decorators.md

Python by Example: Decorators

Decorators wrap a function to add behavior before or after it runs. Write @decorator above a function definition to apply it. Under the hood, @log is shorthand for greet = log(greet). Decorators are used everywhere—logging, timing, authentication, caching.

What you'll learn:

• Defining and applying a decorator
• Preserving arguments with *args, **kwargs
• Decorators that accept their own arguments

def log(func):
    def wrapper(*args, **kwargs):
        print(f"Calling {func.__name__}")
        result = func(*args, **kwargs)
        print(f"Done {func.__name__}")
        return result
    return wrapper


@log
def greet(name):
    print(f"Hello, {name}!")


greet("Alice")


def repeat(n):
    def decorator(func):
        def wrapper(*args, **kwargs):
            for _ in range(n):
                func(*args, **kwargs)
        return wrapper
    return decorator


@repeat(3)
def say_hi():
    print("hi")


say_hi()

log takes a function and returns wrapper, which calls the original. *args, **kwargs passes any arguments through unchanged. repeat(3) returns a decorator—an extra layer of nesting handles the argument.

To run this program:

$ python source/decorators.py
Calling greet
Hello, Alice!
Done greet
hi
hi
hi

Tip: Use functools.wraps(func) inside wrapper to preserve the original function's name and docstring.

Try it: Write a decorator that measures and prints how long a function takes to run.

Source: decorators.py

Next: Range and Enumerate