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Decorators with Arguments

Метки: python

The decorator mechanism behaves quite differently when you pass arguments to the decorator.

Let’s modify the above example to see what happens when we add arguments to the decorator:

# PythonDecorators/decorator_with_arguments.py
class decorator_with_arguments(object):

    def __init__(self, arg1, arg2, arg3):
        """
        If there are decorator arguments, the function
        to be decorated is not passed to the constructor!
        """
        print("Inside __init__()")
        self.arg1 = arg1
        self.arg2 = arg2
        self.arg3 = arg3

    def __call__(self, f):
        """
        If there are decorator arguments, __call__() is only called
        once, as part of the decoration process! You can only give
        it a single argument, which is the function object.
        """
        print("Inside __call__()")
        def wrapped_f(*args):
            print("Inside wrapped_f()")
            print("Decorator arguments:", self.arg1, self.arg2, self.arg3)
            f(*args)
            print("After f(*args)")
        return wrapped_f

@decorator_with_arguments("hello", "world", 42)
def sayHello(a1, a2, a3, a4):
    print('sayHello arguments:', a1, a2, a3, a4)

print("After decoration")

print("Preparing to call sayHello()")
sayHello("say", "hello", "argument", "list")
print("after first sayHello() call")
sayHello("a", "different", "set of", "arguments")
print("after second sayHello() call")

From the output, we can see that the behavior changes quite significantly:

Inside __init__()
Inside __call__()
After decoration
Preparing to call sayHello()
Inside wrapped_f()
Decorator arguments: hello world 42
sayHello arguments: say hello argument list
After f(*args)
after first sayHello() call
Inside wrapped_f()
Decorator arguments: hello world 42
sayHello arguments: a different set of arguments
After f(*args)
after second sayHello() call

Now the process of decoration calls the constructor and then immediately invokes __call__(), which can only take a single argument (the function object) and must return the decorated function object that replaces the original. Notice that __call__() is now only invoked once, during decoration, and after that the decorated function that you return from __call__() is used for the actual calls.

Although this behavior makes sense – the constructor is now used to capture the decorator arguments, but the object __call__() can no longer be used as the decorated function call, so you must instead use __call__() to perform the decoration – it is nonetheless surprising the first time you see it because it’s acting so much differently than the no-argument case, and you must code the decorator very differently from the no-argument case.