## Decorators

### Function Decorators

```python
import functools
import time

def timer(func: Callable) -> Callable:
    """Decorator to time function execution."""
    @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

@timer
def slow_function():
    time.sleep(1)

# slow_function() prints: slow_function took 1.0012s
```

### Parameterized Decorators

```python
def repeat(times: int):
    """Decorator to repeat a function multiple times."""
    def decorator(func: Callable) -> Callable:
        @functools.wraps(func)
        def wrapper(*args, **kwargs):
            results = []
            for _ in range(times):
                results.append(func(*args, **kwargs))
            return results
        return wrapper
    return decorator

@repeat(times=3)
def greet(name: str) -> str:
    return f"Hello, {name}!"

# greet("Alice") returns ["Hello, Alice!", "Hello, Alice!", "Hello, Alice!"]
```

### Class-Based Decorators

```python
class CountCalls:
    """Decorator that counts how many times a function is called."""
    def __init__(self, func: Callable):
        functools.update_wrapper(self, func)
        self.func = func
        self.count = 0

    def __call__(self, *args, **kwargs):
        self.count += 1
        print(f"{self.func.__name__} has been called {self.count} times")
        return self.func(*args, **kwargs)

@CountCalls
def process():
    pass

# Each call to process() prints the call count
```