input: Write a function to generate a Python class that represents a 3-dimensional vector. The class should have an __init__ method to initialize the vector's x, y, and z components, and a method to calculate the length of the vector. If successful, the function should return the class definition.
  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(1, 2, 2).length()==3.0
  assert Vector3D(3, 4, 0).length()==5.0
  assert Vector3D(0, 0, 0).length()==0.0
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