code: def particles(self):
          sim = self._sim.contents
          ps = []
          if self.testparticle>=0:
              N = 1
          else:
              N = sim.N-sim.N_var 
          ParticleList = Particle*N
          ps = ParticleList.from_address(ctypes.addressof(sim._particles.contents)+self.index*ctypes.sizeof(Particle))
          return ps