input: Write a function to create a Python class representing a 3-dimensional vector. The class should have an initialization method that accepts three values for x, y, and z coordinates, and a method to calculate the vector's length. 
  
  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, 0, 1).length() == 1.4142135623730951
  assert Vector3D(0, 0, 0).length() == 0.0
  assert Vector3D(-2, 3, 5).length() == 6.164414002968976
  assert Vector3D(6, 8, 0).length() == 10.0
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