input: Write a function to create a class in Python to represent a complex number, with an initialization method to store its real and imaginary parts, and a string representation method to display the complex number in the format "(real + imaginary*i)".
  class ComplexNumber:
      def __init__(self, real, imaginary):
          self.real = real
          self.imaginary = imaginary
  
      def __str__(self):
          return "({0} + {1}i)".format(self.real, self.imaginary)
  
  assert str(ComplexNumber(1, 2))=="(1 + 2i)"
  assert str(ComplexNumber(-3, 4))=="(-3 + 4i)"
  assert str(ComplexNumber(0, 0))=="(0 + 0i)"
  assert str(ComplexNumber(5, -6))=="(5 + -6i)"
  assert str(ComplexNumber(-2, -7))=="(-2 + -7i)"
output: def __init__(self, real, imaginary):
      self.real = real
      self.imaginary = imaginary
  
  def __str__(self):
      return "({0} + {1}i)".format(self.real, self.imaginary)