How to create a Python decorator that can be used either with or without parameters?

Question

I\'d like to create a Python decorator that can be used either with parameters:

@redirect_output(\"somewhere.log\")
def foo():
    ....

or

Answer 1

I know this is an old question, but I really don't like any of the techniques proposed so I wanted to add another method. I saw that django uses a really clean method in their login_required decorator in django.contrib.auth.decorators. As you can see in the decorator's docs, it can be used alone as @login_required or with arguments, @login_required(redirect_field_name='my_redirect_field').

The way they do it is quite simple. They add a kwarg (function=None) before their decorator arguments. If the decorator is used alone, function will be the actual function it is decorating, whereas if it is called with arguments, function will be None.

Example:

from functools import wraps

def custom_decorator(function=None, some_arg=None, some_other_arg=None):
    def actual_decorator(f):
        @wraps(f)
        def wrapper(*args, **kwargs):
            # Do stuff with args here...
            if some_arg:
                print(some_arg)
            if some_other_arg:
                print(some_other_arg)
            return f(*args, **kwargs)
        return wrapper
    if function:
        return actual_decorator(function)
    return actual_decorator

---

@custom_decorator
def test1():
    print('test1')

>>> test1()
test1

---

@custom_decorator(some_arg='hello')
def test2():
    print('test2')

>>> test2()
hello
test2

---

@custom_decorator(some_arg='hello', some_other_arg='world')
def test3():
    print('test3')

>>> test3()
hello
world
test3

I find this approach that django uses to be more elegant and easier to understand than any of the other techniques proposed here.