input: Write a function to create a Python class that represents a 3-dimensional vector. The class should have attributes for the x, y, and z coordinates, and a method to calculate the length (magnitude) of the vector. 
  class Vector3D:
      def __init__(self, x, y, z):
          self.x = x
          self.y = y
          self.z = z
      
      def length(self):
          return (self.x**2 + self.y**2 + self.z**2)**0.5
  assert Vector3D(3, 4, 5).length() == (3**2 + 4**2 + 5**2)**0.5
  assert Vector3D(6, 8, 0).length() == (6**2 + 8**2 + 0**2)**0.5
  assert Vector3D(0, 0, 1).length() == (0**2 + 0**2 + 1**2)**0.5
  assert Vector3D(1, 1, 1).length() == (1**2 + 1**2 + 1**2)**0.5
  assert Vector3D(0, 0, 0).length() == (0**2 + 0**2 + 0**2)**0.5
output: def __init__(self, x, y, z):
      self.x = x
      self.y = y
      self.z = z
  
  def length(self):
      return (self.x**2 + self.y**2 + self.z**2)**0.5