Making the instance callable

You already have a class D that can be used with the at sign @ syntax of a decorator:

class D:
    def __init__(self, fn):
        self.fn = fn

@D
def greet(who):
    print(f"Hello, {who}!")

However, the class D isn’t a great decorator yet, because it doesn’t even let you use the function! Since the function greet is supposed to greet someone, you’d expect to be able to use it after it being decorated. But you get an exception if you try to use your “function”:

greet("world")
# Exception raised:
Traceback (most recent call last):
  File "<python-input-9>", line 1, in <module>
    greet("world")
    ~~~~~^^^^^^^^^
TypeError: 'D' object is not callable

The problem is that the original function greet was replaced by an instance of D and this instance of D doesn’t look enough like the original function, since you can’t use the instance as a function yet.

To fix this, you must implement the dunder method __call__! If a class implements __call__, its instances will be callable. In this case, you have to forward the arguments given to the instance to the underlying function that was decorated:

class D:
    def __init__(self, fn):
        self.fn = fn

    def __call__(self, *args, **kwargs):
        return self.fn(*args, **kwargs)

With this implementation of D.__call__, the class D is now a suitable do-nothing decorator:

@D
def greet(who):
    print(f"Hello, {who}!")


greet("world")  # Hello, world!