input: Write a function to generate a Python class representing a 3-dimensional vector. The class should have an __init__ method to initialize the vector components, and a method to compute the length 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() == 7.0710678118654755
  assert Vector3D(1, 2, 3).length() == 3.7416573867739413
  assert Vector3D(0, 0, 0).length() == 0.0
  assert Vector3D(-3, -4, 5).length() == 7.0710678118654755
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
  