Define a method liked `ENV.fetch()` which returns specified default value even when a value of specified key was empty string. see: https://mrk21.hatenablog.com/entry/2024/03/09/190844

env.rb

class << ENV
  # This method behavior is almost same as `ENV.fetch()`, but it returns default value even when the value was empty string.
  def fetch2(*args, &block)
    key, default = args
    value = fetch(*args, &block)
    return value if value != ''
    
    raise KeyError, format('key not found: "%s"', key) if args.size == 1 && !block
    warn('block supersedes default value argument', uplevel: 1) if args.size == 2 && block
    block ? yield(key) : default
  end
end

env_spec.rb

require_relative './env'

RSpec.describe ENV do
  before do
    described_class['__DUMMY_FOR_TEST1'] = '1'
    described_class['__DUMMY_FOR_TEST3'] = ''
  end

  after do
    described_class.delete('__DUMMY_FOR_TEST1')
    described_class.delete('__DUMMY_FOR_TEST3')
  end

  describe '.fetch2(key, default, &block)' do
    context 'existed `key`' do
      let(:key) { '__DUMMY_FOR_TEST1' }

      context 'did not specify neither `default` nor `block`' do
        it 'returns value of `key`' do
          expect(described_class.fetch2(key)).to eq '1'
        end
      end

      context 'specified `default`' do
        it 'returns value of `key`' do
          expect(described_class.fetch2(key, '2')).to eq '1'
        end
      end

      context 'specified `block`' do
        it 'returns value of `key`' do
          expect(described_class.fetch2(key) { '3' }).to eq '1'
          described_class.fetch2(key) { |k| expect(k).to eq key }
        end
      end

      context 'specified `default` with `block`' do
        it 'outputs warn, and it returns value of `key`' do
          expect do
            expect(described_class.fetch2(key, '2') { '3' }).to eq '1'
          end.to output(%r{env.rb:\d+: warning: block supersedes default value argument\n}).to_stderr

          described_class.fetch2(key, '2') { |k| expect(k).to eq key }
        end
      end
    end

    context 'did not exist `key`' do
      let(:key) { '__DUMMY_FOR_TEST2' }

      context 'did not specify neither `default` nor `block`' do
        it 'raises exception' do
          expect { described_class.fetch2(key) }.to raise_error(KeyError, 'key not found: "%s"' % key)
        end
      end

      context 'specified `default`' do
        it 'returns `default`' do
          expect(described_class.fetch2(key, '2')).to eq '2'
        end
      end

      context 'specified `block`' do
        it 'returns return value of `block`' do
          expect(described_class.fetch2(key) { '3' }).to eq '3'
          described_class.fetch2(key) { |k| expect(k).to eq key }
        end
      end

      context 'specified `default` with `block`' do
        it 'outputs warn, and it returns return value of `block`' do
          expect do
            expect(described_class.fetch2(key, '2') { '3' }).to eq '3'
          end.to output(%r{env.rb:\d+: warning: block supersedes default value argument\n}).to_stderr

          described_class.fetch2(key, '2') { |k| expect(k).to eq key }
        end
      end
    end

    context 'existed `key`, but the value was empty string' do
      let(:key) { '__DUMMY_FOR_TEST3' }

      context 'did not specify neither `default` nor `block`' do
        it 'raises exception' do
          expect { described_class.fetch2(key) }.to raise_error(KeyError, 'key not found: "%s"' % key)
        end
      end

      context 'specified `default`' do
        it 'returns `default`' do
          expect(described_class.fetch2(key, '2')).to eq '2'
        end
      end

      context 'specified `block`' do
        it 'returns return value of `block`' do
          expect(described_class.fetch2(key) { '3' }).to eq '3'
          described_class.fetch2(key) { |k| expect(k).to eq key }
        end
      end

      context 'specified `default` with `block`' do
        it 'outputs warn, and it returns return value of `block`' do
          expect do
            expect(described_class.fetch2(key, '2') { '3' }).to eq '3'
          end.to output(%r{env.rb:\d+: warning: block supersedes default value argument\n}).to_stderr

          described_class.fetch2(key, '2') { |k| expect(k).to eq key }
        end
      end
    end
  end
end