input: Write a function to generate a Python class that represents a 3-dimensional vector with methods to initialize the vector and calculate its length. The class should be named Vector3D and have an initialization method that takes three parameters: x, y, and z. Additionally, the class should have a method called length that returns the Euclidean distance 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, 0).length() == 5
  assert Vector3D(0, 3, 4).length() == 5
  assert Vector3D(2, 2, 1).length() == 3
  assert Vector3D(1, 1, 1).length() == 1.7320508075688772
  assert Vector3D(-1, -2, 3).length() == 3.7416573867739413
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