5.3Closures

A closure captures variables from its enclosing scope — useful for factory functions that create pre-configured utilities.

API Client Factory

def make_api_client(base_url: str, token: str):
    """Creates a pre-configured API client."""
    def request(method: str, path: str, **kwargs):
        headers = {"Authorization": f"Bearer {token}"}
        headers.update(kwargs.pop("headers", {}))
        return requests.request(method, f"{base_url}{path}", headers=headers, **kwargs)
    return request

# Usage: create clients for different environments
staging_api = make_api_client("https://api.staging.example.com", os.environ["STAGING_TOKEN"])
prod_api = make_api_client("https://api.example.com", os.environ["PROD_TOKEN"])

# Both use the same interface
staging_response = staging_api("GET", "/users/me")
prod_response = prod_api("GET", "/users/me")

Retry Wrapper

def with_retry(max_attempts: int = 3, delay: float = 1.0):
    """Creates a retry wrapper for flaky operations."""
    def decorator(func):
        def wrapper(*args, **kwargs):
            last_exception = None
            for attempt in range(max_attempts):
                try:
                    return func(*args, **kwargs)
                except Exception as e:
                    last_exception = e
                    if attempt < max_attempts - 1:
                        time.sleep(delay * (attempt + 1))
            raise last_exception
        return wrapper
    return decorator

@with_retry(max_attempts=3, delay=0.5)
def fetch_user(user_id: str):
    response = requests.get(f"{base_url}/users/{user_id}")
    response.raise_for_status()
    return response.json()

TypeScript Closures

function makeApiClient(baseUrl: string, token: string) {
    return async (method: string, path: string, body?: object) => {
        const response = await fetch(`${baseUrl}${path}`, {
            method,
            headers: {
                Authorization: `Bearer ${token}`,
                "Content-Type": "application/json",
            },
            body: body ? JSON.stringify(body) : undefined,
        });
        return response;
    };
}

const api = makeApiClient("https://api.staging.example.com", process.env.TOKEN!);
const response = await api("GET", "/users/me");