assert_differences

assert_differences.rb

  def assert_differences(expression_array, by:, message: nil, &block)
    expressions = Array(expression_array)
    change_by = Array(by).map(&:to_i)

    if expressions.size != change_by.size
      raise ArgumentError, 'Each expression must have a corresponding value to change by'
    end

    exps = expressions.map.with_index do |e, i|
      callable = e.respond_to?(:call) ? e : lambda { eval(e, block.binding) }
      {
        callable: callable,
        before: callable.call,
        expected_change: change_by[i]
      }
    end

    retval = yield

    expressions.zip(exps).each do |code, e|
      difference = e[:expected_change]
      error      = "#{code.inspect} didn't change by #{difference}"
      error      = "#{message}.\n#{error}" if message
      assert_equal(e[:before] + difference, e[:callable].call, error)
    end

    retval
  end

I wrote about this and how to implement it in a blog post: https://kierancodes.com/blog/assert-multiple-differences-in-minitest-with-assert_differences