Performance

Memory Optimization

Generators for Large Data

from collections.abc import Iterator

# Bad - loads entire file into memory
def read_all_lines(filepath: str) -> list[str]:
    with open(filepath) as f:
        return f.readlines()

# Good - yields one line at a time
def read_lines(filepath: str) -> Iterator[str]:
    with open(filepath) as f:
        for line in f:
            yield line.strip()

slots and Data Classes

from dataclasses import dataclass

@dataclass(slots=True)
class User:
    id: int
    name: str
    email: str

Bulk Operations

# Bad - N individual inserts
for user in users:
    db.execute("INSERT INTO users (name) VALUES (?)", (user.name,))

# Good - single bulk insert
db.executemany(
    "INSERT INTO users (name) VALUES (?)",
    [(u.name,) for u in users]
)

Async/Concurrent Processing

import asyncio
import httpx

async def fetch_all(urls: list[str]) -> list[dict]:
    async with httpx.AsyncClient() as client:
        tasks = [client.get(url) for url in urls]
        responses = await asyncio.gather(*tasks)
        return [r.json() for r in responses]

# With concurrency limit
async def fetch_with_limit(urls: list[str], limit: int = 10) -> list[dict]:
    semaphore = asyncio.Semaphore(limit)

    async def fetch_one(url: str) -> dict:
        async with semaphore:
            async with httpx.AsyncClient() as client:
                response = await client.get(url)
                return response.json()

    return await asyncio.gather(*[fetch_one(url) for url in urls])

Database Optimization

Avoid N+1 Queries

# Bad - N+1 queries
users = session.query(User).all()
for user in users:
    print(user.posts)  # Each access triggers a query

# Good - eager loading
users = session.query(User).options(joinedload(User.posts)).all()

Cursor Pagination

def get_users_cursor(
    session: Session,
    after_id: int | None = None,
    limit: int = 20,
) -> list[User]:
    stmt = select(User).order_by(User.id).limit(limit)
    if after_id:
        stmt = stmt.where(User.id > after_id)
    return list(session.scalars(stmt))

Lazy Loading

from functools import cached_property

class DataProcessor:
    def __init__(self, config_path: str) -> None:
        self._config_path = config_path

    @cached_property
    def config(self) -> dict:
        return load_config(self._config_path)

Quick Reference

See Also

• conventions — general coding conventions