How to allow parameters for a decorator while keep the function's signature at the same time?

Question

I'm trying to write a decorator which will do some actions based on kwargs while I want to keep the functions signature at the same time.

Here is the unit test I want to pass:

def test_test_deprecator(self):

    @test_deprecator("here is decorator message", 'z'):
    def some_method(x, y, z):
        return x + y

    self.assertEqual(some_method(2, 2, z=6), 4)

    # test the original function's signature
    argspec = inspect.getargspec(some_method)
    self.assertEqual(argspec.args, ["x", "y", "z"])

I wrote something like:

import decorator

def test_decorator(message, args_name):

    @decorator.decorator
    def _test_docorator(f, *args, **kwargs):
        if args_name in kwargs:
            print(message)
        return f(*args, **kwargs)

    return _test_docorator

The unit test can pass, but there is no output message, because kwargs is always empty.

Does anyone know how to achieve this?

Answer 1

I'm not sure what the role of @decorator.decorator is in your code, but here is a decorator doing what you describe:

def test_decorator(message, arg_name):
    def _test_decorator(f):
        def wrapped_f(*args, **kwargs):
            if arg_name in kwargs:
                print(message)
            return f(*args, **kwargs)
        return wrapped_f
    return _test_decorator

Updated answer:

I learned a bit about decorators looking into this. In short, though, I think using the wrapt module would work for you:

def test_decorator(message, arg_name):
    @wrapt.decorator
    def wrapped_f(f, instance, args, kwargs):
        if arg_name in kwargs:
            print(message)
        return f(*args, **kwargs)
    return wrapped_f

Test code:

@test_decorator('found it', 'y')
def foo(x="bad", y="dog"):
    print("x={}, y={}".format(x, y))

def bar(x="bad", y="dog"):
    print("x={}, y={}".format(x, y))

print ("foo argspec={}".format(inspect.getargspec(foo)))
print ("bar argspec={}".format(inspect.getargspec(bar)))

foo(y='cat')

Output:

foo argspec=ArgSpec(args=['x', 'y'], varargs=None, keywords=None, defaults=('bad', 'dog'))
bar argspec=ArgSpec(args=['x', 'y'], varargs=None, keywords=None, defaults=('bad', 'dog'))
found it
x=bad, y=cat