"show_code" does not work when ode integration method is "exp_euler"

[x] Check for duplicate issues.
[x] Provide a complete example of how to reproduce the bug, wrapped in triple backticks like this:
[x] If applicable, include full error messages/tracebacks.

In the following code:

import brainpy as bp
import brainpy.math as bm

Iext=10.;   ENa=50.;   EK=-77.;   EL=-54.387
C=1.0;      gNa=120.;  gK=36.;    gL=0.03

def dm(m, t, V):
    alpha = 0.1 * (V + 40) / (1 - bm.exp(-(V + 40) / 10))
    beta = 4.0 * bm.exp(-(V + 65) / 18)
    dmdt = alpha * (1 - m) - beta * m
    return dmdt

def dh(h, t, V):
    alpha = 0.07 * bm.exp(-(V + 65) / 20.)
    beta = 1 / (1 + bm.exp(-(V + 35) / 10))
    dhdt = alpha * (1 - h) - beta * h
    return dhdt

def dn(n, t, V):
    alpha = 0.01 * (V + 55) / (1 - bm.exp(-(V + 55) / 10))
    beta = 0.125 * bm.exp(-(V + 65) / 80)
    dndt = alpha * (1 - n) - beta * n
    return dndt

def dV(V, t, m, h, n, Iext):
    I_Na = (gNa * m ** 3.0 * h) * (V - ENa)
    I_K = (gK * n ** 4.0) * (V - EK)
    I_leak = gL * (V - EL)
    dVdt = (- I_Na - I_K - I_leak + Iext) / C
    return dVdt

hh_derivative = bp.JointEq([dV, dm, dh, dn])

integral_1 = bp.odeint(hh_derivative, method='rk2', show_code=True)
integral_2 = bp.odeint(hh_derivative, method='exp_euler', show_code=True)

Running the integral_1 = ... line outputs the following in console:

def brainpy_itg_of_ode7_joint_eq(V, m, h, n, t, Iext, dt=0.1):
  dV_k1, dm_k1, dh_k1, dn_k1 = f(V, m, h, n, t, Iext)
  k2_V_arg = V + dt * dV_k1 * 0.6666666666666666
  k2_m_arg = m + dt * dm_k1 * 0.6666666666666666
  k2_h_arg = h + dt * dh_k1 * 0.6666666666666666
  k2_n_arg = n + dt * dn_k1 * 0.6666666666666666
  k2_t_arg = t + dt * 0.6666666666666666
  dV_k2, dm_k2, dh_k2, dn_k2 = f(k2_V_arg, k2_m_arg, k2_h_arg, k2_n_arg, k2_t_arg, Iext)
  V_new = V + dV_k1 * dt * 0.25 + dV_k2 * dt * 0.75
  m_new = m + dm_k1 * dt * 0.25 + dm_k2 * dt * 0.75
  h_new = h + dh_k1 * dt * 0.25 + dh_k2 * dt * 0.75
  n_new = n + dn_k1 * dt * 0.25 + dn_k2 * dt * 0.75
  return V_new, m_new, h_new, n_new

However, running the integral_2 = ... line only outputs this:

{'f': <brainpy._src.integrators.joint_eq.JointEq object at 0x0000026BDD214610>}

If the formatted code is not shown, how can I determine the position of variables and parameters when calling the joint equation?

brainpy / BrainPy

"show_code" does not work when ode integration method is "exp_euler" #457