input: Write a function that generates a Python class to represent a 3-dimensional vector with an initializer method and a method to calculate the length of the vector. The class should be named Vector3D and have a method named length that returns the Euclidean distance of the vector from the origin.
  
  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(0, 0, 0).length() == 0.0
  assert Vector3D(2, 2, 2).length() == (12**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