Using python decorator with or without parentheses

some_decorator in the first code snippet is a regular decorator:

@some_decorator
def some_method():
    pass

is equivalent to

some_method = some_decorator(some_method)

On the other hand, some_decorator in the second code snippet is a callable that returns a decorator:

@some_decorator()
def some_method():
    pass

is equivalent to

some_method = some_decorator()(some_method)

As pointed out by Duncan in comments, some decorators are designed to work both ways. Here's a pretty basic implementation of such decorator:

def some_decorator(arg=None):
    def decorator(func):
        def wrapper(*a, **ka):
            return func(*a, **ka)
        return wrapper

    if callable(arg):
        return decorator(arg) # return 'wrapper'
    else:
        return decorator # ... or 'decorator'

pytest.fixture is a more complex example.

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(*args, **kwargs):
        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.