** (NEVER logrus/zap/zerolog for new code) |
| Testing | `pytest` | `cargo nextest` + `proptest` + `insta` | `bun test` / `vitest` | stdlib `testing` + `testify/require` + `goleak` + `autogold` + `rapid` + `testcontainers` |
| Data / analytics | **polars** + **duckdb** + `numpy` (NEVER pandas) | `polars-rs` or `arrow` | (defer to backend service) | `arrow-go` + DuckDB-Go bindings + `gonum` |
| LLM / agent | **pydantic-ai** | (call out to Python via subprocess) | **Vercel AI SDK** | direct `net/http` + Connect (langchaingo not recommended) |
| TUI | **textual** | `ratatui` | `@clack/prompts` or ink | **bubbletea v2 RC** + `bubbles/v2` + `lipgloss/v2` (v2 mandatory for CJK IME) |
| Config from env | **pydantic-settings** | `figment` or `config` | `zod` + `process.env` | `caarlos0/env/v11` (struct-tag env) |

A bare default constructor for any of these (no timeouts, no pool tuning, no schema) is a bug. See the per-language reference for the canonical production defaults.

---

## Modern toolchain - the only acceptable setup

| Tool category | Python | Rust | TypeScript | Go |
|---|---|---|---|---|
| Package / project manager | **uv** (NEVER pip/poetry/conda) | **cargo** + `cargo-nextest` + `cargo-machete` + `cargo-deny` | **Bun** (runtime + package manager); pnpm if Node is forced | **`go modules`** + `go work` for monorepos |
| Type checker | **basedpyright** with `typeCheckingMode = "all"` | the Rust compiler with `-D warnings` + clippy `pedantic` + `nursery` + `cargo` groups | `tsc --noEmit` (or `tsgo` when available) with `strict` + `noUncheckedIndexedAccess` + `exactOptionalPropertyTypes` + `verbatimModuleSyntax` | the Go compiler + **`golangci-lint v2`** with the strict bundle + **`nilaway`** (nil-deref static analysis) |
| Linter + formatter | **ruff** with `select = ["ALL"]` | `clippy` + `rustfmt` | **oxlint + oxfmt**, with the local copy and suppression-reason rules | **`gofumpt`** (stricter gofmt) + `goimports -local` + `golangci-lint v2` |
| Test runner | **pytest** | **cargo-nextest** | `bun test` / `vitest` | stdlib `go test -race -shuffle=on -count=1` + `goleak` |
| UB / soundness gate | (n/a) | **nightly miri** with strict provenance + Tree Borrows pass | (n/a) | **`nilaway`** + `-race` detector + `goleak` are the equivalent gate |
| Disposable scripts | **PEP 723** inline metadata + `uv run script.py` | **rust-script** with inline `Cargo.toml` block | `bun run script.ts` | `//go:build ignore` + `go run script.go` |
| Bootstrap a new project | `bun run scripts/python/new-project.ts` | `bun run scripts/rust/new-project.ts` | `bun run scripts/typescript/new-project.ts` | `bun run scripts/go/new-project.ts` |
| Pre-commit / CI gate | `ruff check . && basedpyright && pytest` | `cargo clippy --all-targets -- -D warnings && cargo nextest run && cargo test --doc && cargo +nightly miri nextest run` | `bunx oxlint --type-aware --deny-warnings && bunx oxfmt --check && bunx tsc --noEmit && bun test` | `gofumpt -l . && golangci-lint run ./... && nilaway ./... && go test -race -shuffle=on -count=1 ./...` |

For TS/JS, **use oxlint + oxfmt with these two rules by default**: `no-em-dash-copy` and `require-suppression-reason`. Strongly prefer this OXC toolchain for new projects; retain ESLint, Prettier, or Biome only when the repository already uses them. Load [`references/typescript/oxc-lint.md`](references/typescript/oxc-lint.md) for the complete configs, plugin, and type-aware support requirements. Rewrite paired em dashes as asides using parentheses or commas, not colon chains.

