Simple method to mock environment variable in ruby with minitest or other testing framework

mock_env.rb

# in test_helper.rb (for example)
def mock_env(partial_env_hash)
  old_env = ENV.to_hash
  ENV.update partial_env_hash
  begin
    yield
  ensure
    ENV.replace old_env
  end
end

# usage
mock_env('MY_ENV_VAR' => 'Hello') do
  assert something?
end