def process_items( items: list[str], transform: Optional[callable] = None, ) -> list[str]: if transform: return [transform(item) for item in items] return items


## Core Concepts

### Type Hints

```python
from typing import TypeVar, Generic
from collections.abc import Callable, Iterator

def process_items(
    items: list[str],
    transform: Callable[[str], str] | None = None,
) -> list[str]:
    if transform:
        return [transform(item) for item in items]
    return items

# Use TypeVar for generics
T = TypeVar('T')

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

Dataclasses & Pydantic

from dataclasses import dataclass, field
from pydantic import BaseModel, Field

# Dataclass for simple data containers
@dataclass
class User:
    name: str
    email: str
    tags: list[str] = field(default_factory=list)

# Pydantic for validation
class UserCreate(BaseModel):
    name: str = Field(..., min_length=1, max_length=100)
    email: str = Field(..., pattern=r'^[\w\.-]+@[\w\.-]+\.\w+$')

Common Patterns

Async Patterns

import asyncio

async def fetch_all(urls: list[str]) -> list[Response]:
    async with aiohttp.ClientSession() as session:
        tasks = [fetch_one(session, url) for url in urls]
        return await asyncio.gather(*tasks)

# Use TaskGroup for structured concurrency (3.11+)
async def process_batch(items: list[Item]) -> list[Result]:
    results = []
    async with asyncio.TaskGroup() as tg:
        for item in items:
            tg.create_task(process_item(item, results))
    return results

Context Managers

from contextlib import contextmanager, asynccontextmanager

@contextmanager
def managed_resource() -> Iterator[Resource]:
    resource = Resource()
    try:
        yield resource
    finally:
        resource.cleanup()

Best Practices

Do:

Use type hints for function signatures
Use pyproject.toml for project configuration
Use virtual environments (venv, poetry)
Use generators for large datasets

Don't:

Use mutable default arguments (def f(x=[])
Use import * (pollutes namespace)
Catch bare except: (catch specific exceptions)
Use assert for input validation

Troubleshooting

Error	Cause	Solution
`ModuleNotFoundError`	Package not installed	Run `pip install package`
`IndentationError`	Mixed tabs/spaces	Use consistent 4-space indentation
`TypeError: unhashable type`	Using list as dict key	Use tuple instead

Dataclasses & Pydantic

Common Patterns

Async Patterns

Context Managers

Best Practices

Troubleshooting

References

python

Python

When to Use

Quick Start