Python Best Practices Skill for Claude

by aiskillstore

Use when reading or writing Python files (.py, pyproject.toml, requirements.txt).

7k tokens

context cost

the whole folder, loaded on every use

files

instructions only

copies elsewhere

how many repositories repackaged it

404

stars on the repo

on the repository, not the skill itself

Install

one command, takes just this skill from the repository

npx skills add https://github.com/aiskillstore/marketplace --skill python-best-practices

What comes with it

26 396 bytes besides the instruction

skill-report.json

The instruction itself

5 sections, as written by the author

Python Best Practices

Follows type-first, functional, and error handling patterns from CLAUDE.md. This skill covers language-specific idioms only.

Make Illegal States Unrepresentable

Use Python's type system to prevent invalid states at type-check time.

Frozen dataclasses for immutable domain models:

from dataclasses import dataclass
from datetime import datetime

@dataclass(frozen=True)
class User:
    id: str
    email: str
    name: str
    created_at: datetime

# Frozen dataclasses are immutable — no accidental mutation

Discriminated unions with Literal:

from dataclasses import dataclass
from typing import Literal

@dataclass
class Success:
    status: Literal["success"] = "success"
    data: str

@dataclass
class Failure:
    status: Literal["error"] = "error"
    error: Exception

RequestState = Success | Failure

def handle_state(state: RequestState) -> None:
    match state:
        case Success(data=data):
            render(data)
        case Failure(error=err):
            show_error(err)

NewType for domain primitives:

from typing import NewType

UserId = NewType("UserId", str)
OrderId = NewType("OrderId", str)

def get_user(user_id: UserId) -> User:
    # Type checker prevents passing OrderId here
    ...

Protocol for structural typing:

from typing import Protocol

class Readable(Protocol):
    def read(self, n: int = -1) -> bytes: ...

def process_input(source: Readable) -> bytes:
    # Accepts any object with a read() method — no inheritance required
    return source.read()

Python-Specific Error Handling

Chain exceptions with from err to preserve the original traceback:

try:
    data = json.loads(raw)
except json.JSONDecodeError as err:
    raise ValueError(f"invalid JSON payload: {err}") from err

Structured Logging

Use a module-level logger with %s formatting (deferred string interpolation):

import logging

logger = logging.getLogger("myapp.widgets")

def create_widget(name: str) -> Widget:
    logger.debug("creating widget: %s", name)
    widget = Widget(name=name)
    logger.debug("created widget id=%s", widget.id)
    return widget

Optional: ty

For fast type checking, consider ty from Astral (creators of ruff and uv). Written in Rust, significantly faster than mypy or pyright.

uvx ty check          # run directly, no install needed
uvx ty check src/     # check specific path

# pyproject.toml
[tool.ty]
python-version = "3.12"

When to choose:

ty — fastest, good for CI and large codebases (early stage, rapidly evolving)
pyright — most complete type inference, VS Code integration
mypy — mature, extensive plugin ecosystem

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How to use it

Copy the folder

Take aiskillstore/python-best-practices from the repository into ~/.claude/skills for personal use, or into .claude/skills inside a project.

Check the name does not clash

The agent identifies a skill by the name field in its header. Two skills with the same name cannot sit side by side — one of them will be ignored.

Install what it needs

The instructions reference uvx. Without those the skill loads but fails at the first command.