Python

Python

Readability counts. Explicit is better than implicit. If your code needs a comment to explain its control flow, restructure it.

Python 3.14+ is the baseline. Use modern syntax unconditionally — no backward compatibility with older Python versions unless the project explicitly requires it.

References

• [${CLAUDE_SKILL_DIR}/references/typing.md] — Type annotation patterns, generics, overloads, TypeVar, variance: full annotation examples, generic class patterns, Protocol implementation, TypeVar usage
• [${CLAUDE_SKILL_DIR}/references/packaging.md] — Project layout, pyproject.toml, uv, dependency management: pyproject.toml templates, uv workflows, src layout, dependency groups, build backends
• [${CLAUDE_SKILL_DIR}/references/modules.md] — Module system, imports, namespace packages, __init__.py: import resolution order, circular import fixes, lazy imports, namespace packages
• [${CLAUDE_SKILL_DIR}/references/concurrency.md] — asyncio, TaskGroup, cancellation, timeouts, threading interop: TaskGroup error handling, timeout scopes, cancellation semantics, to_thread, eager task factory

Naming

• Variables, functions, methods — snake_case: user_name, fetch_data

• Classes, type aliases — PascalCase: UserService, HttpClient

• Constants — UPPER_SNAKE_CASE: MAX_RETRIES, API_BASE_URL

• Modules, packages — snake_case, short: user_store, auth

• Private attributes/methods — _ prefix: _internal_cache, _validate()

• Name-mangled attributes — __ prefix: __secret (rarely needed)

• Type variables — PascalCase, short: T, KT, VT, ResponseT

• Protocols — PascalCase, -able/-ible suffix: Renderable, Serializable

• Descriptive names. user_count not n. Short names (i, x) only in tiny scopes (comprehensions, simple lambdas).

• No redundant context. car.make not car.car_make.

• Boolean names: is_/has_/can_/should_ prefix: is_valid, has_access.

• Dunder methods are reserved for the data model. Never invent custom dunder names.

• Avoid single-character names outside loop indices, comprehension variables, and well-established conventions (f for file, e for exception, k/v for key/value).

Type Annotations

Python 3.14+ uses modern annotation syntax natively. No from __future__ import annotations needed — all annotations are evaluated lazily by default.

Core Rules

• Annotate all public API boundaries — function signatures, class attributes, module-level variables. Internal code often needs fewer annotations; types flow from context.
• Use built-in generics: list[str], dict[str, int], tuple[int, ...], set[float]. Never import List, Dict, Tuple, Set from typing.
• Union with |: str | None, int | float. Never Optional[X] or Union[X, Y].
• type statement for aliases: type Vector = list[float]. Not TypeAlias annotation.
• None return: annotate -> None on functions that return nothing. Omit return type only on __init__.
• Avoid Any — it disables type checking. Use object when you mean "any type but still type-safe." Use Any only at true interop boundaries with untyped code.

Generics

• type parameter syntax (3.12+): class Stack[T]: and def first[T](items: list[T]) -> T: instead of TypeVar declarations.
• Constrained type parameters: def process[T: (str, bytes)](data: T) -> T: for a finite set of allowed types.
• Bounded type parameters: def sort[T: Comparable](items: list[T]) -> list[T]: for upper-bound constraints.
• Variance is inferred from usage in 3.12+ generics. No manual covariant/contravariant flags needed.

Protocols (Structural Typing)

• Prefer protocols over ABCs when you don't control the implementing types or when structural compatibility is sufficient.
• @runtime_checkable only when you need isinstance() checks — it adds overhead and only validates method presence, not signatures.
• Keep protocols small — one to three methods. A protocol with many methods is a sign you need an ABC or a concrete base class.

from typing import Protocol, runtime_checkable

@runtime_checkable
class Renderable(Protocol):
    def render(self) -> str: ...

