Decorator for adding obvious assertions to functions

obviously.py

def obviously(v, msg=''):
    def outer(f):
        def inner(*args, **kwargs):
            r = f(*args, **kwargs)
            if not v(r):
                if msg:
                    errmsg = f.__name__ + ': ' + msg
                else:
                    errmsg = f.__name__ + ': obviously not true'
                raise Exception(errmsg)
            return r
        return inner
    return outer

@obviously(lambda x: 0 <= x <= 1, "Probabilities must be between 0 and 1")
def my_probability_thingy(x):
    return x

my_probability_thingy(0)
my_probability_thingy(0.5)
my_probability_thingy(1)
my_probability_thingy(2)
my_probability_thingy(-1)