How to use the rebound.set_dt function in rebound

def simulation(par):
    integrator, run, trial = par
    rebound.reset()
    k = 0.01720209895    
    Gfac = 1./k
    rebound.set_dt(dt*pow(2.,trial))
    rebound.set_integrator(integrator)
    rebound.set_force_is_velocitydependent(0)

    massfac = 1.
    rebound.add_particle(m=1.00000597682, x=-4.06428567034226e-3, y=-6.08813756435987e-3, z=-1.66162304225834e-6,      vx=+6.69048890636161e-6*Gfac, vy=-6.33922479583593e-6*Gfac, vz=-3.13202145590767e-9*Gfac)   # Sun
    rebound.add_particle(m=massfac/1407.355,   x=+3.40546614227466e+0, y=+3.62978190075864e+0, z=+3.42386261766577e-2, vx=-5.59797969310664e-3*Gfac, vy=+5.51815399480116e-3*Gfac, vz=-2.66711392865591e-6*Gfac)   # Jupiter
    rebound.add_particle(m=massfac/3501.6,     x=+6.60801554403466e+0, y=+6.38084674585064e+0, z=-1.36145963724542e-1, vx=-4.17354020307064e-3*Gfac, vy=+3.99723751748116e-3*Gfac, vz=+1.67206320571441e-5*Gfac)   # Saturn
    rebound.add_particle(m=massfac/22869.,     x=+1.11636331405597e+1, y=+1.60373479057256e+1, z=+3.61783279369958e-1, vx=-3.25884806151064e-3*Gfac, vy=+2.06438412905916e-3*Gfac, vz=-2.17699042180559e-5*Gfac)   # Uranus
    rebound.add_particle(m=massfac/19314.,     x=-3.01777243405203e+1, y=+1.91155314998064e+0, z=-1.53887595621042e-1, vx=-2.17471785045538e-4*Gfac, vy=-3.11361111025884e-3*Gfac, vz=+3.58344705491441e-5*Gfac)   # Neptune
    N = rebound.get_N()
    particles = rebound.get_particles()
    np.random.seed(run)
    for i in xrange(N):
        particles[i].m *= 1.+1e-3*np.random.rand()
        particles[i].x *= 1.+1e-3*np.random.rand()
        particles[i].y *= 1.+1e-3*np.random.rand()

rebound.set_G(k*k)      # Gravitational constant

# Setup particles (data taken from NASA Horizons)
# This could also be easily read in from a file.
rebound.add_particle( m=1.00000597682, x=-4.06428567034226e-3, y=-6.08813756435987e-3, z=-1.66162304225834e-6, vx=+6.69048890636161e-6, vy=-6.33922479583593e-6, vz=-3.13202145590767e-9 )  # Sun
rebound.add_particle( m=1./1047.355,   x=+3.40546614227466e+0, y=+3.62978190075864e+0, z=+3.42386261766577e-2, vx=-5.59797969310664e-3, vy=+5.51815399480116e-3, vz=-2.66711392865591e-6 )  # Jupiter
rebound.add_particle( m=1./3501.6,     x=+6.60801554403466e+0, y=+6.38084674585064e+0, z=-1.36145963724542e-1, vx=-4.17354020307064e-3, vy=+3.99723751748116e-3, vz=+1.67206320571441e-5 )  # Saturn
rebound.add_particle( m=1./22869.,     x=+1.11636331405597e+1, y=+1.60373479057256e+1, z=+3.61783279369958e-1, vx=-3.25884806151064e-3, vy=+2.06438412905916e-3, vz=-2.17699042180559e-5 )  # Uranus
rebound.add_particle( m=1./19314.,     x=-3.01777243405203e+1, y=+1.91155314998064e+0, z=-1.53887595621042e-1, vx=-2.17471785045538e-4, vy=-3.11361111025884e-3, vz=+3.58344705491441e-5 )  # Neptune

# Set the center of momentum to be at the origin
rebound.move_to_center_of_momentum()

rebound.set_integrator("mercury")
#rebound.set_integrator("mikkola")
rebound.set_dt(40.)
rebound.integrate(1e7)
print(rebound.get_timing())
print(rebound.status())
rebound.integrate(2e7)
print(rebound.get_timing())
print(rebound.status())

def simulation(par):
    integrator, run, trial = par
    rebound.reset()
    k = 0.01720209895    
    G = k*k
    rebound.set_G(G)     
    rebound.set_dt(dt)
    rebound.set_integrator(integrator)
    rebound.set_force_is_velocitydependent(0)

    rebound.add_particle(m=1.00000597682, x=-4.06428567034226e-3, y=-6.08813756435987e-3, z=-1.66162304225834e-6, vx=+6.69048890636161e-6, vy=-6.33922479583593e-6, vz=-3.13202145590767e-9)   # Sun
    rebound.add_particle(m=1./1047.355,   x=+3.40546614227466e+0, y=+3.62978190075864e+0, z=+3.42386261766577e-2, vx=-5.59797969310664e-3, vy=+5.51815399480116e-3, vz=-2.66711392865591e-6)   # Jupiter
    rebound.add_particle(m=1./3501.6,     x=+6.60801554403466e+0, y=+6.38084674585064e+0, z=-1.36145963724542e-1, vx=-4.17354020307064e-3, vy=+3.99723751748116e-3, vz=+1.67206320571441e-5)   # Saturn
    rebound.add_particle(m=1./22869.,     x=+1.11636331405597e+1, y=+1.60373479057256e+1, z=+3.61783279369958e-1, vx=-3.25884806151064e-3, vy=+2.06438412905916e-3, vz=-2.17699042180559e-5)   # Uranus
    rebound.add_particle(m=1./19314.,     x=-3.01777243405203e+1, y=+1.91155314998064e+0, z=-1.53887595621042e-1, vx=-2.17471785045538e-4, vy=-3.11361111025884e-3, vz=+3.58344705491441e-5)   # Neptune
#    rebound.add_particle(m=0,             x=-2.13858977531573e+1, y=+3.20719104739886e+1, z=+2.49245689556096e+0, vx=-1.76936577252484e-3, vy=-2.06720938381724e-3, vz=+6.58091931493844e-4)   # Pluto
    N = rebound.get_N()
    particles = rebound.get_particles()
    np.random.seed(run)
    for i in xrange(N):
        particles[i].m *= 1.+1e-3*np.random.rand()
        particles[i].x *= 1.+1e-3*np.random.rand()
        particles[i].y *= 1.+1e-3*np.random.rand()