Callable Types

• collections.abc.Callable for callable annotations: Callable[[int, str], bool].
• ParamSpec for decorators that preserve signatures: def decorator[**P, R](fn: Callable[P, R]) -> Callable[P, R]:.
• Use Protocol for complex callable signatures with keyword arguments or overloads.

TypeGuard and TypeIs

• TypeIs (3.13+) for narrowing that refines the input type: def is_str_list(val: list[object]) -> TypeIs[list[str]]:.
• TypeGuard for narrowing where the output type is unrelated to input: def is_valid_config(data: object) -> TypeGuard[Config]:.

See ${CLAUDE_SKILL_DIR}/references/typing.md for full annotation patterns, generics, overloads, and variance.

Data Classes and Structured Data

dataclasses

• Use @dataclass for data containers — classes that primarily hold data with minimal behavior.
• frozen=True for immutable data: @dataclass(frozen=True). Default to frozen unless mutation is required.
• slots=True for memory efficiency and attribute safety: @dataclass(slots=True, frozen=True).
• kw_only=True when constructors have more than 3 fields — prevents positional argument ordering bugs.
• field(default_factory=list) for mutable defaults. Never use mutable default arguments.
• Post-init processing: __post_init__ for derived fields and validation.

@dataclass(frozen=True, slots=True, kw_only=True)
class User:
    name: str
    email: str
    roles: list[str] = field(default_factory=list)

When NOT to Use dataclasses

• Simple value containers with 1-2 fields: use NamedTuple or plain tuples.
• Config/settings with validation: use Pydantic or attrs with validators.
• Persistence/ORM models: use the ORM's model base class.

NamedTuple

• Use class syntax over functional form: class Point(NamedTuple): x: float; y: float.
• NamedTuples are immutable and iterable — useful as dict keys and in destructuring.

Enums

• Use enum.Enum for categorical constants. Never use bare strings or ints as pseudo-enums.
• enum.StrEnum when the enum must interoperate with string APIs (JSON, config keys).
• enum.IntEnum only when integer interop is mandatory (legacy protocols). Prefer Enum otherwise.
• @enum.unique to prevent duplicate values.
• Access by value: Color(1). Access by name: Color["RED"]. Iteration: for c in Color:.
• Never subclass enums with members. Enums with members are final.

from enum import StrEnum, unique

@unique
class Status(StrEnum):
    ACTIVE = "active"
    INACTIVE = "inactive"
    SUSPENDED = "suspended"

Pattern Matching

match/case (3.10+) is the preferred dispatch mechanism for structural patterns.

• Use match for structural dispatch — matching on type, shape, or destructured values. Don't use match as a substitute for simple if/elif chains on a single value.
• Always include a wildcard case _: arm unless the match is provably exhaustive.
• Guard clauses with if: case Point(x, y) if x > 0:.
• Use | for alternatives: case "quit" | "exit" | "q":.
• Capture with walrus: case {"error": str() as msg}: captures while matching type.
• Class patterns require __match_args__ or keyword patterns: case Point(x=0, y=y):.

match command:
    case {"action": "move", "direction": str() as direction}:
        move(direction)
    case {"action": "attack", "target": str() as target}:
        attack(target)
    case _:
        raise ValueError(f"Unknown command: {command}")

Functions

• Early return. Guard clauses first, happy path flat. Reduce nesting.
• One function, one job. If the name contains "and", split it.
• Type-annotate all parameters and return types on public functions.
• Default arguments: immutable values only. Use None + conditional for mutable defaults: def f(items: list[int] | None = None): then items = items or [] in body. Never def f(items: list[int] = []):.
• * to force keyword-only arguments after positional params: def connect(host: str, *, port: int = 443):.
• / to force positional-only for parameters that callers shouldn't name: def sqrt(x: float, /) -> float:.
• Prefer returning values over mutating arguments. Functions should be referentially transparent when possible.
• None means absent, not error. Return T | None for optional results. Raise exceptions for errors.

