Question

So I’m still kind of new to Python decorators – I’ve used them before, but I’ve never made my own. I’m reading this tutorial (that particular paragraph) and I don’t seem to understand why do we need three levels of functions? Why can’t we do something like this:

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

Thanks 🙂

Answer 1

Well, what would happen if you called that decorator on a function?

@decorator
def foo(): pass

This code would immediately call foo, which we don’t want. Decorators are called and their return value replaces the function. It’s the same as saying

def foo(): pass
foo = decorator(foo)

So if we have a decorator that calls foo, we probably want to have a function that returns a function that calls foo — that function that it returns will replace foo.

def decorator(f):
    def g(*args, **kwargs):
        return f(*args, **kwargs)
    return g

Now, if we want to pass options to the decorator, we can’t exactly pass them ins ide-by-side with the function like in your example. There’s no syntax for it. So we define a function that returns a parameterized decorator. The decorator it returns will be a closure.

def argument_decorator(will_I_call_f):
    def decorator(f):
        def g(*args, **kwargs):
            if will_I_call_f: return f(*args, **kwargs)
        return g
    return decorator

so we can do

decorator = argument_decorator(True)
@decorator
def foo(): pass

And Python offers the convenience syntax where you inline the function call:

@argument_decorator(True)
def foo(): pass

And all this is syntax sugar for the non-decorator syntax of

def foo(): pass
foo = argument_decorator(True)(foo)