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Here's an expansion on Michael Speer's answer to take it a few steps further:

An instance method decorator which takes arguments and acts on a function with arguments and a return value.

class Test(object):
 "Prints if x == y. Throws an error otherwise."
 def __init__(self, x):
 self.x = x
 
 def _outer_decorator(y):
 def _decorator(foo):
 def magic(self, *args, **kwargs) :
 print("start magic")
 if self.x == y:
 return foo(self, *args, **kwargs)
 else:
 raise ValueError("x ({}) != y ({})".format(self.x, y))
 print("end magic")
 return magic
 
 return _decorator
 @_outer_decorator(y=3)
 def bar(self, *args, **kwargs) :
 print("normal call")
 print("args: {}".format(args))
 print("kwargs: {}".format(kwargs))
 
 return 27

And then

In [2]:
 
 test = Test(3)
 test.bar(
 13,
 'Test',
 q=9,
 lollipop=[1,2,3]
 )
 ​
 start magic
 normal call
 args: (13, 'Test')
 kwargs: {'q': 9, 'lollipop': [1, 2, 3]}
Out[2]:
 27
In [3]:
 
 test = Test(4)
 test.bar(
 13,
 'Test',
 q=9,
 lollipop=[1,2,3]
 )
 ​
 start magic
 ---------------------------------------------------------------------------
 ValueError Traceback (most recent call last)
 <ipython-input-3-576146b3d37e> in <module>()
 4 'Test',
 5 q=9,
 ----> 6 lollipop=[1,2,3]
 7 )
 
 <ipython-input-1-428f22ac6c9b> in magic(self, *args, **kwargs)
 11 return foo(self, *args, **kwargs)
 12 else:
 ---> 13 raise ValueError("x ({}) != y ({})".format(self.x, y))
 14 print("end magic")
 15 return magic
 
 ValueError: x (4) != y (3)