Decorators intro

A decorator wraps a function or class to extend behavior—syntax @decorator above def. Functions are first-class objects in Python, enabling higher-order patterns like middleware in the Django track.

Function decorator

def log_calls(fn):
    def wrapper(*args, **kwargs):
        print(f"calling {fn.__name__}")
        return fn(*args, **kwargs)
    return wrapper

@log_calls
def add(a, b):
    return a + b

functools.wraps

from functools import wraps

def deco(fn):
    @wraps(fn)
    def wrapper(*args, **kwargs):
        return fn(*args, **kwargs)
    return wrapper

@wraps preserves metadata (__name__, docstring) on the wrapper—important for debugging and introspection.

Important interview questions and answers

1. Q: What does @decorator do?
   A: Rebinds the name to decorator(original_function)—syntactic sugar for wrapper assignment.
2. Q: Decorator with arguments?
   A: Requires an outer factory returning the actual decorator—three nested functions.

Self-check

1. Why use functools.wraps?
2. Are decorators compile-time or runtime?

Tip: Use @wraps(fn) in decorators—preserves __name__ for pytest and debugging.