Helper to have temporary environment variables (ENV), ie for testing.

temp_env.rb

# In order to make testing of environment variables easier,
# you can use this helper to temporarily change the ENV to something you expect.
#
# As an example:
#
# Imagine you have some ENV variable that changes based on the current runtime environment.
#
# def user_id
#   ENV['TEST_USER_ID'].present? ? ENV['TEST_USER_ID'] : session[:user_id]
# end
#
# Then in your test it would be used like
#
# it 'should return the test-user-id when it is passed through the env' do
#   ENV.with_env('TEST_USER_ID' => '123') do
#     user_id.must_equal('123')
#   end
# end
ENV.define_singleton_method(:with_env) do |new_env, &block|
  old_env = to_hash
  clear
  update(new_env)
  begin
    block.call
  ensure
    clear
    update(old_env)
  end
end

if __FILE__ == $0
  require 'minitest/autorun'

  describe ENV do
    it 'should have the defined arguments only in the scope of the block' do
      old_env = ENV.to_hash
      ENV.with_env('hi' => 'there') do
        ENV['hi'].must_equal('there')
      end
      ENV.to_hash.must_equal(old_env)
    end

    it 'should not have existing arguments in the scope of the block' do
      ENV['predefined'] = 'yes'
      ENV.with_env('hi' => 'there') do
        ENV['predefined'].must_equal(nil)
      end
      ENV['predefined'].must_equal('yes')
    end
  end
end