Decorators

• Preserve signatures with functools.wraps:

  def retry[**P, R](fn: Callable[P, R]) -> Callable[P, R]:
      @functools.wraps(fn)
      def wrapper(*args: P.args, **kwargs: P.kwargs) -> R:
          ...
      return wrapper
  

• Decorator order matters. Decorators apply bottom-up. @staticmethod and @classmethod must be outermost (topmost in source).
• Parametric decorators return a decorator: @retry(attempts=3) means retry returns the actual decorator function.
• Don't over-abstract with decorators. If the decorator hides important control flow (error handling, transaction management), make it explicit instead.

Context Managers

• contextlib.contextmanager for simple resource management:

  @contextmanager
  def managed_connection(url: str) -> Iterator[Connection]:
      conn = Connection(url)
      try:
          yield conn
      finally:
          conn.close()
  

• Class-based context managers when state management is complex — implement __enter__ and __exit__.
• contextlib.suppress(ExceptionType) instead of empty except: pass.
• contextlib.closing(thing) for objects with .close() but no __exit__.
• contextlib.asynccontextmanager for async resource management.
• Always use with for files, locks, database connections, and any resource that needs deterministic cleanup.

Generators and Iterators

• Generators for lazy sequences. Use yield to produce values on demand instead of building full lists in memory.
• Generator expressions over list comprehensions when the result is iterated only once: sum(x * x for x in range(1000)).
• yield from to delegate to sub-generators — preserves .send(), .throw(), .close() protocol.
• itertools for composition: chain, islice, groupby, batched (3.12+), pairwise (3.10+).
• Annotate generators: def gen() -> Iterator[int]: for simple generators, Generator[YieldType, SendType, ReturnType] when using .send().
• Never exhaust a generator twice. Generators are single-pass. If you need multiple passes, materialize to a list or use itertools.tee.

Comprehensions

• List/dict/set comprehensions for simple transforms: [x.name for x in users if x.active].
• One level of nesting maximum. Two nested for clauses are the absolute limit. Beyond that, extract to a function.
• Don't use comprehensions for side effects. [print(x) for x in items] is wrong — use a for loop.
• Walrus operator in comprehensions for compute-once-filter-and-use: [y for x in data if (y := transform(x)) is not None].
• Dict comprehensions for key transformations: {k.lower(): v for k, v in headers.items()}.

Exception Handling

• Be specific. Catch the narrowest exception type: except ValueError: not except Exception:.
• Never bare except:. It catches SystemExit, KeyboardInterrupt, and GeneratorExit. At minimum use except Exception:.
• except* ExceptionGroup (3.11+) for handling multiple concurrent exceptions from TaskGroup and similar.
• Wrap with context. raise AppError("context") from err chains the original cause.
• Don't use exceptions for flow control. if key in dict: not try: dict[key] except KeyError: (unless the miss is rare and lookup is expensive).
• Custom exceptions inherit from a project-specific base that extends Exception:

  class AppError(Exception): ...
  class NotFoundError(AppError): ...
  class ValidationError(AppError): ...
  

• Error strings: lowercase, no trailing punctuation. They compose in chains: "parse config: invalid format".
• else clause runs only when no exception was raised — use for code that should execute on success but isn't part of the try body.
• finally for unconditional cleanup — prefer context managers when possible.
• Exception groups (3.11+): use ExceptionGroup to bundle multiple errors. Handle with except* which matches by type and re-raises unhandled exceptions.
• Add notes with .add_note() (3.11+) to attach context without creating new exception types.

Strings

• f-strings for interpolation. Never % formatting or .format() in new code.
• f-string expressions must be simple. No function calls with multiple arguments, no nested f-strings, no complex expressions. Extract to a variable first.
• str.removeprefix() / str.removesuffix() (3.9+) over slicing.
• Triple-quoted strings for multiline. Use textwrap.dedent when indentation matters.
• "".join(parts) for building strings in loops — never += in a loop.
• Raw strings r"..." for regex patterns and Windows paths.

