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.to_hash
  ENV.update partial_env_hash
  begin
    yield
  ensure
    ENV.replace old
  end
end

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

Nice, I just modified it a bit so I can test methods that use Rails.env.production? etc and added defaults.

def mock_enviroment(env: "test", partial_env_hash: {})
  old_env_mode = Rails.env
  old_env = ENV.to_hash
	
  Rails.env = env
  ENV.update(partial_env_hash)
	
  begin
    yield
  ensure
     Rails.env = old_env_mode
     ENV.replace(old_env)
  end
end


# usage
# mock_enviroment(env:"production", partial_env_hash:{"SECRET"=>"123"}) do
#   assert something?
# end