text: import pytest
  from calculator import Calculator
  
  correct_expression = '-1.2 * (2 + 6 / 3) + 6 / (5 + (-6 + 8) / 2)'  # -3.8
  correct_expression2 = '6 / (5 + (-6 + 8) / 2)'  # 1
  correct_expression3 = '-1.2 * (2 + 6 / 3)'  # -4.8
  wrong_expression = '-1.2 * 2 + 6 / 3)'
  
  
  def test_simple_calc():
      res = Calculator(correct_expression).simple_calc()
      assert res == -3.8
  
  
  def test_calc():
      res = Calculator(correct_expression).calc()
      assert res == -3.8
      res = Calculator(correct_expression2).calc()
      assert res == 1.0
      res = Calculator(correct_expression3).calc()
      assert res == -4.8
      res = Calculator(wrong_expression).calc()
      assert res == None
  
  