What is a decorator and how to use it?

Question

Accepted Answer

**Decorator** is a function that **wraps another function** to extend its functionality without changing the code.

**Simple decorator:**
```python
def timer(func):
    import time
    def wrapper(*args, **kwargs):
        start = time.time()
        result = func(*args, **kwargs)
        end = time.time()
        print(f"{func.__name__} took {end - start:.2f}s")
        return result
    return wrapper

@timer
def slow_function():
    time.sleep(1)
    return "Done"

slow_function()  # "slow_function took 1.00s"
```

**Decorator with arguments:**
```python
def repeat(n):
    def decorator(func):
        def wrapper(*args, **kwargs):
            for _ in range(n):
                result = func(*args, **kwargs)
            return result
        return wrapper
    return decorator

@repeat(3)
def say_hello():
    print("Hello!")
```

**Popular decorators:**
- `@staticmethod`, `@classmethod`.
- `@property` — getter/setter.
- `@lru_cache` — caching.
- `@login_required` (Django).

Python: What is a decorator and how to use it?