input: Write a function to create a Python class that represents a 3-dimensional vector with attributes for the x, y, and z components. Include a method to calculate the length of the vector. Name the class Vector3D and implement the above functionality.
  
  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, 0).length() == 5.0
  assert Vector3D(1, 1, 1).length() == 3**0.5
  assert Vector3D(2, 2, 2).length() == 3 * 2**0.5
output: 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