bmi_cal.py

import time

class BmiData:
  def __init__(self):
    self.weight = 0
    self.height = 0

def main(bmiData):
  for i in range(100):
    bmiData.height += 1
    time.sleep(1)

gui.py

import time
from bmi_cal import *
import threading

bmiData = BmiData()
threading.Thread(target=main, args=(bmiData,)).start()

while True:
  print(f"Weight = {bmiData.weight}, height = {bmiData.height}")
  time.sleep(3)