Using python decorator with or without parentheses

Question

In Python, what is the difference between using the same decorator with and without parentheses?

For example:

Without parentheses:

@some         


        

Answer 1

Briefly speaking, decorators allow adding rich features to groups of functions and classes without modifying them at all.

The key to understand the difference between @some_decorator and @some_decorator() is that the former is decorator, while the latter is a function (or callable) that returns a decorator.

I believe that seeing an implementation of each case facilitates understanding the difference:

@some_decorator

def some_decorator(func):
    def wrapper(func):
        return func(*args, **kwargs)
    return wrapper

Application:

@some_decorator
def some_method():
    pass

Equivalence:

some_method = some_decorator(some_method)

@some_decorator()

def some_decorator():
    def decorator(func):
        def wrapper(*args, **kwargs):
            return func(*args, **kwargs)
        return wrapper
    return decorator

Application:

@some_decorator()
def some_method():
    pass

Equivalence:

some_method = some_decorator()(some_method)

Notice that now it is easier to see that @some_decorator() is a function returning a decorator while some_decorator is just a decorator. Keep in mind that some decorators are written to work both ways.

So now you might be wondering why we have these two cases when the former version seems simpler. The answer is that if you want to pass arguments to a decorator, using @some_decorator() will allow you to do this. Let's see some code in action:

def some_decorator(arg1, arg2):
    def decorator(func):
        def wrapper(*args, **kwargs):
            print(arg1)
            print(arg2)
            return func(*args, **kwargs)
        return wrapper
    return decorator

Application:

@some_decorator('hello', 'bye')
def some_method():
    pass

Equivalence:

some_method = some_decorator('hello', 'bye')(some_method)

Note: I think that it is worth to mention that a decorator can be implemented as a function or as a class. Check this for more information.