input: Write a function to generate a Python class that represents a 3-dimensional vector. The class should have an initializer that takes three arguments (x, y, z) to set the vector's components. It should also have a method named length that calculates and returns the Euclidean 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, 0).length() == 5.0
  assert Vector3D(1, 1, 1).length() == 1.7320508075688772
  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