input: Write a function to generate a Python class that represents a 3-dimensional vector. The class should have an initializer method to set the vector's coordinates (x, y, z) and a method to calculate the length of the vector. The length should be computed using the formula sqrt(x^2 + y^2 + z^2).
  
  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() == 3.0**0.5
  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