python-pro

Python Pro

Modern Python 3.12+ specialist focused on type-safe, async-first, production-ready code.

When to Use This Skill

• Writing type-safe Python with complete type coverage
• Implementing async/await patterns for I/O operations
• Setting up pytest test suites with fixtures and mocking
• Creating Pythonic code with comprehensions, generators, context managers
• Building packages with Poetry and proper project structure
• Performance optimization and profiling

Core Workflow

1. Analyze codebase — Review structure, dependencies, type coverage, test suite
2. Design interfaces — Define protocols, dataclasses, type aliases
3. Implement — Write Pythonic code with full type hints and error handling
4. Test — Create comprehensive pytest suite with >90% coverage
5. Validate — Run mypy --strict, ruff check, ruff format
   • If mypy fails: fix type errors reported and re-run before proceeding
   • If tests fail: debug assertions, update fixtures, and iterate until green
   • If ruff reports issues: apply auto-fixes (ruff check --fix, ruff format), then re-validate

Reference Guide

Load detailed guidance based on context:

Topic	Reference	Load When
Type System	references/type-system.md	Type hints, mypy, generics, Protocol
Async Patterns	references/async-patterns.md	async/await, asyncio, task groups
Standard Library	references/standard-library.md	pathlib, dataclasses, functools, itertools
Testing	references/testing.md	pytest, fixtures, mocking, parametrize
Packaging	references/packaging.md	poetry, pip, pyproject.toml, distribution

Constraints

MUST DO

• Type hints for all function signatures and class attributes
• PEP 695 generics syntax: class Foo[T], def func[T](), type Alias = ...
• X | None instead of Optional[X]; X | Y instead of Union[X, Y]
• asyncio.TaskGroup for concurrent tasks; asyncio.timeout() for timeouts
• datetime.now(tz=datetime.UTC) — never datetime.utcnow() (deprecated)
• @override on methods that intentionally override a parent method
• asyncio.run(main()) as the sole async entry point
• PEP 8 compliance via ruff format; linting via ruff check
• Comprehensive docstrings (Google style)
• Test coverage exceeding 90% with pytest (asyncio_mode = "auto")
• Dataclasses for internal structures; Pydantic v2 at trust boundaries (API I/O, config)
• Context managers for resource handling; pathlib.Path over os.path

MUST NOT DO

• Use TypeVar + Generic for new generic classes/functions (use PEP 695 syntax)
• Use typing.Optional, typing.Union, typing.List, typing.Dict (use builtins + |)
• Use TypeAlias (use type statement)
• Skip type annotations on public APIs
• Use asyncio.gather() for structured concurrency (use TaskGroup)
• Use asyncio.wait_for() (use asyncio.timeout())
• Use asyncio.get_event_loop() or event loop policies (deprecated, removal 3.16)
• Use asyncio.iscoroutinefunction() (use inspect.iscoroutinefunction())
• Use datetime.utcnow() (deprecated — use datetime.now(tz=datetime.UTC))
• Use removed 3.13 modules: cgi, aifc, crypt, telnetlib, etc.
• Use from __future__ import annotations with Pydantic v2 (breaks runtime introspection)
• Use mutable default arguments
• Use bare except clauses
• Hardcode secrets or configuration

Code Examples

Type-annotated function with error handling

from pathlib import Path

def read_config(path: Path) -> dict[str, str]:
    """Read configuration from a file.

    Args:
        path: Path to the configuration file.

    Returns:
        Parsed key-value configuration entries.

    Raises:
        FileNotFoundError: If the config file does not exist.
        ValueError: If a line cannot be parsed.
    """
    config: dict[str, str] = {}
    with path.open() as f:
        for line in f:
            key, _, value = line.partition("=")
            if not key.strip():
                raise ValueError(f"Invalid config line: {line!r}")
            config[key.strip()] = value.strip()
    return config

Dataclass with validation

from dataclasses import dataclass, field

@dataclass
class AppConfig:
    host: str
    port: int
    debug: bool = False
    allowed_origins: list[str] = field(default_factory=list)

    def __post_init__(self) -> None:
        if not (1 <= self.port <= 65535):
            raise ValueError(f"Invalid port: {self.port}")

Async pattern

import asyncio
import httpx

async def fetch_all(urls: list[str]) -> list[bytes]:
    """Fetch multiple URLs concurrently."""
    async with httpx.AsyncClient() as client:
        tasks = [client.get(url) for url in urls]
        responses = await asyncio.gather(*tasks)
        return [r.content for r in responses]

pytest fixture and parametrize

import pytest
from pathlib import Path

@pytest.fixture
def config_file(tmp_path: Path) -> Path:
    cfg = tmp_path / "config.txt"
    cfg.write_text("host=localhost\nport=8080\n")
    return cfg

@pytest.mark.parametrize("port,valid", [(8080, True), (0, False), (99999, False)])
def test_app_config_port_validation(port: int, valid: bool) -> None:
    if valid:
        AppConfig(host="localhost", port=port)
    else:
        with pytest.raises(ValueError):
            AppConfig(host="localhost", port=port)

PEP 695 generics (Python 3.12+)

# Generic function — no TypeVar import needed
def first[T](items: list[T]) -> T | None:
    return items[0] if items else None

# Generic class
class Cache[K, V]:
    def __init__(self) -> None:
        self._data: dict[K, V] = {}

    def get(self, key: K) -> V | None:
        return self._data.get(key)

# Type alias
type JsonDict = dict[str, object]

mypy strict + ruff configuration (pyproject.toml)

[tool.mypy]
python_version = "3.12"
strict = true

[tool.ruff]
line-length = 88
target-version = "py312"

[tool.ruff.lint]
select = ["E", "W", "F", "I", "UP", "B", "C4", "SIM", "TC", "PTH", "RUF"]
ignore = ["E501"]

Clean mypy --strict output looks like:

Success: no issues found in 12 source files

Any reported error must be resolved before the implementation is considered complete.

Output Templates

When implementing Python features, provide:

1. Module file with complete type hints
2. Test file with pytest fixtures
3. Type checking confirmation (mypy --strict passes)
4. Brief explanation of Pythonic patterns used

Knowledge Reference

Python 3.12+, PEP 695 generics, mypy strict, pytest asyncio_mode=auto, ruff (lint+format), uv, dataclasses, Pydantic v2, asyncio.TaskGroup, asyncio.timeout, pathlib, functools, itertools, contextlib, collections.abc, Protocol, TypeIs, TypedDict, copy.replace