How to retrieve method arguments of a decorated python method

Question

I have the following code in python2.7 where the decorator is a third party code and shall not be changed:

# tasks.py
def test(val):
    print val

@app.task
def add(a, b):
    return a + b

#main.py:
import mypackage.tasks as tasks
for method in tasks.__dict__.values():
    if hasattr(method, '_decorated') and method._decorated:
        print method.name.split('.')[-1]
        ### here I want to get the method arguments

The problem is that I cannot get the method arguments of a decorated method. I've already tried:

inspect.getargspec(method)
#and
tasks.test.func_code.co_varnames

they do not work for decorated methods. Any ideas?