I'm struggling to understand how to add this test case.  

 

19. Implement circ_area
Now that you have fixed the header for circ_area, it is time to implement the function. Again, pay close attention to the specification. If you are having trouble understanding the specification, look at the test cases in tests.py. When you have implemented the function, test your answer with the test script. You should pass all tests (for now).

def circ_area(**kw):    # The parameter is MISSING.  Add it back.
   """
   Returns the area of the specified circle, defined by the keywords in kwd
   
   The area of a circle is PI r*r where r is the radius
   
   The circle may be specified by 'radius' or 'diameter', but not both.  This function
   should intentionally crash (with an AssertionError) if neither 'radius' nor 'diameter'
   are specified, or if they both are.
   
   Any keyword arguments other than 'radius' or 'diameter' are ignored.
   
   Examples:
       circ_area(radius=3) returns approx 28.27433
       circ_area(diameter=4) returns approx 12.56637
       circ_area(radius=3,foo=20) returns approx 28.27433
       circ_area() crashes with AssertionError
       circ_area(radius=2,diameter=4) crashes with AssertionError
   
   Parameter kwd: the function keyword arguments
   Precondition: the arguments are all numbers (int or float)
   """
   assert ('radius' in kw) ^ ('diameter' in kw), 'Either radius or diameter must be specified, but not both'
   import math
   if 'radius' in kw:
       return math.pi * kw['radius']**2
   else:
       return math.pi * (kw['diameter']/2)**2

def test_circ_area():
   """
   Test procedure for function circ_area().
   """
   print('Testing circ_area()')
   
   result = func.circ_area(radius=3)
   introcs.assert_floats_equal(28.27433,result)
   
   result = func.circ_area(radius=2)
   introcs.assert_floats_equal(12.56637,result)
   
   result = func.circ_area(diameter=4)
   introcs.assert_floats_equal(12.56637,result)
   
   # Test for crashes
   try:
       func.circ_area()
       introcs.assert_true(False) # We should never reach this line!
   except:
       pass
   
   try:
       func.circ_area(radius=3,diameter=6)
       introcs.assert_true(False) # We should never reach this line!
   except:
       pass
   
   # Add a test with additional arguments