Pathlib

• pathlib.Path for all filesystem operations. Never os.path in new code.
• / operator for path joining: base / "subdir" / "file.txt".
• Common operations: path.exists(), path.is_file(), path.is_dir(), path.read_text(), path.write_text(), path.mkdir(parents=True, exist_ok=True), path.iterdir(), path.glob("*.py"), path.rglob("**/*.py").
• path.resolve() for absolute paths. path.relative_to(base) for relative paths.
• Accept str | Path in public APIs, convert to Path internally.

Imports

• Absolute imports by default: from mypackage.utils import helper.
• Relative imports only within packages for tightly coupled modules: from .models import User.
• Import grouping (separated by blank lines):
  1. Standard library (import os, from pathlib import Path)
  2. Third-party (import httpx, from pydantic import BaseModel)
  3. Local (from myapp.models import User)
• Import specific names: from collections import defaultdict not import collections (unless you use many names from the module).
• Never from module import * — pollutes namespace, breaks type checkers, hides dependencies.
• if TYPE_CHECKING: block for imports used only in annotations — avoids circular imports and runtime overhead. In 3.14+ with lazy annotations, this is less necessary but still useful for avoiding circular import side effects.
• Lazy imports in function bodies when a top-level import would create a circular dependency or when the import is expensive and rarely needed.

Classes

Slots

• Always use __slots__ on classes that will have many instances — prevents __dict__ creation, saves memory, catches typos in attribute names.
• @dataclass(slots=True) adds slots automatically.
• Slots and inheritance: every class in the hierarchy must declare __slots__. Missing slots on a parent reintroduces __dict__.

Dunder Methods

• __repr__ on every class — must be unambiguous: def __repr__(self) -> str: return f"User(name={self.name!r})".
• __str__ only when a human-readable form differs from repr.
• __eq__ and __hash__ — if you define __eq__, define __hash__ too (or set __hash__ = None to make unhashable). Mutable objects should not be hashable.
• __bool__ — define when truthiness of instances has meaningful semantics.
• __enter__/__exit__ for context manager protocol.
• __init_subclass__ for class registration patterns without metaclasses.
• __class_getitem__ to make classes subscriptable for generic type hints.

Inheritance

• Composition over inheritance. Use inheritance only for true "is-a" relationships.
• ABCs for interfaces when you control both sides and need enforced implementation: from abc import ABC, abstractmethod.
• Protocols for duck typing when you don't control implementations.
• super() — always use super() (no arguments in 3.x). Never hardcode parent class names.
• MRO awareness. Understand method resolution order in diamond inheritance. When in doubt, avoid multiple inheritance.

Class Methods and Static Methods

• @classmethod for alternative constructors: User.from_dict(data).
• @staticmethod for utility functions that don't need class or instance state — but prefer module-level functions unless the function is logically part of the class's API.

Packaging and Toolchain

pyproject.toml

• pyproject.toml is the single source of truth for project metadata, dependencies, tool configuration. Never setup.py or setup.cfg in new projects.
• Build backend: use hatchling, flit-core, or setuptools with [build-system] table.
• Dependency specification: pin with >= lower bound, avoid upper bounds unless genuinely incompatible: httpx>=0.27.

uv

• uv is the preferred Python package manager and environment tool.
• uv sync to install dependencies from lock file.
• uv add <package> to add dependencies.
• uv run <command> to run commands in the project environment.
• uv lock to generate/update the lock file.
• uv venv to create virtual environments.
• uv python install 3.14 to install Python versions.

Project Layout

my-project/
├── pyproject.toml
├── uv.lock
├── src/
│   └── my_package/
│       ├── __init__.py
│       └── ...
└── tests/
    ├── conftest.py
    └── ...

• src layout — package code lives under src/. Prevents accidental imports from the project root during testing.
• __init__.py — keep minimal. Define __all__ for public API. Don't put substantial logic in init files.

