input: Write a function to create a class in Python to represent a complex number. The class should have an initializer to accept the real and imaginary parts of the complex number. Additionally, the class should have a string representation method to display the complex number in the form "(a + bi)".
  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(3, 4))=="(3 + 4i)"
  assert str(ComplexNumber(-1, 2))=="(-1 + 2i)"
  assert str(ComplexNumber(0, 0))=="(0 + 0i)"
  assert str(ComplexNumber(1, -1))=="(1 + -1i)"
  assert str(ComplexNumber(5, 0))=="(5 + 0i)"
output: def __init__(self, real, imaginary):
      self.real = real
      self.imaginary = imaginary
  
  def __str__(self):
      return "({0} + {1}i)".format(self.real, self.imaginary)