A `tsconfig.json` with `"strict": true` alone is **not** strict. The reference enumerates the additional flags. Same for `pyproject.toml` and `Cargo.toml` - the references contain the canonical full configuration.

---

## CODE SMELLS — AUTOMATIC REVIEW TRIGGERS

Most smells below are design review triggers: STOP, re-examine the code, and either fix the smell or justify carrying it with a SPECIFIC reason. **The 250 pure LOC ceiling is stricter: >250 is a DEFECT. Refactor before adding lines except for rare SIZE_OK or pure-data-table exceptions.**

Full rationale, measurement methods, workaround detection, and split examples: **[`references/code-smells.md`](references/code-smells.md)**.

### Smell 1 — File exceeds 250 pure LOC

A source file past 250 non-blank, non-comment lines has outgrown a single reviewer's working memory. The module is almost certainly doing more than one thing. Measure: `awk '!/^[[:space:]]*$/ && !/^[[:space:]]*(\/\/|#|--)/' <file> | wc -l`.

**When detected:** Name what the file owns in one short noun phrase. If the answer needs "and", the file needs splitting. Load `/refactor` and split by responsibility. If the file genuinely cannot be split (generated parser, indivisible state machine), mark it in the first 10 lines with `# noqa: SIZE_OK — <reason>` in Python (the form `scripts/python/check-no-excuse-rules.py` reads) or `// allow: SIZE_OK — <reason>` in `//`-comment languages.

### Smell 2 — Function with more than 3 parameters

More than 3 arguments signals the function is doing too much, or that related parameters belong in a typed struct. **Workarounds count as the same smell** — passing `dict`/`Record<string, unknown>`/`map[string]any`/`**kwargs`/`...args` to smuggle parameters through one argument, or a throwaway "config" object with 6+ fields that exists solely to wrap what would otherwise be positional args (genuine reusable domain types like `HttpClientConfig` are fine).

**When detected:** Group related parameters into a typed value object with a domain name. If 4+ independent inputs are genuinely required, the justification must be SPECIFIC. See [`references/code-smells.md` Smell 2](references/code-smells.md#smell-2--function-with-more-than-3-parameters) for Python, TypeScript, and Go examples.

### Smell 3 — Redundant verification after a destructive action

Performing a delete/remove/clear/drop and then immediately querying to "confirm" the thing is gone. **The operation's contract IS the verification.** Re-checking is AI-generated defensive bloat that wastes cycles and teaches the reader the operation is unreliable — which it is not. Same smell: calling a setter then getting to "confirm", writing a file then reading it back, inserting a row then SELECT-ing it, pushing to an array then checking `.length`.

**When detected:** Delete the verification code. Trust the operation's contract. If the operation can genuinely fail silently, fix the operation — do not paper over it with a post-check. See [`references/code-smells.md` Smell 3](references/code-smells.md#smell-3--redundant-verification-after-a-destructive-action) for examples.

### Smell 4 — Negative-form names and conditions

Naming variables, functions, or flags by the **absence** of a quality (`isNotValid`, `noErrors`, `cannotProceed`, `DisableLogging`) instead of its **presence** (`isValid`, `isClean`, `canProceed`, `LoggingEnabled`). Every negation forces the reader to invert mentally; two negations (`if !isNotReady`) become a logic puzzle nobody reviews confidently.

**When detected:** Rename to the positive form and invert the branch logic. Negation IS appropriate in guard clauses (`if !authorized { return }`) and filters (`items.filter(|x| !x.is_expired())`) — the negative form is the intent there. See [`references/code-smells.md` Smell 4](references/code-smells.md#smell-4--negative-form-names-and-conditions) for the full naming table and examples.

---

## LOGGING — CROSS-CUTTING RULES

Logging is part of the code you ship, and it has iron rules of its own: levels chosen by naming the consumer (never by severity vibes), placement at decision points (never inside helpers), stable message