Linting and Formatting

• ruff for both linting and formatting. Single tool, fast.
• ruff check to lint. ruff format to format.
• Configure in pyproject.toml under [tool.ruff].

See ${CLAUDE_SKILL_DIR}/references/packaging.md for pyproject.toml templates, uv workflows, and dependency management patterns.

Concurrency

asyncio

• async/await for I/O-bound concurrency.
• asyncio.TaskGroup (3.11+) for structured concurrency — replaces asyncio.gather() with better error handling.
• Never use asyncio.gather() in new code — it has inconsistent error semantics. Use TaskGroup instead.
• asyncio.run() as the single entry point. Never loop.run_until_complete().
• Cancel via asyncio.CancelledError — always clean up resources in finally blocks.

threading

• concurrent.futures.ThreadPoolExecutor for CPU-light I/O-bound parallel work.
• threading.Lock for shared mutable state. Always use with lock: context manager.
• GIL note: in CPython, threads don't achieve true parallelism for CPU-bound work. Use multiprocessing or ProcessPoolExecutor for CPU-bound tasks.
• Free-threaded Python (3.13+): when running with --disable-gil, standard thread-safety practices become critical. Guard all shared mutable state with locks.

General Rules

• Structured concurrency preferred. TaskGroup and context managers over bare create_task().
• Never fire-and-forget tasks or threads — always track completion.
• Cancellation must be cooperative. Check for cancellation and clean up.

Logging

• logging module over print() for anything beyond quick debugging.
• logger = logging.getLogger(__name__) at module level.
• Lazy formatting: logger.info("User %s logged in", user_id) not logger.info(f"User {user_id} logged in") — f-string evaluates even when level is disabled.
• Use appropriate levels: DEBUG for diagnostics, INFO for operational events, WARNING for degraded but working, ERROR for failures, CRITICAL for system-down.

Application

When writing Python code: apply all conventions silently — don't narrate each rule. If an existing codebase contradicts a convention, follow the codebase and flag the divergence once.

When reviewing Python code: cite the specific violation and show the fix inline. Don't lecture — state what's wrong and how to fix it.

Bad:  "According to Python best practices, you should use type unions
       with the pipe operator instead of Optional..."
Good: "Optional[str] -> str | None"

Code Navigation — LSP Required

A pyright-langserver LSP server is configured for .py and .pyi files. Always use LSP tools for code navigation instead of Grep or Glob. LSP understands Python's module system, type inference, scope rules, and package boundaries — text search does not.

Tool Routing

• goToDefinition — find where a function/class/variable is defined: resolves imports, re-exports, aliases
• findReferences — find all usages of a symbol: scope-aware, no false positives from string matches
• hover — get type signature, docs, or return types: instant type info without reading source files
• documentSymbol — list all symbols in a file: structured output vs grepping for def/class
• workspaceSymbol — find a symbol by name across the project: searches all packages, respects __all__
• goToImplementation — find implementations of a Protocol or ABC: knows the type system and structural subtyping
• incomingCalls — find what calls a function: precise call graph across module boundaries
• outgoingCalls — find what a function calls: structured dependency map

Grep/Glob remain appropriate for: text in comments, string literals, log messages, TODO markers, config values, env vars, file name patterns, URLs, error message text — anything that isn't a Python identifier.

When spawning subagents for Python codebase exploration, instruct them to use LSP tools. Subagents have access to the same LSP server.

Toolchain

• ruff: single entry point for linting and formatting. Must pass before committing.
  • ruff check — lint. ruff check --fix — auto-fix.
  • ruff format — format.
• uv: package management, virtual environments, Python version management.
• mypy or pyright: static type checking. Configure in pyproject.toml.

Integration

The coding skill governs workflow (discovery, planning, verification); this skill governs Python implementation choices. The pytest skill governs testing conventions — both are active simultaneously when writing Python tests.

Readability counts. If you read a function twice to understand it, rewrite it once to make it clear.