Explain Python decorators — how do they work, and when would you write a custom decorator?

Question

At a fintech company:

> "We need to add timing, logging, and access control to our API handlers without modifying each function. Can you implement decorators for these use cases?"

Interview OS Community · Accepted Answer

## What Are Decorators?

Decorators are **higher-order functions** that wrap another function to extend its behavior.

```python
def timer(func):
    import time
    def wrapper(*args, **kwargs):
        start = time.time()
        result = func(*args, **kwargs)
        elapsed = time.time() - start
        print(f"{func.__name__} took {elapsed:.3f}s")
        return result
    return wrapper

@timer
def fetch_data(query):
    # ... slow operation
    return results

# Equivalent to: fetch_data = timer(fetch_data)
```

## Decorators with Arguments

```python
def require_role(role):
    def decorator(func):
        def wrapper(user, *args, **kwargs):
            if user.role != role:
                raise PermissionError(f"Requires {role}")
            return func(user, *args, **kwargs)
        return wrapper
    return decorator

@require_role("admin")
def delete_user(user, user_id):
    db.delete(user_id)
```

## Class-Based Decorator (stateful)

```python
class RateLimit:
    def __init__(self, max_calls, period):
        self.max_calls = max_calls
        self.period = period
        self.calls = []

def __call__(self, func):
        def wrapper(*args, **kwargs):
            now = time.time()
            self.calls = [t for t in self.calls if now - t < self.period]
            if len(self.calls) >= self.max_calls:
                raise Exception("Rate limit exceeded")
            self.calls.append(now)
            return func(*args, **kwargs)
        return wrapper

@RateLimit(max_calls=100, period=60)
def api_endpoint():
    return {"status": "ok"}
```

## functools.wraps (preserve metadata)

```python
from functools import wraps

def logged(func):
    @wraps(func)  # Preserves __name__, __doc__, etc.
    def wrapper(*args, **kwargs):
        print(f"Calling {func.__name__}")
        return func(*args, **kwargs)
    return wrapper
```

| Pattern | Use Case |
|---------|----------|
| Simple decorator | Logging, timing, caching |
| Decorator with args | Role-based access, config |
| Class decorator | Stateful (rate limiting, retry) |
| `@wraps` | Always use to preserve function identity |

Interview OS

Explain Python decorators — how do they work, and when would you write a custom decorator?

Question Details

Suggested Solution

What Are Decorators?

Equivalent to: fetchdata = timer(fetchdata)

Decorators with Arguments

Class-Based Decorator (stateful)

functools.wraps (preserve metadata)

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