def simulation(par):
    integrator = par
    rebound.reset()
    k = 0.01720209895    
    G = k*k
    rebound.set_G(G)     
    rebound.set_dt(dt)
    rebound.set_integrator(integrator)

    rebound.add_particle(m=1.00000597682, x=-4.06428567034226e-3, y=-6.08813756435987e-3, z=-1.66162304225834e-6, vx=+6.69048890636161e-6, vy=-6.33922479583593e-6, vz=-3.13202145590767e-9)   # Sun
    rebound.add_particle(m=1./1047.355,   x=+3.40546614227466e+0, y=+3.62978190075864e+0, z=+3.42386261766577e-2, vx=-5.59797969310664e-3, vy=+5.51815399480116e-3, vz=-2.66711392865591e-6)   # Jupiter
#    rebound.add_particle(m=1./3501.6,     x=+6.60801554403466e+0, y=+6.38084674585064e+0, z=-1.36145963724542e-1, vx=-4.17354020307064e-3, vy=+3.99723751748116e-3, vz=+1.67206320571441e-5)   # Saturn
#    rebound.add_particle(m=1./22869.,     x=+1.11636331405597e+1, y=+1.60373479057256e+1, z=+3.61783279369958e-1, vx=-3.25884806151064e-3, vy=+2.06438412905916e-3, vz=-2.17699042180559e-5)   # Uranus
#    rebound.add_particle(m=1./19314.,     x=-3.01777243405203e+1, y=+1.91155314998064e+0, z=-1.53887595621042e-1, vx=-2.17471785045538e-4, vy=-3.11361111025884e-3, vz=+3.58344705491441e-5)   # Neptune
#    rebound.add_particle(m=0,             x=-2.13858977531573e+1, y=+3.20719104739886e+1, z=+2.49245689556096e+0, vx=-1.76936577252484e-3, vy=-2.06720938381724e-3, vz=+6.58091931493844e-4)   # Pluto
    N = rebound.get_N()

    def move_to_heliocentric():
        particles = rebound.get_particles()
        
        for i in xrange(1,N):
            particles[i].x -= particles[0].x
            particles[i].y -= particles[0].y

# Import the rebound module
import sys; sys.path.append('../')
import rebound
from rebound import Particle

# Set variables (defaults are G=1, t=0, dt=0.01)
k = 0.01720209895       # Gaussian constant 
rebound.set_G(k*k)      # Gravitational constant

# Choose the symplectic WHFast integrator 
rebound.set_integrator("whfast")

# This integrator is not adaptive, so we need to set the timestep
rebound.set_dt(40.)     # 40 days (the time unit depends on the unit of G, see above).

# Setup particles (data taken from NASA Horizons)
# This could also be easily read in from a file.
rebound.particle_add( Particle( m=1.00000597682, x=-4.06428567034226e-3, y=-6.08813756435987e-3, z=-1.66162304225834e-6, vx=+6.69048890636161e-6, vy=-6.33922479583593e-6, vz=-3.13202145590767e-9) )  # Sun
rebound.particle_add( Particle( m=1./1047.355,   x=+3.40546614227466e+0, y=+3.62978190075864e+0, z=+3.42386261766577e-2, vx=-5.59797969310664e-3, vy=+5.51815399480116e-3, vz=-2.66711392865591e-6) )  # Jupiter
rebound.particle_add( Particle( m=1./3501.6,     x=+6.60801554403466e+0, y=+6.38084674585064e+0, z=-1.36145963724542e-1, vx=-4.17354020307064e-3, vy=+3.99723751748116e-3, vz=+1.67206320571441e-5) )  # Saturn
rebound.particle_add( Particle( m=1./22869.,     x=+1.11636331405597e+1, y=+1.60373479057256e+1, z=+3.61783279369958e-1, vx=-3.25884806151064e-3, vy=+2.06438412905916e-3, vz=-2.17699042180559e-5) )  # Uranus
rebound.particle_add( Particle( m=1./19314.,     x=-3.01777243405203e+1, y=+1.91155314998064e+0, z=-1.53887595621042e-1, vx=-2.17471785045538e-4, vy=-3.11361111025884e-3, vz=+3.58344705491441e-5) )  # Neptune
rebound.particle_add( Particle( m=0,             x=-2.13858977531573e+1, y=+3.20719104739886e+1, z=+2.49245689556096e+0, vx=-1.76936577252484e-3, vy=-2.06720938381724e-3, vz=+6.58091931493844e-4) )  # Pluto

# Set the center of momentum to be at the origin
rebound.move_to_center_of_momentum()

# Get the particle data
# Note: this is a pointer and will automatically update as the simulation progresses
particles = rebound.particles_get()