Agent Skills: Python Development (3.12+)

Python Development (3.12+)

Core Philosophy

1. Stdlib and Mature Libraries First

   • Always prefer Python stdlib solutions
   • External deps only when stdlib insufficient
   • Prefer dataclasses over attrs, pathlib over os.path

2. Type Hints Everywhere (No Any)

   • Use X | Y union syntax (3.10+), PEP 695 generics (3.12+)
   • Use Protocol for structural typing (duck typing)
   • Avoid Any—use concrete types or generics

3. Protocol Over ABC

   • Protocol for implicit interface satisfaction
   • ABC only when runtime isinstance() needed

4. Flat Control Flow

   • Guard clauses with early returns
   • Pattern matching to flatten conditionals
   • Maximum 2 levels of nesting

5. Explicit Error Handling

   • Custom exception hierarchy for domain errors
   • Raise early, handle at boundaries
   • Specific exception types (never bare except Exception)

6. Structured Logging

   • Use structlog for structured, contextualized logging
   • Never use print() for operational output

Quick Patterns

Protocol-Based Dependency Injection

from typing import Protocol

class UserStore(Protocol):
    def get(self, id: str) -> User | None: ...
    def save(self, user: User) -> None: ...

class UserService:
    def __init__(self, store: UserStore):
        self.store = store  # accepts any matching impl

PEP 695 Generics (3.12+)

# NEW: type parameter syntax (no TypeVar boilerplate)
def first[T](items: list[T]) -> T | None:
    return items[0] if items else None

type Vector = list[float]  # type alias statement

Dataclasses with Performance

from dataclasses import dataclass

@dataclass(frozen=True, slots=True)
class StatusUpdate:
    raw_text: str
    display_label: str
    is_interactive: bool = False

Flat Control Flow (No Nesting)

def process(user: User | None) -> Result:
    if user is None:
        raise ValueError("user required")
    if not user.email:
        raise ValueError("email required")
    if not is_valid_email(user.email):
        raise ValueError("invalid email")
    return do_work(user)  # happy path at end

Structured Logging

import structlog
logger = structlog.get_logger()

logger.info("processing_started", user_id=user.id, count=len(items))
logger.error("operation_failed", error=str(e), context=ctx)

Error Handling

class NotFoundError(AppError):
    def __init__(self, resource: str, id: str):
        self.resource = resource
        self.id = id
        super().__init__(f"{resource} not found: {id}")

# Exception chaining
raise ProcessingError("failed") from original_error

Python 3.12+ Features (Adopt Now)

• PEP 695 type params: def first[T](lst: list[T]) -> T (no TypeVar)
• F-string improvements: nested quotes, multiline expressions
• Pattern matching: structural pattern matching (3.10+)

Python 3.14 Features (When Targeting 3.14)

• Deferred annotations: No from __future__ import annotations needed
• Template strings (t""): t"Hello {name}" returns Template (safe interpolation)
• except without parens: except ValueError, TypeError: (PEP 758)
• concurrent.interpreters: True multi-core parallelism
• compression.zstd: Zstandard in stdlib

References

• PATTERNS.md - Code patterns and style
• CLI.md - CLI application patterns (Click)
• TESTING.md - Testing with pytest

Tooling

uv sync                              # Install deps from pyproject.toml
uv add pkg                           # Add dependency
uv run python script.py              # Run in project env
uv run --with pkg python script.py   # Run with one-off dep
uv run --extra dev pytest -v         # Run tests (dev group)
ruff check --fix .                   # Lint and autofix
ruff format .                        # Format
pyright src/                         # Type check (preferred)
deptry src                           # Dependency check

Makefile Targets (Convention)

make fmt          # ruff format
make lint         # ruff check
make typecheck    # pyright
make deptry       # dependency check
make test         # unit tests only
make check        # fmt + lint + typecheck + deptry + test

Verify Generated Code

After generating code, always verify it passes checks:

ruff check . && ruff format --check . && pyright