How can one swap elements which come from a function?

Question

So I have defined a class called "Particles":

class Particle:
    def __init__(self, posInit, momInit, spin):
        self.posInit = posInit
        self.momInit = momInit
        self.spin = spin
    def momfT(self, t):
        return self.momInit*(math.cos(t))-self.posInit*(math.sin(t))
    def posfT(self, t):
        return self.posInit*(math.cos(t))+self.momInit*(math.sin(t))

P = [Particle(posInit = i, momInit = j, spin = choice(["up", "down"])) for i,j in zip(Zinitial,Pinitial)]

What I now want to do is switch the positions of the particles if a certain condition is met. So something like the following:

if cond==True:
  P[1].posfT[t], P[2].posfT[t1] = P[2].posfT[t1], P[1].posfT[t]

But the above does not work since I am trying to assign to a function a value.

So I am not sure how to do this?