RPN Calculator kata solution (Ruby)

rpn.rb

class RPN

  def initialize(operation)
    @operation = operation.split(' ')
  end

  def calculate
    stack = []
    @operation.each do |value|
      if is_operator?(value)
        stack.push(eval_subexpression(stack, value))
      elsif is_number?(value)
        stack.push(value)
      else
        raise 'Malformed expression'
      end
    end
    stack * ' '
  end

  private

  def eval_subexpression(stack, value)
    raise 'Malformed expression' unless stack.length >= 2
    b = stack.pop
    a = stack.pop
    operator = value == '^' ? '**' : value
    eval("#{a} #{operator} #{b}").to_s rescue raise 'Division by 0'
  end

  def is_number?(value)
    value.to_i.to_s == value rescue false
  end

  def is_operator?(value)
    %w(+ - * / ^).include? value
  end

end

rpn_test.rb

require 'minitest/autorun'
require './rpn'

class TestRPN < Minitest::Test

  def test_when_empty_input_returns_empty_result
    rpn = RPN.new('')
    assert_equal '', rpn.calculate
  end

  def test_single_number_returns_same_number
    rpn = RPN.new('5')
    assert_equal '5', rpn.calculate
  end

  def test_add_two_numbers_returns_the_addition
    rpn = RPN.new('5 3 +')
    assert_equal '8', rpn.calculate
  end

  def test_with_three_operands_returns_correct_value
    rpn = RPN.new('4 2 + 3 -')
    assert_equal '3', rpn.calculate
  end

  def test_with_multiple_operands_with_priorities_returns_correct_result
    rpn = RPN.new('3 5 8 * 7 + *')
    assert_equal '141', rpn.calculate
  end

  def test_with_extra_values_without_operator_returns_appended_results
    rpn = RPN.new('7 2 - 3 4')
    assert_equal '5 3 4', rpn.calculate
  end

  def test_with_exponentiation_returns_exponent_result
    rpn = RPN.new('7 2 ^')
    assert_equal '49', rpn.calculate
  end

  def test_when_divide_by_zero_raise_custom_divison_exception
    rpn = RPN.new('7 0 /')
    assert_raises 'Division by 0' do
      rpn.calculate
    end
  end

  def test_when_non_valid_characters_raise_custom_malformed_expression
    rpn = RPN.new('7 2 &')
    assert_raises 'Malformed expression' do
      rpn.calculate
    end
  end

  def test_when_operators_placed_wrong_raise_custom_malformed_expression
    rpn = RPN.new('7 2 + +')
    assert_raises 'Malformed expression' do
      rpn.calculate
    end
  end

end