input: Write a function to generate a Python class that represents a 3-dimensional vector with attributes x, y, and z. The class should also have a method to calculate and return 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() == (50)**0.5
  assert Vector3D(1, 1, 1).length() == (3)**0.5
  assert Vector3D(0, 0, 0).length() == 0
  assert Vector3D(-2, 2, -1).length() == (9)**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