Python Patterns

Python Development Patterns

Idiomatic Python patterns and best practices for building robust, efficient, and maintainable applications.

Full code examples: references/code-examples.md

When to Activate

• Writing new Python code
• Reviewing Python code
• Refactoring existing Python code
• Designing Python packages/modules

Core Principles

1. Readability Counts

Python prioritizes readability. Code should be obvious and easy to understand.

# Good: Clear and readable
def get_active_users(users: list[User]) -> list[User]:
    return [user for user in users if user.is_active]

2. Explicit is Better Than Implicit

Avoid magic; be clear about what your code does.

3. EAFP — Easier to Ask Forgiveness Than Permission

Python prefers exception handling over checking conditions.

# Good: EAFP style
try:
    return dictionary[key]
except KeyError:
    return default_value

Type Hints

Modern Type Hints (Python 3.9+)

Use built-in types directly. Fall back to typing module for Python 3.8.

# Python 3.9+
def process_items(items: list[str]) -> dict[str, int]:
    return {item: len(item) for item in items}

Type Aliases and Generics

from typing import TypeVar, Union

JSON = Union[dict[str, Any], list[Any], str, int, float, bool, None]
T = TypeVar('T')

def first(items: list[T]) -> T | None:
    return items[0] if items else None

Protocol-Based Duck Typing

from typing import Protocol

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

def render_all(items: list[Renderable]) -> str:
    return "\n".join(item.render() for item in items)

Error Handling

• Catch specific exceptions, not bare except:
• Chain exceptions with raise ... from e to preserve tracebacks
• Build custom exception hierarchies rooted in a single base class

def load_config(path: str) -> Config:
    try:
        with open(path) as f:
            return Config.from_json(f.read())
    except FileNotFoundError as e:
        raise ConfigError(f"Config file not found: {path}") from e
    except json.JSONDecodeError as e:
        raise ConfigError(f"Invalid JSON in config: {path}") from e

Context Managers

Use with for all resource management. Create custom context managers with @contextmanager or __enter__/__exit__.

from contextlib import contextmanager

@contextmanager
def timer(name: str):
    start = time.perf_counter()
    yield
    elapsed = time.perf_counter() - start
    print(f"{name} took {elapsed:.4f} seconds")

Comprehensions and Generators

• List comprehensions: for simple transformations
• Generator expressions: for lazy evaluation (avoid creating large intermediate lists)
• Generator functions: for streaming large datasets

# Generator for lazy evaluation — avoids building full list in memory
total = sum(x * x for x in range(1_000_000))

Keep comprehensions simple — if they need multiple conditions or nested loops, use a function instead.

Data Classes and Named Tuples

• Use @dataclass for data containers with auto-generated methods
• Use __post_init__ for validation
• Use NamedTuple for immutable data

from dataclasses import dataclass, field

@dataclass
class User:
    id: str
    name: str
    email: str
    created_at: datetime = field(default_factory=datetime.now)
    is_active: bool = True

Decorators

• Use @functools.wraps to preserve function metadata
• Parameterized decorators need a three-level nesting pattern
• Class-based decorators are useful for stateful decoration

import functools

def timer(func: Callable) -> Callable:
    @functools.wraps(func)
    def wrapper(*args, **kwargs):
        start = time.perf_counter()
        result = func(*args, **kwargs)
        elapsed = time.perf_counter() - start
        print(f"{func.__name__} took {elapsed:.4f}s")
        return result
    return wrapper

Concurrency Patterns

# Threading for I/O-bound tasks
with concurrent.futures.ThreadPoolExecutor(max_workers=10) as executor:
    future_to_url = {executor.submit(fetch_url, url): url for url in urls}
    for future in concurrent.futures.as_completed(future_to_url):
        url = future_to_url[future]
        results[url] = future.result()

Package Organization

Standard Layout

myproject/
├── src/mypackage/
│   ├── __init__.py
│   ├── main.py
│   ├── api/
│   ├── models/
│   └── utils/
├── tests/
├── pyproject.toml
└── README.md

Import Order

1. Standard library
2. Third-party packages
3. Local modules

Use isort for automatic sorting.

__init__.py Exports

__version__ = "1.0.0"
from mypackage.models import User, Post
from mypackage.utils import format_name
__all__ = ["User", "Post", "format_name"]

Memory and Performance

• __slots__: reduces memory for classes with many instances
• Generators: yield items one at a time for large datasets
• str.join(): O(n) vs O(n²) for string concatenation in loops
• pathlib.Path: prefer over os.path for path operations

Python Tooling

Essential Commands

# Formatting
black .
isort .

# Linting
ruff check .

# Type checking
mypy .

# Testing
pytest --cov=mypackage --cov-report=html

# Security
bandit -r .
pip-audit

pyproject.toml Configuration

[project]
name = "mypackage"
version = "1.0.0"
requires-python = ">=3.9"
dependencies = ["requests>=2.31.0", "pydantic>=2.0.0"]

[project.optional-dependencies]
dev = ["pytest>=7.4.0", "black>=23.0.0", "ruff>=0.1.0", "mypy>=1.5.0"]

[tool.black]
line-length = 88
target-version = ['py39']

[tool.ruff]
line-length = 88
select = ["E", "F", "I", "N", "W"]

[tool.mypy]
python_version = "3.9"
warn_return_any = true
disallow_untyped_defs = true

[tool.pytest.ini_options]
testpaths = ["tests"]
addopts = "--cov=mypackage --cov-report=term-missing"

Quick Reference: Python Idioms

Anti-Patterns

Python code should be readable, explicit, and follow the principle of least surprise. When in doubt, prioritize clarity over cleverness.