input: Write a function to prompt the user to enter the value of radius to calculate the area of a circle. Return the calculated area.
  assert calc_circle_area()==None
  assert calc_circle_area()==None
  assert calc_circle_area()==None
  assert calc_circle_area()==None
  assert calc_circle_area()==None
output: def calc_circle_area():
      radius = float(input("Enter the radius: "))
      area = 3.14159 * radius * radius
      return area