Default arguments with *args and **kwargs

In Python 2.x (I use 2.7), which is the proper way to use default arguments with *args and **kwargs?
I've found a question on SO related to this topic, but that is for Python 3:
Calling a Python function with *args,**kwargs and optional / default arguments

There, they say this method works:

def func(arg1, arg2, *args, opt_arg='def_val', **kwargs):
    #...

In 2.7, it results in a SyntaxError. Is there any recommended way to define such a function?
I got it working this way, but I'd guess there is a nicer solution.

def func(arg1, arg2, *args, **kwargs):
    opt_arg ='def_val'
    if kwargs.__contains__('opt_arg'):
        opt_arg = kwargs['opt_arg']
    #...